Remove NUL macro

66 files changed, 2543 insertions, 2544 deletions

diff --git a/src/arabic.c b/src/arabic.c
index 54f88f8757..f62a1a9c05 100644
--- a/src/arabic.c
+++ b/src/arabic.c
@@ -1418,7 +1418,7 @@ int arabic_shape(int c, int *ccp, int *c1p, int prev_c, int prev_c1,
 
   /* Sanity check -- curr_c should, in the future, never be 0.
    * We should, in the future, insert a fatal error here. */
-  if (curr_c == NUL) {
+  if (curr_c == '\0') {
     curr_c = c;
   }
 
@@ -1467,7 +1467,7 @@ int arabic_maycombine(int two)
  */
 static int A_firstc_laa(int c, int c1)
 {
-  if ((c1 != NUL) && (c == a_LAM) && !A_is_harakat(c1)) {
+  if ((c1 != '\0') && (c == a_LAM) && !A_is_harakat(c1)) {
     return c1;
   }
   return 0;
diff --git a/src/ascii.h b/src/ascii.h
index cf488c56a7..e63bdb2fde 100644
--- a/src/ascii.h
+++ b/src/ascii.h
@@ -22,7 +22,6 @@
 #define CharOrdUp(x)    ((x) - 'A')
 #define ROT13(c, a)     (((((c) - (a)) + 13) % 26) + (a))
 
-#define NUL             '\000'
 #define BELL            '\007'
 #define BS              '\010'
 #define TAB             '\011'
diff --git a/src/blowfish.c b/src/blowfish.c
index 67712a2fbc..707d3e53ee 100644
--- a/src/blowfish.c
+++ b/src/blowfish.c
@@ -567,7 +567,7 @@ void bf_crypt_decode(char_u *ptr, long len)
 void bf_crypt_init_keys(char_u *passwd)
 {
   char_u *p;
-  for (p = passwd; *p != NUL; p++) {
+  for (p = passwd; *p != '\0'; p++) {
     BF_CFB_UPDATE(*p);
   }
 }
diff --git a/src/buffer.c b/src/buffer.c
index 8723e31be3..c4455bfe8e 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -747,7 +747,7 @@ do_bufdel (
           break;
       } else {    /* addr_count == 1 */
         arg = skipwhite(arg);
-        if (*arg == NUL)
+        if (*arg == '\0')
           break;
         if (!VIM_ISDIGIT(*arg)) {
           p = skiptowhite_esc(arg);
@@ -1232,7 +1232,7 @@ void enter_buffer(buf_T *buf)
     /* If there is no filetype, allow for detecting one.  Esp. useful for
      * ":ball" used in a autocommand.  If there already is a filetype we
      * might prefer to keep it. */
-    if (*curbuf->b_p_ft == NUL)
+    if (*curbuf->b_p_ft == '\0')
       did_filetype = FALSE;
 
     open_buffer(FALSE, NULL, 0);
@@ -1265,7 +1265,7 @@ void enter_buffer(buf_T *buf)
     (void)keymap_init();
   /* May need to set the spell language.  Can only do this after the buffer
    * has been properly setup. */
-  if (!curbuf->b_help && curwin->w_p_spell && *curwin->w_s->b_p_spl != NUL)
+  if (!curbuf->b_help && curwin->w_p_spell && *curwin->w_s->b_p_spl != '\0')
     (void)did_set_spelllang(curwin);
 
   redraw_later(NOT_VALID);
@@ -1768,7 +1768,7 @@ buflist_findpat (
       for (attempt = 0; attempt <= 3; ++attempt) {
         /* may add '^' and '$' */
         if (toggledollar)
-          *patend = (attempt < 2) ? NUL : '$';           /* add/remove '$' */
+          *patend = (attempt < 2) ? '\0' : '$';           /* add/remove '$' */
         p = pat;
         if (*p == '^' && !(attempt & 1))                 /* add/remove '^' */
           ++p;
@@ -2227,7 +2227,7 @@ setfname (
   struct stat st;
 #endif
 
-  if (ffname == NULL || *ffname == NUL) {
+  if (ffname == NULL || *ffname == '\0') {
     /* Removing the name. */
     vim_free(buf->b_ffname);
     vim_free(buf->b_sfname);
@@ -2443,7 +2443,7 @@ static int otherfile_buf(buf_T *buf, char_u *ffname
 )
 {
   /* no name is different */
-  if (ffname == NULL || *ffname == NUL || buf->b_ffname == NULL)
+  if (ffname == NULL || *ffname == '\0' || buf->b_ffname == NULL)
     return TRUE;
   if (fnamecmp(ffname, buf->b_ffname) == 0)
     return FALSE;
@@ -2653,7 +2653,7 @@ void maketitle(void)
     }
 
     t_str = buf;
-    if (*p_titlestring != NUL) {
+    if (*p_titlestring != '\0') {
       if (stl_syntax & STL_IN_TITLE) {
         int use_sandbox = FALSE;
         int save_called_emsg = called_emsg;
@@ -2714,7 +2714,7 @@ void maketitle(void)
           vim_strncpy(buf + off, (char_u *)_("help"),
               (size_t)(SPACE_FOR_DIR - off - 1));
         else
-          *p = NUL;
+          *p = '\0';
 
         /* Translate unprintable chars and concatenate.  Keep some
          * room for the server name.  When there is no room (very long
@@ -2745,7 +2745,7 @@ void maketitle(void)
 
   if (p_icon) {
     i_str = buf;
-    if (*p_iconstring != NUL) {
+    if (*p_iconstring != '\0') {
       if (stl_syntax & STL_IN_ICON) {
         int use_sandbox = FALSE;
         int save_called_emsg = called_emsg;
@@ -2766,7 +2766,7 @@ void maketitle(void)
         i_name = buf_spname(curbuf);
       else                          /* use file name only in icon */
         i_name = path_tail(curbuf->b_ffname);
-      *i_str = NUL;
+      *i_str = '\0';
       /* Truncate name at 100 bytes. */
       len = (int)STRLEN(i_name);
       if (len > 100) {
@@ -2914,7 +2914,7 @@ build_stl_str_hl (
   /* Get line & check if empty (cursorpos will show "0-1").  Note that
    * p will become invalid when getting another buffer line. */
   p = ml_get_buf(wp->w_buffer, wp->w_cursor.lnum, FALSE);
-  empty_line = (*p == NUL);
+  empty_line = (*p == '\0');
 
   /* Get the byte value now, in case we need it below. This is more
    * efficient than making a copy of the line. */
@@ -2939,22 +2939,22 @@ build_stl_str_hl (
       break;
     }
 
-    if (*s != NUL && *s != '%')
+    if (*s != '\0' && *s != '%')
       prevchar_isflag = prevchar_isitem = FALSE;
 
     /*
      * Handle up to the next '%' or the end.
      */
-    while (*s != NUL && *s != '%' && p + 1 < out + outlen)
+    while (*s != '\0' && *s != '%' && p + 1 < out + outlen)
       *p++ = *s++;
-    if (*s == NUL || p + 1 >= out + outlen)
+    if (*s == '\0' || p + 1 >= out + outlen)
       break;
 
     /*
      * Handle one '%' item.
      */
     s++;
-    if (*s == NUL)      /* ignore trailing % */
+    if (*s == '\0')      /* ignore trailing % */
       break;
     if (*s == '%') {
       if (p + 1 >= out + outlen)
@@ -2984,7 +2984,7 @@ build_stl_str_hl (
       groupdepth--;
 
       t = item[groupitem[groupdepth]].start;
-      *p = NUL;
+      *p = '\0';
       l = vim_strsize(t);
       if (curitem > groupitem[groupdepth] + 1
           && item[groupitem[groupdepth]].minwid == 0) {
@@ -3144,7 +3144,7 @@ build_stl_str_hl (
     case STL_VIM_EXPR:     /* '{' */
       itemisflag = TRUE;
       t = p;
-      while (*s != '}' && *s != NUL && p + 1 < out + outlen)
+      while (*s != '}' && *s != '\0' && p + 1 < out + outlen)
         *p++ = *s++;
       if (*s != '}')            /* missing '}' or out of space */
         break;
@@ -3167,7 +3167,7 @@ build_stl_str_hl (
       do_unlet((char_u *)"g:actual_curbuf", TRUE);
 
       if (str != NULL && *str != 0) {
-        if (*skipdigits(str) == NUL) {
+        if (*skipdigits(str) == '\0') {
           num = atoi((char *)str);
           vim_free(str);
           str = NULL;
@@ -3194,7 +3194,7 @@ build_stl_str_hl (
     case STL_VIRTCOL_ALT:
       /* In list mode virtcol needs to be recomputed */
       virtcol = wp->w_virtcol;
-      if (wp->w_p_list && lcs_tab1 == NUL) {
+      if (wp->w_p_list && lcs_tab1 == '\0') {
         wp->w_p_list = FALSE;
         getvcol(wp, &wp->w_cursor, NULL, &virtcol, NULL);
         wp->w_p_list = TRUE;
@@ -3273,7 +3273,7 @@ build_stl_str_hl (
       break;
 
     case STL_FILETYPE:
-      if (*wp->w_buffer->b_p_ft != NUL
+      if (*wp->w_buffer->b_p_ft != '\0'
           && STRLEN(wp->w_buffer->b_p_ft) < TMPLEN - 3) {
         vim_snprintf((char *)tmp, sizeof(tmp), "[%s]",
             wp->w_buffer->b_p_ft);
@@ -3283,7 +3283,7 @@ build_stl_str_hl (
 
     case STL_FILETYPE_ALT:
       itemisflag = TRUE;
-      if (*wp->w_buffer->b_p_ft != NUL
+      if (*wp->w_buffer->b_p_ft != '\0'
           && STRLEN(wp->w_buffer->b_p_ft) < TMPLEN - 2) {
         vim_snprintf((char *)tmp, sizeof(tmp), ",%s",
             wp->w_buffer->b_p_ft);
@@ -3325,7 +3325,7 @@ build_stl_str_hl (
 
     case STL_HIGHLIGHT:
       t = s;
-      while (*s != '#' && *s != NUL)
+      while (*s != '#' && *s != '\0')
         ++s;
       if (*s == '#') {
         item[curitem].type = Highlight;
@@ -3333,7 +3333,7 @@ build_stl_str_hl (
         item[curitem].minwid = -syn_namen2id(t, (int)(s - t));
         curitem++;
       }
-      if (*s != NUL)
+      if (*s != '\0')
         ++s;
       continue;
     }
@@ -3432,7 +3432,7 @@ build_stl_str_hl (
       prevchar_isflag = FALSE;              /* Item not NULL, but not a flag */
     curitem++;
   }
-  *p = NUL;
+  *p = '\0';
   itemcnt = curitem;
 
   if (usefmt != fmt)
@@ -3497,7 +3497,7 @@ build_stl_str_hl (
       while (++width < maxwidth) {
         s = s + STRLEN(s);
         *s++ = fillchar;
-        *s = NUL;
+        *s = '\0';
       }
 
       --n;              /* count the '<' */
@@ -4131,7 +4131,7 @@ chk_modeline (
   scid_T save_SID;
 
   prev = -1;
-  for (s = ml_get(lnum); *s != NUL; ++s) {
+  for (s = ml_get(lnum); *s != '\0'; ++s) {
     if (prev == -1 || vim_isspace(prev)) {
       if ((prev != -1 && STRNCMP(s, "ex:", (size_t)3) == 0)
           || STRNCMP(s, "vi:", (size_t)3) == 0)
@@ -4174,17 +4174,17 @@ chk_modeline (
     end = FALSE;
     while (end == FALSE) {
       s = skipwhite(s);
-      if (*s == NUL)
+      if (*s == '\0')
         break;
 
       /*
        * Find end of set command: ':' or end of line.
        * Skip over "\:", replacing it with ":".
        */
-      for (e = s; *e != ':' && *e != NUL; ++e)
+      for (e = s; *e != ':' && *e != '\0'; ++e)
         if (e[0] == '\\' && e[1] == ':')
           STRMOVE(e, e + 1);
-      if (*e == NUL)
+      if (*e == '\0')
         end = TRUE;
 
       /*
@@ -4201,9 +4201,9 @@ chk_modeline (
         end = TRUE;
         s = vim_strchr(s, ' ') + 1;
       }
-      *e = NUL;                         /* truncate the set command */
+      *e = '\0';                         /* truncate the set command */
 
-      if (*s != NUL) {                  /* skip over an empty "::" */
+      if (*s != '\0') {                  /* skip over an empty "::" */
         save_SID = current_SID;
         current_SID = SID_MODELINE;
         retval = do_set(s, OPT_MODELINE | OPT_LOCAL | flags);
diff --git a/src/charset.c b/src/charset.c
index 89ce5d38eb..a77a442499 100644
--- a/src/charset.c
+++ b/src/charset.c
@@ -169,7 +169,7 @@ int buf_init_chartab(buf_T *buf, int global)
       tilde = FALSE;
       do_isalpha = FALSE;
 
-      if ((*p == '^') && (p[1] != NUL)) {
+      if ((*p == '^') && (p[1] != '\0')) {
         tilde = TRUE;
         ++p;
       }
@@ -183,7 +183,7 @@ int buf_init_chartab(buf_T *buf, int global)
       }
       c2 = -1;
 
-      if ((*p == '-') && (p[1] != NUL)) {
+      if ((*p == '-') && (p[1] != '\0')) {
         ++p;
 
         if (VIM_ISDIGIT(*p)) {
@@ -199,7 +199,7 @@ int buf_init_chartab(buf_T *buf, int global)
           || (c >= 256)
           || ((c2 < c) && (c2 != -1))
           || (c2 >= 256)
-          || !((*p == NUL) || (*p == ','))) {
+          || !((*p == '\0') || (*p == ','))) {
         return FAIL;
       }
 
@@ -271,7 +271,7 @@ int buf_init_chartab(buf_T *buf, int global)
       c = *p;
       p = skip_to_option_part(p);
 
-      if ((c == ',') && (*p == NUL)) {
+      if ((c == ',') && (*p == '\0')) {
         // Trailing comma is not allowed.
         return FAIL;
       }
@@ -344,7 +344,7 @@ char_u *transstr(char_u *s)
     len = 0;
     p = s;
 
-    while (*p != NUL) {
+    while (*p != '\0') {
       if ((l = (*mb_ptr2len)(p)) > 1) {
         c = (*mb_ptr2char)(p);
         p += l;
@@ -371,10 +371,10 @@ char_u *transstr(char_u *s)
     res = alloc((unsigned)(vim_strsize(s) + 1));
   }
 
-  *res = NUL;
+  *res = '\0';
   p = s;
 
-  while (*p != NUL) {
+  while (*p != '\0') {
     if (has_mbyte && ((l = (*mb_ptr2len)(p)) > 1)) {
       c = (*mb_ptr2char)(p);
 
@@ -435,14 +435,14 @@ char_u* str_foldcase(char_u *str, int orglen, char_u *buf, int buflen)
   }
 
   if (buf == NULL) {
-    GA_CHAR(len) = NUL;
+    GA_CHAR(len) = '\0';
   } else {
-    buf[len] = NUL;
+    buf[len] = '\0';
   }
 
   // Make each character lower case.
   i = 0;
-  while (STR_CHAR(i) != NUL) {
+  while (STR_CHAR(i) != '\0') {
     if (enc_utf8 || (has_mbyte && (MB_BYTE2LEN(STR_CHAR(i)) > 1))) {
       if (enc_utf8) {
         int c = utf_ptr2char(STR_PTR(i));
@@ -530,7 +530,7 @@ char_u* transchar(int c)
       || ((c < 256) && vim_isprintc_strict(c))) {
     // printable character
     transchar_buf[i] = c;
-    transchar_buf[i + 1] = NUL;
+    transchar_buf[i + 1] = '\0';
   } else {
     transchar_nonprint(transchar_buf + i, c);
   }
@@ -562,7 +562,7 @@ void transchar_nonprint(char_u *buf, int c)
 {
   if (c == NL) {
     // we use newline in place of a NUL
-    c = NUL;
+    c = '\0';
   } else if ((c == CAR) && (get_fileformat(curbuf) == EOL_MAC)) {
     // we use CR in place of  NL in this case
     c = NL;
@@ -577,14 +577,14 @@ void transchar_nonprint(char_u *buf, int c)
     // DEL displayed as ^?
     buf[1] = c ^ 0x40;
 
-    buf[2] = NUL;
+    buf[2] = '\0';
   } else if (enc_utf8 && (c >= 0x80)) {
     transchar_hex(buf, c);
   } else if ((c >= ' ' + 0x80) && (c <= '~' + 0x80)) {
     // 0xa0 - 0xfe
     buf[0] = '|';
     buf[1] = c - 0x80;
-    buf[2] = NUL;
+    buf[2] = '\0';
   } else {
     // 0x80 - 0x9f and 0xff
     // TODO: EBCDIC I don't know what to do with this chars, so I display
@@ -592,7 +592,7 @@ void transchar_nonprint(char_u *buf, int c)
     buf[0] = '~';
     buf[1] = (c - 0x80) ^ 0x40;
     // 0xff displayed as ~?
-    buf[2] = NUL;
+    buf[2] = '\0';
   }
 }
 
@@ -612,7 +612,7 @@ void transchar_hex(char_u *buf, int c)
   buf[++i] = nr2hex((unsigned)c >> 4);
   buf[++i] = nr2hex((unsigned)c);
   buf[++i] = '>';
-  buf[++i] = NUL;
+  buf[++i] = '\0';
 }
 
 /// Convert the lower 4 bits of byte "c" to its hex character.
@@ -724,7 +724,7 @@ int vim_strnsize(char_u *s, int len)
 {
   assert(s != NULL);
   int size = 0;
-  while (*s != NUL && --len >= 0) {
+  while (*s != '\0' && --len >= 0) {
     if (has_mbyte) {
       int l = (*mb_ptr2len)(s);
       size += ptr2cells(s);
@@ -794,7 +794,7 @@ int linetabsize_col(int startcol, char_u *s)
 {
   colnr_T col = startcol;
 
-  while (*s != NUL) {
+  while (*s != '\0') {
     col += lbr_chartabsize_adv(&s, col);
   }
   return (int)col;
@@ -812,7 +812,7 @@ int win_linetabsize(win_T *wp, char_u *p, colnr_T len)
   colnr_T col = 0;
   char_u *s;
 
-  for (s = p; *s != NUL && (len == MAXCOL || s < p + len); mb_ptr_adv(s)) {
+  for (s = p; *s != '\0' && (len == MAXCOL || s < p + len); mb_ptr_adv(s)) {
     col += win_lbr_chartabsize(wp, s, col, NULL);
   }
   return (int)col;
@@ -901,7 +901,7 @@ int vim_isfilec_or_wc(int c)
 {
   char_u buf[2];
   buf[0] = (char_u)c;
-  buf[1] = NUL;
+  buf[1] = '\0';
   return vim_isfilec(c) || c == ']' || mch_has_wildcard(buf);
 }
 
@@ -946,7 +946,7 @@ int vim_isprintc_strict(int c)
 /// @return The number of characters taken up on the screen.
 int lbr_chartabsize(unsigned char *s, colnr_T col)
 {
-  if (!curwin->w_p_lbr && (*p_sbr == NUL)) {
+  if (!curwin->w_p_lbr && (*p_sbr == '\0')) {
     if (curwin->w_p_wrap) {
       return win_nolbr_chartabsize(curwin, s, col, NULL);
     }
@@ -994,7 +994,7 @@ int win_lbr_chartabsize(win_T *wp, char_u *s, colnr_T col, int *headp)
   int n;
 
   // No 'linebreak' and 'showbreak': return quickly.
-  if (!wp->w_p_lbr && (*p_sbr == NUL)) {
+  if (!wp->w_p_lbr && (*p_sbr == '\0')) {
     if (wp->w_p_wrap) {
       return win_nolbr_chartabsize(wp, s, col, headp);
     }
@@ -1032,7 +1032,7 @@ int win_lbr_chartabsize(win_T *wp, char_u *s, colnr_T col, int *headp)
       mb_ptr_adv(s);
       c = *s;
 
-      if (!((c != NUL)
+      if (!((c != '\0')
             && (vim_isbreak(c)
                 || (!vim_isbreak(c)
                     && ((col2 == col) || !vim_isbreak(*ps)))))) {
@@ -1061,7 +1061,7 @@ int win_lbr_chartabsize(win_T *wp, char_u *s, colnr_T col, int *headp)
   // Set *headp to the size of what we add.
   added = 0;
 
-  if ((*p_sbr != NUL) && wp->w_p_wrap && (col != 0)) {
+  if ((*p_sbr != '\0') && wp->w_p_wrap && (col != 0)) {
     numberextra = win_col_off(wp);
     col += numberextra + mb_added;
 
@@ -1193,15 +1193,15 @@ void getvcol(win_T *wp, pos_T *pos, colnr_T *start, colnr_T *cursor,
   // This function is used very often, do some speed optimizations.
   // When 'list', 'linebreak' and 'showbreak' are not set use a simple loop.
    // Also use this when 'list' is set but tabs take their normal size.
-  if ((!wp->w_p_list || (lcs_tab1 != NUL))
+  if ((!wp->w_p_list || (lcs_tab1 != '\0'))
       && !wp->w_p_lbr
-      && (*p_sbr == NUL)) {
+      && (*p_sbr == '\0')) {
     for (;;) {
       head = 0;
       c = *ptr;
 
       // make sure we don't go past the end of the line
-      if (c == NUL) {
+      if (c == '\0') {
         // NUL at end of line only takes one column
         incr = 1;
         break;
@@ -1250,7 +1250,7 @@ void getvcol(win_T *wp, pos_T *pos, colnr_T *start, colnr_T *cursor,
       incr = win_lbr_chartabsize(wp, ptr, vcol, &head);
 
       // make sure we don't go past the end of the line
-      if (*ptr == NUL) {
+      if (*ptr == '\0') {
         // NUL at end of line only takes one column
         incr = 1;
         break;
@@ -1455,7 +1455,7 @@ char_u* skiphex(char_u *q)
 char_u* skiptodigit(char_u *q)
 {
   char_u *p = q;
-  while (*p != NUL && !VIM_ISDIGIT(*p)) {
+  while (*p != '\0' && !VIM_ISDIGIT(*p)) {
     // skip to next digit
     p++;
   }
@@ -1470,7 +1470,7 @@ char_u* skiptodigit(char_u *q)
 char_u* skiptohex(char_u *q)
 {
   char_u *p = q;
-  while (*p != NUL && !vim_isxdigit(*p)) {
+  while (*p != '\0' && !vim_isxdigit(*p)) {
     // skip to next digit
     p++;
   }
@@ -1654,7 +1654,7 @@ int vim_tolower(int c)
 /// @return Pointer to the next whitespace or NUL character.
 char_u* skiptowhite(char_u *p)
 {
-  while (*p != ' ' && *p != '\t' && *p != NUL) {
+  while (*p != ' ' && *p != '\t' && *p != '\0') {
     p++;
   }
   return p;
@@ -1666,8 +1666,8 @@ char_u* skiptowhite(char_u *p)
 ///
 /// @return Pointer to the next whitespace character.
 char_u* skiptowhite_esc(char_u *p) {
-  while (*p != ' ' && *p != '\t' && *p != NUL) {
-    if (((*p == '\\') || (*p == Ctrl_V)) && (*(p + 1) != NUL)) {
+  while (*p != ' ' && *p != '\t' && *p != '\0') {
+    if (((*p == '\\') || (*p == Ctrl_V)) && (*(p + 1) != '\0')) {
       ++p;
     }
     ++p;
@@ -1705,7 +1705,7 @@ long getdigits(char_u **pp)
 int vim_isblankline(char_u *lbuf)
 {
   char_u *p = skipwhite(lbuf);
-  return *p == NUL || *p == '\r' || *p == '\n';
+  return *p == '\0' || *p == '\r' || *p == '\n';
 }
 
 /// Convert a string into a long and/or unsigned long, taking care of
@@ -1880,13 +1880,13 @@ int rem_backslash(char_u *str)
   return str[0] == '\\'
          && str[1] < 0x80
          && (str[1] == ' '
-             || (str[1] != NUL
+             || (str[1] != '\0'
                  && str[1] != '*'
                  && str[1] != '?'
                  && !vim_isfilec(str[1])));
 
 #else  // ifdef BACKSLASH_IN_FILENAME
-  return str[0] == '\\' && str[1] != NUL;
+  return str[0] == '\\' && str[1] != '\0';
 #endif  // ifdef BACKSLASH_IN_FILENAME
 }
 
diff --git a/src/crypt.c b/src/crypt.c
index 72d4a89c63..09c67d527d 100644
--- a/src/crypt.c
+++ b/src/crypt.c
@@ -75,7 +75,7 @@ int crypt_method_from_string(char_u *s)
 
 int get_crypt_method(buf_T *buf)
 {
-  return crypt_method_from_string(*buf->b_p_cm == NUL ? p_cm : buf->b_p_cm);
+  return crypt_method_from_string(*buf->b_p_cm == '\0' ? p_cm : buf->b_p_cm);
 }
 
 void set_crypt_method(buf_T *buf, int method)
@@ -156,7 +156,7 @@ void crypt_decode(char_u *ptr, long len)
 
 void crypt_init_keys(char_u *passwd)
 {
-  if ((passwd != NULL) && (*passwd != NUL)) {
+  if ((passwd != NULL) && (*passwd != '\0')) {
     if (use_crypt_method == 0) {
       char_u *p;
 
@@ -165,7 +165,7 @@ void crypt_init_keys(char_u *passwd)
       keys[1] = 591751049L;
       keys[2] = 878082192L;
 
-      for (p = passwd; *p != NUL; p++) {
+      for (p = passwd; *p != '\0'; ++p) {
         UPDATE_KEYS_ZIP((int)*p);
       }
     } else {
@@ -179,7 +179,7 @@ void free_crypt_key(char_u *key)
   char_u *p;
 
   if (key != NULL) {
-    for (p = key; *p != NUL; p++) {
+    for (p = key; *p != '\0'; ++p) {
       *p = 0;
     }
     free(key);
@@ -198,7 +198,7 @@ char_u *get_crypt_key(int store, int twice)
     char_u *prompt = (round == 0)
         ? (char_u *) _("Enter encryption key: ")
         : (char_u *) _("Enter same key again: ");
-    p1 = getcmdline_prompt(NUL, prompt, 0, EXPAND_NOTHING, NULL);
+    p1 = getcmdline_prompt('\0', prompt, 0, EXPAND_NOTHING, NULL);
     cmdline_star = FALSE;
     if (p1 == NULL) {
       break;
diff --git a/src/cursor_shape.c b/src/cursor_shape.c
index 05d51e1c9d..14f22c5a6f 100644
--- a/src/cursor_shape.c
+++ b/src/cursor_shape.c
@@ -61,7 +61,7 @@ char_u *parse_shape_opt(int what)
      * Repeat for all comma separated parts.
      */
     modep = p_guicursor;
-    while (*modep != NUL) {
+    while (*modep != '\0') {
       colonp = vim_strchr(modep, ':');
       if (colonp == NULL)
         return (char_u *)N_("E545: Missing colon");
diff --git a/src/diff.c b/src/diff.c
index 74e145f96c..e06f2612a3 100644
--- a/src/diff.c
+++ b/src/diff.c
@@ -735,7 +735,7 @@ void ex_diffupdate(exarg_T *eap)
     }
 
     // When using 'diffexpr' break here.
-    if (*p_dex != NUL) {
+    if (*p_dex != '\0') {
       break;
     }
 
@@ -813,7 +813,7 @@ theend:
 /// @param tmp_diff
 static void diff_file(char_u *tmp_orig, char_u *tmp_new, char_u *tmp_diff)
 {
-  if (*p_dex != NUL) {
+  if (*p_dex != '\0') {
     // Use 'diffexpr' to generate the diff file.
     eval_diff(tmp_orig, tmp_new, tmp_diff);
   } else {
@@ -906,7 +906,7 @@ void ex_diffpatch(exarg_T *eap)
   // return to the current.
   if ((os_dirname(dirbuf, MAXPATHL) != OK)
       || (os_chdir((char *)dirbuf) != 0)) {
-    dirbuf[0] = NUL;
+    dirbuf[0] = '\0';
   } else {
 # ifdef TEMPDIRNAMES
     if (vim_tempdir != NULL) {
@@ -921,7 +921,7 @@ void ex_diffpatch(exarg_T *eap)
   }
 #endif  // ifdef UNIX
 
-  if (*p_pex != NUL) {
+  if (*p_pex != '\0') {
     // Use 'patchexpr' to generate the new file.
 #ifdef UNIX
     eval_patch(tmp_orig, fullname != NULL ? fullname : eap->arg, tmp_new);
@@ -945,7 +945,7 @@ void ex_diffpatch(exarg_T *eap)
   }
 
 #ifdef UNIX
-  if (dirbuf[0] != NUL) {
+  if (dirbuf[0] != '\0') {
     if (os_chdir((char *)dirbuf) != 0) {
       EMSG(_(e_prev_dir));
     }
@@ -1614,7 +1614,7 @@ static int diff_cmp(char_u *s1, char_u *s2)
   char_u *p1 = s1;
   char_u *p2 = s2;
 
-  while (*p1 != NUL && *p2 != NUL) {
+  while (*p1 != '\0' && *p2 != '\0') {
     if (vim_iswhite(*p1) && vim_iswhite(*p2)) {
       p1 = skipwhite(p1);
       p2 = skipwhite(p2);
@@ -1650,7 +1650,7 @@ static int diff_cmp(char_u *s1, char_u *s2)
   p1 = skipwhite(p1);
   p2 = skipwhite(p2);
 
-  if ((*p1 != NUL) || (*p2 != NUL)) {
+  if ((*p1 != '\0') || (*p2 != '\0')) {
     return 1;
   }
   return 0;
@@ -1802,7 +1802,7 @@ int diffopt_changed(void)
   int diff_foldcolumn_new = 2;
 
   char_u *p = p_dip;
-  while (*p != NUL) {
+  while (*p != '\0') {
     if (STRNCMP(p, "filler", 6) == 0) {
       p += 6;
       diff_flags_new |= DIFF_FILLER;
@@ -1826,7 +1826,7 @@ int diffopt_changed(void)
       diff_foldcolumn_new = getdigits(&p);
     }
 
-    if ((*p != ',') && (*p != NUL)) {
+    if ((*p != ',') && (*p != '\0')) {
       return FAIL;
     }
 
@@ -1924,7 +1924,7 @@ int diff_find_change(win_T *wp, linenr_T lnum, int *startp, int *endp)
       // Search for start of difference
       si_org = si_new = 0;
 
-      while (line_org[si_org] != NUL) {
+      while (line_org[si_org] != '\0') {
         if ((diff_flags & DIFF_IWHITE)
             && vim_iswhite(line_org[si_org])
             && vim_iswhite(line_new[si_new])) {
@@ -1951,7 +1951,7 @@ int diff_find_change(win_T *wp, linenr_T lnum, int *startp, int *endp)
       }
 
       // Search for end of difference, if any.
-      if ((line_org[si_org] != NUL) || (line_new[si_new] != NUL)) {
+      if ((line_org[si_org] != '\0') || (line_new[si_new] != '\0')) {
         ei_org = (int)STRLEN(line_org);
         ei_new = (int)STRLEN(line_new);
 
@@ -2094,7 +2094,7 @@ void ex_diffgetput(exarg_T *eap)
     return;
   }
 
-  if (*eap->arg == NUL) {
+  if (*eap->arg == '\0') {
     // No argument: Find the other buffer in the list of diff buffers.
     for (idx_other = 0; idx_other < DB_COUNT; ++idx_other) {
       if ((curtab->tp_diffbuf[idx_other] != curbuf)
diff --git a/src/digraph.c b/src/digraph.c
index 88f183be4f..f4f8e2a711 100644
--- a/src/digraph.c
+++ b/src/digraph.c
@@ -1400,7 +1400,7 @@ static digr_T digraphdefault[] =
   { 'u', '^', 251 },  // û
   { 'y', '"', 255 },  // x XX
 
-  { NUL, NUL, NUL }
+  { '\0', '\0', '\0' }
 };
 
 /// handle digraphs after typing a character
@@ -1469,7 +1469,7 @@ int get_digraph(int cmdline)
       return getdigraph(c, cc, TRUE);
     }
   }
-  return NUL;
+  return '\0';
 }
 
 /// Lookup the pair "char1", "char2" in the digraph tables.
@@ -1579,10 +1579,10 @@ void putdigraph(char_u *str)
   int i;
   digr_T *dp;
 
-  while (*str != NUL) {
+  while (*str != '\0') {
     str = skipwhite(str);
 
-    if (*str == NUL) {
+    if (*str == '\0') {
       return;
     }
     char1 = *str++;
@@ -1637,7 +1637,7 @@ void listdigraphs(void)
 
   dp = digraphdefault;
 
-  for (i = 0; dp->char1 != NUL && !got_int; ++i) {
+  for (i = 0; dp->char1 != '\0' && !got_int; ++i) {
     digr_T tmp;
 
     // May need to convert the result to 'encoding'.
@@ -1731,7 +1731,7 @@ char_u* keymap_init(void)
 {
   curbuf->b_kmap_state &= ~KEYMAP_INIT;
 
-  if (*curbuf->b_p_keymap == NUL) {
+  if (*curbuf->b_p_keymap == '\0') {
     // Stop any active keymap and clear the table.  Also remove
     // b:keymap_name, as no keymap is active now.
     keymap_unload();
@@ -1808,7 +1808,7 @@ void ex_loadkeymap(exarg_T *eap)
 
     p = skipwhite(line);
 
-    if ((*p != '"') && (*p != NUL)) {
+    if ((*p != '"') && (*p != '\0')) {
       ga_grow(&curbuf->b_kmap_ga, 1);
       kp = (kmap_T *)curbuf->b_kmap_ga.ga_data + curbuf->b_kmap_ga.ga_len;
       s = skiptowhite(p);
@@ -1820,9 +1820,9 @@ void ex_loadkeymap(exarg_T *eap)
       if ((kp->from == NULL)
           || (kp->to == NULL)
           || (STRLEN(kp->from) + STRLEN(kp->to) >= KMAP_LLEN)
-          || (*kp->from == NUL)
-          || (*kp->to == NUL)) {
-        if ((kp->to != NULL) && (*kp->to == NUL)) {
+          || (*kp->from == '\0')
+          || (*kp->to == '\0')) {
+        if ((kp->to != NULL) && (*kp->to == '\0')) {
           EMSG(_("E791: Empty keymap entry"));
         }
         vim_free(kp->from);
diff --git a/src/edit.c b/src/edit.c
index 702d117580..c2b0ead2eb 100644
--- a/src/edit.c
+++ b/src/edit.c
@@ -400,7 +400,7 @@ edit (
      * line number is still valid (lines may have been deleted).
      * Do not restore if v:char was set to a non-empty string. */
     if (!equalpos(curwin->w_cursor, save_cursor)
-        && *get_vim_var_str(VV_CHAR) == NUL
+        && *get_vim_var_str(VV_CHAR) == '\0'
         && save_cursor.lnum <= curbuf->b_ml.ml_line_count) {
       int save_state = State;
 
@@ -431,7 +431,7 @@ edit (
   if (!did_ai)
     ai_col = 0;
 
-  if (cmdchar != NUL && restart_edit == 0) {
+  if (cmdchar != '\0' && restart_edit == 0) {
     ResetRedobuff();
     AppendNumberToRedobuff(count);
     if (cmdchar == 'V' || cmdchar == 'v') {
@@ -519,12 +519,12 @@ edit (
     update_curswant();
     if (((ins_at_eol && curwin->w_cursor.lnum == o_lnum)
          || curwin->w_curswant > curwin->w_virtcol)
-        && *(ptr = ml_get_curline() + curwin->w_cursor.col) != NUL) {
-      if (ptr[1] == NUL)
+        && *(ptr = ml_get_curline() + curwin->w_cursor.col) != '\0') {
+      if (ptr[1] == '\0')
         ++curwin->w_cursor.col;
       else if (has_mbyte) {
         i = (*mb_ptr2len)(ptr);
-        if (ptr[i] == NUL)
+        if (ptr[i] == '\0')
           curwin->w_cursor.col += i;
       }
     }
@@ -708,7 +708,7 @@ edit (
       /* BS: Delete one character from "compl_leader". */
       if ((c == K_BS || c == Ctrl_H)
           && curwin->w_cursor.col > compl_col
-          && (c = ins_compl_bs()) == NUL)
+          && (c = ins_compl_bs()) == '\0')
         continue;
 
       /* When no match was selected or it was edited. */
@@ -732,7 +732,7 @@ edit (
           char_u *p;
 
           if (str != NULL) {
-            for (p = str; *p != NUL; mb_ptr_adv(p))
+            for (p = str; *p != '\0'; mb_ptr_adv(p))
               ins_compl_addleader(PTR2CHAR(p));
             vim_free(str);
           } else
@@ -864,7 +864,7 @@ doESCkey:
        */
       /* Always update o_lnum, so that a "CTRL-O ." that adds a line
        * still puts the cursor back after the inserted text. */
-      if (ins_at_eol && gchar_cursor() == NUL)
+      if (ins_at_eol && gchar_cursor() == '\0')
         o_lnum = curwin->w_cursor.lnum;
 
       if (ins_esc(&count, cmdchar, nomove)) {
@@ -917,11 +917,11 @@ doESCkey:
 
 
     case K_ZERO:        /* Insert the previously inserted text. */
-    case NUL:
+    case '\0':
     case Ctrl_A:
       /* For ^@ the trailing ESC will end the insert, unless there is an
        * error.  */
-      if (stuff_inserted(NUL, 1L, (c == Ctrl_A)) == FAIL
+      if (stuff_inserted('\0', 1L, (c == Ctrl_A)) == FAIL
           && c != Ctrl_A && !p_im)
         goto doESCkey;                  /* quit insert mode */
       inserted_space = FALSE;
@@ -1155,7 +1155,7 @@ doESCkey:
         break;
       }
       c = ins_digraph();
-      if (c == NUL)
+      if (c == '\0')
         break;
       goto normalchar;
 
@@ -1195,7 +1195,7 @@ doESCkey:
     case Ctrl_N:
       /* if 'complete' is empty then plain ^P is no longer special,
        * but it is under other ^X modes */
-      if (*curbuf->b_p_cpt == NUL
+      if (*curbuf->b_p_cpt == '\0'
           && ctrl_x_mode != 0
           && !(compl_cont_status & CONT_LOCAL))
         goto normalchar;
@@ -1228,9 +1228,9 @@ normalchar:
         char_u *p;
 
         if (str != NULL) {
-          if (*str != NUL && stop_arrow() != FAIL) {
+          if (*str != '\0' && stop_arrow() != FAIL) {
             /* Insert the new value of v:char literally. */
-            for (p = str; *p != NUL; mb_ptr_adv(p)) {
+            for (p = str; *p != '\0'; mb_ptr_adv(p)) {
               c = PTR2CHAR(p);
               if (c == CAR || c == K_KENTER || c == NL)
                 ins_eol(c);
@@ -1240,12 +1240,12 @@ normalchar:
             AppendToRedobuffLit(str, -1);
           }
           vim_free(str);
-          c = NUL;
+          c = '\0';
         }
 
         /* If the new value is already inserted or an empty string
          * then don't insert any character. */
-        if (c == NUL)
+        if (c == '\0')
           break;
       }
       /* Try to perform smart-indenting. */
@@ -1654,7 +1654,7 @@ change_indent (
       i = (int)curwin->w_virtcol - vcol;
       ptr = alloc((unsigned)(i + 1));
       new_cursor_col += i;
-      ptr[i] = NUL;
+      ptr[i] = '\0';
       while (--i >= 0)
         ptr[i] = ' ';
       ins_str(ptr);
@@ -1706,10 +1706,10 @@ change_indent (
       --start_col;
     }
     while (start_col < (int)curwin->w_cursor.col || replaced) {
-      replace_push(NUL);
+      replace_push('\0');
       if (replaced) {
         replace_push(replaced);
-        replaced = NUL;
+        replaced = '\0';
       }
       ++start_col;
     }
@@ -1732,7 +1732,7 @@ change_indent (
       return;
 
     /* We only put back the new line up to the cursor */
-    new_line[curwin->w_cursor.col] = NUL;
+    new_line[curwin->w_cursor.col] = '\0';
 
     /* Put back original line */
     ml_replace(curwin->w_cursor.lnum, orig_line, FALSE);
@@ -1762,7 +1762,7 @@ void truncate_spaces(char_u *line)
     if (State & REPLACE_FLAG)
       replace_join(0);              /* remove a NUL from the replace stack */
   }
-  line[i + 1] = NUL;
+  line[i + 1] = '\0';
 }
 
 #if defined(FEAT_VREPLACE) || defined(FEAT_INS_EXPAND) \
@@ -1806,7 +1806,7 @@ static int del_char_after_col(int limit_col)
         break;
       curwin->w_cursor.col += l;
     }
-    if (*ml_get_cursor() == NUL || curwin->w_cursor.col == ecol)
+    if (*ml_get_cursor() == '\0' || curwin->w_cursor.col == ecol)
       return FALSE;
     del_bytes((long)((int)ecol - curwin->w_cursor.col), FALSE, TRUE);
   } else
@@ -1841,10 +1841,10 @@ static void ins_ctrl_x(void)
  */
 static int has_compl_option(int dict_opt)
 {
-  if (dict_opt ? (*curbuf->b_p_dict == NUL && *p_dict == NUL
+  if (dict_opt ? (*curbuf->b_p_dict == '\0' && *p_dict == '\0'
                   && !curwin->w_p_spell
                   )
-      : (*curbuf->b_p_tsr == NUL && *p_tsr == NUL)) {
+      : (*curbuf->b_p_tsr == '\0' && *p_tsr == '\0')) {
     ctrl_x_mode = 0;
     edit_submode = NULL;
     msg_attr(dict_opt ? (char_u *)_("'dictionary' option is empty")
@@ -1970,7 +1970,7 @@ int ins_compl_add_infercase(char_u *str, int len, int icase, char_u *fname, int
     if (has_mbyte) {
       p = str;
       actual_len = 0;
-      while (*p != NUL) {
+      while (*p != '\0') {
         mb_ptr_adv(p);
         ++actual_len;
       }
@@ -1981,7 +1981,7 @@ int ins_compl_add_infercase(char_u *str, int len, int icase, char_u *fname, int
     if (has_mbyte) {
       p = compl_orig_text;
       actual_compl_length = 0;
-      while (*p != NUL) {
+      while (*p != '\0') {
         mb_ptr_adv(p);
         ++actual_compl_length;
       }
@@ -2067,7 +2067,7 @@ int ins_compl_add_infercase(char_u *str, int len, int icase, char_u *fname, int
         p += (*mb_char2bytes)(wca[i++], p);
       else
         *(p++) = wca[i++];
-    *p = NUL;
+    *p = '\0';
 
     vim_free(wca);
 
@@ -2112,7 +2112,7 @@ ins_compl_add (
     do {
       if (    !(match->cp_flags & ORIGINAL_TEXT)
               && STRNCMP(match->cp_str, str, len) == 0
-              && match->cp_str[len] == NUL)
+              && match->cp_str[len] == '\0')
         return NOTDONE;
       match = match->cp_next;
     } while (match != NULL && match != compl_first_match);
@@ -2155,7 +2155,7 @@ ins_compl_add (
     int i;
 
     for (i = 0; i < CPT_COUNT; ++i)
-      if (cptext[i] != NULL && *cptext[i] != NUL)
+      if (cptext[i] != NULL && *cptext[i] != '\0')
         match->cp_text[i] = vim_strsave(cptext[i]);
   }
 
@@ -2227,7 +2227,7 @@ static void ins_compl_longest_match(compl_T *match)
     /* Reduce the text if this match differs from compl_leader. */
     p = compl_leader;
     s = match->cp_str;
-    while (*p != NUL) {
+    while (*p != '\0') {
       if (has_mbyte) {
         c1 = mb_ptr2char(p);
         c2 = mb_ptr2char(s);
@@ -2247,9 +2247,9 @@ static void ins_compl_longest_match(compl_T *match)
       }
     }
 
-    if (*p != NUL) {
+    if (*p != '\0') {
       /* Leader was shortened, need to change the inserted text. */
-      *p = NUL;
+      *p = '\0';
       had_match = (curwin->w_cursor.col > compl_col);
       ins_compl_delete();
       ins_bytes(compl_leader + ins_compl_len());
@@ -2567,7 +2567,7 @@ ins_compl_dictionaries (
   int save_p_scs;
   int dir = compl_direction;
 
-  if (*dict == NUL) {
+  if (*dict == '\0') {
     /* When 'dictionary' is empty and spell checking is enabled use
      * "spell". */
     if (!thesaurus && curwin->w_p_spell)
@@ -2607,7 +2607,7 @@ ins_compl_dictionaries (
 
   /* ignore case depends on 'ignorecase', 'smartcase' and "pat" */
   regmatch.rm_ic = ignorecase(pat);
-  while (*dict != NUL && !got_int && !compl_interrupted) {
+  while (*dict != '\0' && !got_int && !compl_interrupted) {
     /* copy one dictionary file name into buf */
     if (flags == DICT_EXACT) {
       count = 1;
@@ -2692,7 +2692,7 @@ static void ins_compl_files(int count, char_u **files, int thesaurus, int flags,
               /* Find start of the next word.  Skip white
                * space and punctuation. */
               ptr = find_word_start(ptr);
-              if (*ptr == NUL || *ptr == NL)
+              if (*ptr == '\0' || *ptr == NL)
                 break;
               wstart = ptr;
 
@@ -2701,7 +2701,7 @@ static void ins_compl_files(int count, char_u **files, int thesaurus, int flags,
                 /* Japanese words may have characters in
                  * different classes, only separate words
                  * with single-byte non-word characters. */
-                while (*ptr != NUL) {
+                while (*ptr != '\0') {
                   int l = (*mb_ptr2len)(ptr);
 
                   if (l < 2 && !vim_iswordc(*ptr))
@@ -2743,10 +2743,10 @@ static void ins_compl_files(int count, char_u **files, int thesaurus, int flags,
 char_u *find_word_start(char_u *ptr)
 {
   if (has_mbyte)
-    while (*ptr != NUL && *ptr != '\n' && mb_get_class(ptr) <= 1)
+    while (*ptr != '\0' && *ptr != '\n' && mb_get_class(ptr) <= 1)
       ptr += (*mb_ptr2len)(ptr);
   else
-    while (*ptr != NUL && *ptr != '\n' && !vim_iswordc(*ptr))
+    while (*ptr != '\0' && *ptr != '\n' && !vim_iswordc(*ptr))
       ++ptr;
   return ptr;
 }
@@ -2762,7 +2762,7 @@ char_u *find_word_end(char_u *ptr)
   if (has_mbyte) {
     start_class = mb_get_class(ptr);
     if (start_class > 1)
-      while (*ptr != NUL) {
+      while (*ptr != '\0') {
         ptr += (*mb_ptr2len)(ptr);
         if (mb_get_class(ptr) != start_class)
           break;
@@ -2880,7 +2880,7 @@ static int ins_compl_bs(void)
     if (compl_shown_match != NULL)
       /* Make sure current match is not a hidden item. */
       compl_curr_match = compl_shown_match;
-    return NUL;
+    return '\0';
   }
   return K_BS;
 }
@@ -2961,7 +2961,7 @@ static void ins_compl_addleader(int c)
     char_u buf[MB_MAXBYTES + 1];
 
     (*mb_char2bytes)(c, buf);
-    buf[cc] = NUL;
+    buf[cc] = '\0';
     ins_char_bytes(buf, cc);
     if (compl_opt_refresh_always)
       AppendToRedobuff(buf);
@@ -3233,9 +3233,9 @@ static int ins_compl_prep(int c)
         if (prev_col > 0)
           dec_cursor();
         if (stop_arrow() == OK)
-          insertchar(NUL, 0, -1);
+          insertchar('\0', 0, -1);
         if (prev_col > 0
-            && ml_get_curline()[curwin->w_cursor.col] != NUL)
+            && ml_get_curline()[curwin->w_cursor.col] != '\0')
           inc_cursor();
       }
 
@@ -3313,11 +3313,11 @@ static void ins_compl_fixRedoBufForLeader(char_u *ptr_arg)
   }
   if (compl_orig_text != NULL) {
     p = compl_orig_text;
-    for (len = 0; p[len] != NUL && p[len] == ptr[len]; ++len)
+    for (len = 0; p[len] != '\0' && p[len] == ptr[len]; ++len)
       ;
     if (len > 0)
       len -= (*mb_head_off)(p, p + len);
-    for (p += len; *p != NUL; mb_ptr_adv(p))
+    for (p += len; *p != '\0'; mb_ptr_adv(p))
       AppendCharToRedobuff(K_BS);
   } else
     len = 0;
@@ -3380,7 +3380,7 @@ expand_by_function (
   typval_T rettv;
 
   funcname = (type == CTRL_X_FUNCTION) ? curbuf->b_p_cfu : curbuf->b_p_ofu;
-  if (*funcname == NUL)
+  if (*funcname == '\0')
     return;
 
   /* Call 'completefunc' to obtain the list of matches. */
@@ -3506,7 +3506,7 @@ int ins_compl_add_tv(typval_T *tv, int dir)
     word = get_tv_string_chk(tv);
     memset(cptext, 0, sizeof(cptext));
   }
-  if (word == NULL || (!aempty && *word == NUL))
+  if (word == NULL || (!aempty && *word == '\0'))
     return FAIL;
   return ins_compl_add(word, -1, icase, NULL, cptext, dir, 0, adup);
 }
@@ -3607,7 +3607,7 @@ static int ins_compl_get_exp(pos_T *ini)
             ? ins_buf->b_fname
             : ins_buf->b_sfname);
         (void)msg_trunc_attr(IObuff, TRUE, hl_attr(HLF_R));
-      } else if (*e_cpt == NUL)
+      } else if (*e_cpt == '\0')
         break;
       else {
         if (ctrl_x_mode == CTRL_X_WHOLE_LINE)
@@ -3617,7 +3617,7 @@ static int ins_compl_get_exp(pos_T *ini)
             type = CTRL_X_DICTIONARY;
           else
             type = CTRL_X_THESAURUS;
-          if (*++e_cpt != ',' && *e_cpt != NUL) {
+          if (*++e_cpt != ',' && *e_cpt != '\0') {
             dict = e_cpt;
             dict_f = DICT_FIRST;
           }
@@ -3659,10 +3659,10 @@ static int ins_compl_get_exp(pos_T *ini)
       ins_compl_dictionaries(
           dict != NULL ? dict
           : (type == CTRL_X_THESAURUS
-             ? (*curbuf->b_p_tsr == NUL
+             ? (*curbuf->b_p_tsr == '\0'
                 ? p_tsr
                 : curbuf->b_p_tsr)
-             : (*curbuf->b_p_dict == NUL
+             : (*curbuf->b_p_dict == '\0'
                 ? p_dict
                 : curbuf->b_p_dict)),
           compl_pattern,
@@ -3830,7 +3830,7 @@ static int ins_compl_get_exp(pos_T *ini)
                 len += (int)(tmp_ptr - ptr);
                 flags |= CONT_S_IPOS;
               }
-              IObuff[len] = NUL;
+              IObuff[len] = '\0';
               ptr = IObuff;
             }
             if (len == compl_length)
@@ -3878,7 +3878,7 @@ static int ins_compl_get_exp(pos_T *ini)
   compl_started = TRUE;
 
   if ((ctrl_x_mode == 0 || ctrl_x_mode == CTRL_X_WHOLE_LINE)
-      && *e_cpt == NUL)                 /* Got to end of 'complete' */
+      && *e_cpt == '\0')                 /* Got to end of 'complete' */
     found_new_match = FAIL;
 
   i = -1;               /* total of matches, unknown */
@@ -4137,7 +4137,7 @@ void ins_compl_check_keys(int frequency)
   /* Check for a typed key.  Do use mappings, otherwise vim_is_ctrl_x_key()
    * can't do its work correctly. */
   c = vpeekc_any();
-  if (c != NUL) {
+  if (c != '\0') {
     if (vim_is_ctrl_x_key(c) && c != Ctrl_X && c != Ctrl_R) {
       c = safe_vgetc();         /* Eat the character */
       compl_shows_dir = ins_compl_key2dir(c);
@@ -4463,7 +4463,7 @@ static int ins_complete(int c)
        * string */
       funcname = ctrl_x_mode == CTRL_X_FUNCTION
                  ? curbuf->b_p_cfu : curbuf->b_p_ofu;
-      if (*funcname == NUL) {
+      if (*funcname == '\0') {
         EMSG2(_(e_notset), ctrl_x_mode == CTRL_X_FUNCTION
             ? "completefunc" : "omnifunc");
         return FAIL;
@@ -4795,7 +4795,7 @@ static unsigned quote_meta(char_u *dest, char_u *src, int len)
     }
   }
   if (dest != NULL)
-    *dest = NUL;
+    *dest = '\0';
 
   return m;
 }
@@ -4921,7 +4921,7 @@ insert_special (
     if (len > 2) {
       if (stop_arrow() == FAIL)
         return;
-      p[len - 1] = NUL;
+      p[len - 1] = '\0';
       ins_str(p);
       AppendToRedobuffLit(p, -1);
       ctrlv = FALSE;
@@ -4990,7 +4990,7 @@ insertchar (
           || (!vim_iswhite(c)
               && !((State & REPLACE_FLAG)
                    && !(State & VREPLACE_FLAG)
-                   && *ml_get_cursor() != NUL)
+                   && *ml_get_cursor() != '\0')
               && (curwin->w_cursor.lnum != Insstart.lnum
                   || ((!has_format_option(FO_INS_LONG)
                        || Insstart_textlen <= (colnr_T)textwidth)
@@ -5001,17 +5001,17 @@ insertchar (
      * when 'formatexpr' isn't set or it returns non-zero. */
     int do_internal = TRUE;
 
-    if (*curbuf->b_p_fex != NUL && (flags & INSCHAR_NO_FEX) == 0) {
+    if (*curbuf->b_p_fex != '\0' && (flags & INSCHAR_NO_FEX) == 0) {
       do_internal = (fex_format(curwin->w_cursor.lnum, 1L, c) != 0);
       /* It may be required to save for undo again, e.g. when setline()
        * was called. */
       ins_need_undo = TRUE;
     }
     if (do_internal)
-      internal_format(textwidth, second_indent, flags, c == NUL, c);
+      internal_format(textwidth, second_indent, flags, c == '\0', c);
   }
 
-  if (c == NUL)             /* only formatting was wanted */
+  if (c == '\0')             /* only formatting was wanted */
     return;
 
   /* Check whether this character should end a comment. */
@@ -5062,7 +5062,7 @@ insertchar (
       }
     }
   }
-  end_comment_pending = NUL;
+  end_comment_pending = '\0';
 
   did_ai = FALSE;
   did_si = FALSE;
@@ -5084,7 +5084,7 @@ insertchar (
 
   if (       !ISSPECIAL(c)
              && (!has_mbyte || (*mb_char2len)(c) == 1)
-             && vpeekc() != NUL
+             && vpeekc() != '\0'
              && !(State & REPLACE_FLAG)
              && !cindent_on()
              && !p_ri
@@ -5107,7 +5107,7 @@ insertchar (
      * - running into the 'textwidth' boundary
      * - need to check for abbreviation: A non-word char after a word-char
      */
-    while (    (c = vpeekc()) != NUL
+    while (    (c = vpeekc()) != '\0'
                && !ISSPECIAL(c)
                && (!has_mbyte || MB_BYTE2LEN_CHECK(c) == 1)
                && i < INPUT_BUFLEN
@@ -5124,14 +5124,14 @@ insertchar (
 
     do_digraph(-1);                     /* clear digraphs */
     do_digraph(buf[i-1]);               /* may be the start of a digraph */
-    buf[i] = NUL;
+    buf[i] = '\0';
     ins_str(buf);
     if (flags & INSCHAR_CTRLV) {
       redo_literal(*buf);
       i = 1;
     } else
       i = 0;
-    if (buf[i] != NUL)
+    if (buf[i] != '\0')
       AppendToRedobuffLit(buf + i, -1);
   } else {
     int cc;
@@ -5140,7 +5140,7 @@ insertchar (
       char_u buf[MB_MAXBYTES + 1];
 
       (*mb_char2bytes)(c, buf);
-      buf[cc] = NUL;
+      buf[cc] = '\0';
       ins_char_bytes(buf, cc);
       AppendCharToRedobuff(c);
     } else {
@@ -5169,7 +5169,7 @@ internal_format (
 )
 {
   int cc;
-  int save_char = NUL;
+  int save_char = '\0';
   int haveto_redraw = FALSE;
   int fo_ins_blank = has_format_option(FO_INS_BLANK);
   int fo_multibyte = has_format_option(FO_MBYTE_BREAK);
@@ -5209,7 +5209,7 @@ internal_format (
     colnr_T end_col;
 
     virtcol = get_nolist_virtcol()
-              + char2cells(c != NUL ? c : gchar_cursor());
+              + char2cells(c != '\0' ? c : gchar_cursor());
     if (virtcol <= (colnr_T)textwidth)
       break;
 
@@ -5254,7 +5254,7 @@ internal_format (
            || (flags & INSCHAR_FORMAT)
            || curwin->w_cursor.lnum != Insstart.lnum
            || curwin->w_cursor.col >= Insstart.col) {
-      if (curwin->w_cursor.col == startcol && c != NUL)
+      if (curwin->w_cursor.col == startcol && c != '\0')
         cc = c;
       else
         cc = gchar_cursor();
@@ -5377,7 +5377,7 @@ internal_format (
       curwin->w_cursor.col = orig_col;
       if (saved_text == NULL)
         break;          /* Can't do it, out of memory */
-      saved_text[startcol] = NUL;
+      saved_text[startcol] = '\0';
 
       /* Backspace over characters that will move to the next line */
       if (!fo_white_par)
@@ -5416,7 +5416,7 @@ internal_format (
         if (second_indent >= 0) {
           if (State & VREPLACE_FLAG)
             change_indent(INDENT_SET, second_indent,
-                FALSE, NUL, TRUE);
+                FALSE, '\0', TRUE);
           else if (leader_len > 0 && second_indent - leader_len > 0) {
             int i;
             int padding = second_indent - leader_len;
@@ -5466,7 +5466,7 @@ internal_format (
     line_breakcheck();
   }
 
-  if (save_char != NUL)                 /* put back space after cursor */
+  if (save_char != '\0')                 /* put back space after cursor */
     pchar_cursor(save_char);
 
   if (!format_only && haveto_redraw) {
@@ -5510,7 +5510,7 @@ auto_format (
    * Otherwise the line would be broken and when typing another non-white
    * next they are not joined back together. */
   wasatend = (pos.col == (colnr_T)STRLEN(old));
-  if (*old != NUL && !trailblank && wasatend) {
+  if (*old != '\0' && !trailblank && wasatend) {
     dec_cursor();
     cc = gchar_cursor();
     if (!WHITECHAR(cc) && curwin->w_cursor.col > 0
@@ -5567,7 +5567,7 @@ auto_format (
     if (curwin->w_cursor.col == len) {
       pnew = vim_strnsave(new, len + 2);
       pnew[len] = ' ';
-      pnew[len + 1] = NUL;
+      pnew[len + 1] = '\0';
       ml_replace(curwin->w_cursor.lnum, pnew, FALSE);
       /* remove the space later */
       did_add_space = TRUE;
@@ -5603,7 +5603,7 @@ check_auto_format (
         c = gchar_cursor();
         dec_cursor();
       }
-      if (c != NUL) {
+      if (c != '\0') {
         /* The space is no longer at the end of the line, delete it. */
         del_char(FALSE);
         did_add_space = FALSE;
@@ -5789,7 +5789,7 @@ stop_insert (
        * formatting will move it to the following word.  Avoid that by
        * moving the cursor onto the space. */
       cc = 'x';
-      if (curwin->w_cursor.col > 0 && gchar_cursor() == NUL) {
+      if (curwin->w_cursor.col > 0 && gchar_cursor() == '\0') {
         dec_cursor();
         cc = gchar_cursor();
         if (!vim_iswhite(cc))
@@ -5799,11 +5799,11 @@ stop_insert (
       auto_format(TRUE, FALSE);
 
       if (vim_iswhite(cc)) {
-        if (gchar_cursor() != NUL)
+        if (gchar_cursor() != '\0')
           inc_cursor();
         /* If the cursor is still at the same character, also keep
          * the "coladd". */
-        if (gchar_cursor() == NUL
+        if (gchar_cursor() == '\0'
             && curwin->w_cursor.lnum == tpos.lnum
             && curwin->w_cursor.col == tpos.col)
           curwin->w_cursor.coladd = tpos.coladd;
@@ -5827,7 +5827,7 @@ stop_insert (
       curwin->w_cursor = *end_insert_pos;
       check_cursor_col();        /* make sure it is not past the line */
       for (;; ) {
-        if (gchar_cursor() == NUL && curwin->w_cursor.col > 0)
+        if (gchar_cursor() == '\0' && curwin->w_cursor.col > 0)
           --curwin->w_cursor.col;
         cc = gchar_cursor();
         if (!vim_iswhite(cc))
@@ -5837,7 +5837,7 @@ stop_insert (
       }
       if (curwin->w_cursor.lnum != tpos.lnum)
         curwin->w_cursor = tpos;
-      else if (cc != NUL)
+      else if (cc != '\0')
         ++curwin->w_cursor.col;         /* put cursor back on the NUL */
 
       /* <C-S-Right> may have started Visual mode, adjust the position for
@@ -5882,7 +5882,7 @@ void set_last_insert(int c)
     *s++ = Ctrl_V;
   s = add_char2buf(c, s);
   *s++ = ESC;
-  *s++ = NUL;
+  *s++ = '\0';
   last_insert_skip = 0;
 }
 
@@ -5941,7 +5941,7 @@ void beginline(int flags)
       char_u  *ptr;
 
       for (ptr = ml_get_curline(); vim_iswhite(*ptr)
-           && !((flags & BL_FIX) && ptr[1] == NUL); ++ptr)
+           && !((flags & BL_FIX) && ptr[1] == '\0'); ++ptr)
         ++curwin->w_cursor.col;
     }
     curwin->w_set_curswant = TRUE;
@@ -5977,7 +5977,7 @@ int oneright(void)
   }
 
   ptr = ml_get_cursor();
-  if (*ptr == NUL)
+  if (*ptr == '\0')
     return FAIL;            /* already at the very end */
 
   if (has_mbyte)
@@ -5987,7 +5987,7 @@ int oneright(void)
 
   /* move "l" bytes right, but don't end up on the NUL, unless 'virtualedit'
    * contains "onemore". */
-  if (ptr[l] == NUL
+  if (ptr[l] == '\0'
       && (ve_flags & VE_ONEMORE) == 0
       )
     return FAIL;
@@ -6012,7 +6012,7 @@ int oneleft(void)
       coladvance(v - width);
       /* getviscol() is slow, skip it when 'showbreak' is empty and
        * there are no multi-byte characters */
-      if ((*p_sbr == NUL
+      if ((*p_sbr == '\0'
            && !has_mbyte
            ) || getviscol() < v)
         break;
@@ -6163,7 +6163,7 @@ stuff_inserted (
   char_u      *esc_ptr;
   char_u      *ptr;
   char_u      *last_ptr;
-  char_u last = NUL;
+  char_u last = '\0';
 
   ptr = get_last_insert();
   if (ptr == NULL) {
@@ -6172,10 +6172,10 @@ stuff_inserted (
   }
 
   /* may want to stuff the command character, to start Insert mode */
-  if (c != NUL)
+  if (c != '\0')
     stuffcharReadbuff(c);
   if ((esc_ptr = (char_u *)vim_strrchr(ptr, ESC)) != NULL)
-    *esc_ptr = NUL;         /* remove the ESC */
+    *esc_ptr = '\0';         /* remove the ESC */
 
   /* when the last char is either "0" or "^" it will be quoted if no ESC
    * comes after it OR if it will inserted more than once and "ptr"
@@ -6185,7 +6185,7 @@ stuff_inserted (
   if (last_ptr >= ptr && (*last_ptr == '0' || *last_ptr == '^')
       && (no_esc || (*ptr == Ctrl_D && count > 1))) {
     last = *last_ptr;
-    *last_ptr = NUL;
+    *last_ptr = '\0';
   }
 
   do {
@@ -6232,7 +6232,7 @@ char_u *get_last_insert_save(void)
   if (s != NULL) {
     len = (int)STRLEN(s);
     if (len > 0 && s[len - 1] == ESC)           /* remove trailing ESC */
-      s[len - 1] = NUL;
+      s[len - 1] = '\0';
   }
   return s;
 }
@@ -6342,7 +6342,7 @@ replace_join (
   int i;
 
   for (i = replace_stack_nr; --i >= 0; )
-    if (replace_stack[i] == NUL && off-- <= 0) {
+    if (replace_stack[i] == '\0' && off-- <= 0) {
       --replace_stack_nr;
       memmove(replace_stack + i, replace_stack + i + 1,
           (size_t)(replace_stack_nr - i));
@@ -6496,7 +6496,7 @@ static void replace_do_bs(int limit_col)
  */
 static int cindent_on(void) {
   return !p_paste && (curbuf->b_p_cin
-                      || *curbuf->b_p_inde != NUL
+                      || *curbuf->b_p_inde != '\0'
                       );
 }
 
@@ -6547,11 +6547,11 @@ int in_cinkeys(int keytyped, int when, int line_is_empty)
   int icase;
   int i;
 
-  if (keytyped == NUL)
+  if (keytyped == '\0')
     /* Can happen with CTRL-Y and CTRL-E on a short line. */
     return FALSE;
 
-  if (*curbuf->b_p_inde != NUL)
+  if (*curbuf->b_p_inde != '\0')
     look = curbuf->b_p_indk;            /* 'indentexpr' set: use 'indentkeys' */
   else
     look = curbuf->b_p_cink;            /* 'indentexpr' empty: use 'cinkeys' */
@@ -6667,7 +6667,7 @@ int in_cinkeys(int keytyped, int when, int line_is_empty)
     /*
      * Is it a word: "=word"?
      */
-    else if (*look == '=' && look[1] != ',' && look[1] != NUL) {
+    else if (*look == '=' && look[1] != ',' && look[1] != '\0') {
       ++look;
       if (*look == '~') {
         icase = TRUE;
@@ -6873,7 +6873,7 @@ static void ins_reg(void)
       im_set_active(TRUE);
 # endif
   }
-  if (regname == NUL || !valid_yank_reg(regname, FALSE)) {
+  if (regname == '\0' || !valid_yank_reg(regname, FALSE)) {
     vim_beep();
     need_redraw = TRUE;         /* remove the '"' */
   } else {
@@ -7051,7 +7051,7 @@ ins_esc (
 
   /* When an autoindent was removed, curswant stays after the
    * indent */
-  if (restart_edit == NUL && (colnr_T)temp == curwin->w_cursor.col)
+  if (restart_edit == '\0' && (colnr_T)temp == curwin->w_cursor.col)
     curwin->w_set_curswant = TRUE;
 
   /* Remember the last Insert position in the '^ mark. */
@@ -7066,15 +7066,15 @@ ins_esc (
       && (curwin->w_cursor.col != 0
           || curwin->w_cursor.coladd > 0
           )
-      && (restart_edit == NUL
-          || (gchar_cursor() == NUL
+      && (restart_edit == '\0'
+          || (gchar_cursor() == '\0'
               && !VIsual_active
               ))
       && !revins_on
       ) {
     if (curwin->w_cursor.coladd > 0 || ve_flags == VE_ALL) {
       oneleft();
-      if (restart_edit != NUL)
+      if (restart_edit != '\0')
         ++curwin->w_cursor.coladd;
     } else {
       --curwin->w_cursor.col;
@@ -7104,7 +7104,7 @@ ins_esc (
    * When recording or for CTRL-O, need to display the new mode.
    * Otherwise remove the mode message.
    */
-  if (Recording || restart_edit != NUL)
+  if (Recording || restart_edit != '\0')
     showmode();
   else if (p_smd)
     MSG("");
@@ -7119,7 +7119,7 @@ ins_esc (
 static void ins_ctrl_(void)
 {
   if (revins_on && revins_chars && revins_scol >= 0) {
-    while (gchar_cursor() != NUL && revins_chars--)
+    while (gchar_cursor() != '\0' && revins_chars--)
       ++curwin->w_cursor.col;
   }
   p_ri = !p_ri;
@@ -7182,7 +7182,7 @@ static int ins_start_select(int c)
         buf[0] = K_SPECIAL;
         buf[1] = KS_MODIFIER;
         buf[2] = mod_mask;
-        buf[3] = NUL;
+        buf[3] = '\0';
         stuffReadbuff(buf);
       }
       stuffcharReadbuff(c);
@@ -7230,7 +7230,7 @@ static void ins_ctrl_o(void)
   if (virtual_active())
     ins_at_eol = FALSE;         /* cursor always keeps its column */
   else
-    ins_at_eol = (gchar_cursor() == NUL);
+    ins_at_eol = (gchar_cursor() == '\0');
 }
 
 /*
@@ -7262,7 +7262,7 @@ static void ins_shift(int c, int lastc)
   } else
     change_indent(c == Ctrl_D ? INDENT_DEC : INDENT_INC, 0, TRUE, 0, TRUE);
 
-  if (did_ai && *skipwhite(ml_get_curline()) != NUL)
+  if (did_ai && *skipwhite(ml_get_curline()) != '\0')
     did_ai = FALSE;
   did_si = FALSE;
   can_si = FALSE;
@@ -7276,7 +7276,7 @@ static void ins_del(void)
 
   if (stop_arrow() == FAIL)
     return;
-  if (gchar_cursor() == NUL) {          /* delete newline */
+  if (gchar_cursor() == '\0') {          /* delete newline */
     temp = curwin->w_cursor.col;
     if (!can_bs(BS_EOL)                 /* only if "eol" included */
         || do_join(2, FALSE, TRUE, FALSE) == FAIL)
@@ -7353,7 +7353,7 @@ static int ins_bs(int c, int mode, int *inserted_space_p)
   in_indent = inindent(0);
   if (in_indent)
     can_cindent = FALSE;
-  end_comment_pending = NUL;    /* After BS, don't auto-end comment */
+  end_comment_pending = '\0';    /* After BS, don't auto-end comment */
   if (revins_on)            /* put cursor after last inserted char */
     inc_cursor();
 
@@ -7418,11 +7418,11 @@ static int ins_bs(int c, int mode, int *inserted_space_p)
 
           len = (int)STRLEN(ptr);
           if (len > 0 && ptr[len - 1] == ' ')
-            ptr[len - 1] = NUL;
+            ptr[len - 1] = '\0';
         }
 
         (void)do_join(2, FALSE, FALSE, FALSE);
-        if (temp == NUL && gchar_cursor() != NUL)
+        if (temp == '\0' && gchar_cursor() != '\0')
           inc_cursor();
       } else
         dec_cursor();
@@ -7526,7 +7526,7 @@ static int ins_bs(int c, int mode, int *inserted_space_p)
         else {
           ins_str((char_u *)" ");
           if ((State & REPLACE_FLAG))
-            replace_push(NUL);
+            replace_push('\0');
         }
         getvcol(curwin, &curwin->w_cursor, &vcol, NULL, NULL);
       }
@@ -7569,13 +7569,13 @@ static int ins_bs(int c, int mode, int *inserted_space_p)
            * move the cursor back.  Don't back up before the base
            * character.
            */
-          if (enc_utf8 && p_deco && cpc[0] != NUL)
+          if (enc_utf8 && p_deco && cpc[0] != '\0')
             inc_cursor();
           if (revins_chars) {
             revins_chars--;
             revins_legal++;
           }
-          if (revins_on && gchar_cursor() == NUL)
+          if (revins_on && gchar_cursor() == '\0')
             break;
         }
         /* Just a single backspace?: */
@@ -7790,7 +7790,7 @@ static void ins_right(void)
   if ((fdo_flags & FDO_HOR) && KeyTyped)
     foldOpenCursor();
   undisplay_dollar();
-  if (gchar_cursor() != NUL
+  if (gchar_cursor() != '\0'
       || virtual_active()
       ) {
     start_arrow(&curwin->w_cursor);
@@ -7826,7 +7826,7 @@ static void ins_s_right(void)
     foldOpenCursor();
   undisplay_dollar();
   if (curwin->w_cursor.lnum < curbuf->b_ml.ml_line_count
-      || gchar_cursor() != NUL) {
+      || gchar_cursor() != '\0') {
     start_arrow(&curwin->w_cursor);
     (void)fwd_word(1L, FALSE, 0);
     curwin->w_set_curswant = TRUE;
@@ -7984,7 +7984,7 @@ static int ins_tab(void)
     else {
       ins_str((char_u *)" ");
       if (State & REPLACE_FLAG)             /* no char replaced */
-        replace_push(NUL);
+        replace_push('\0');
     }
   }
 
@@ -8135,7 +8135,7 @@ static int ins_eol(int c)
   if ((State & REPLACE_FLAG)
       && !(State & VREPLACE_FLAG)
       )
-    replace_push(NUL);
+    replace_push('\0');
 
   /*
    * In VREPLACE mode, a NL replaces the rest of the line, and starts
@@ -8208,7 +8208,7 @@ static int ins_digraph(void)
   if (IS_SPECIAL(c) || mod_mask) {          /* special key */
     clear_showcmd();
     insert_special(c, TRUE, FALSE);
-    return NUL;
+    return '\0';
   }
   if (c != ESC) {
     did_putchar = FALSE;
@@ -8240,7 +8240,7 @@ static int ins_digraph(void)
     }
   }
   clear_showcmd();
-  return NUL;
+  return '\0';
 }
 
 /*
@@ -8255,7 +8255,7 @@ int ins_copychar(linenr_T lnum)
 
   if (lnum < 1 || lnum > curbuf->b_ml.ml_line_count) {
     vim_beep();
-    return NUL;
+    return '\0';
   }
 
   /* try to advance to the cursor column */
@@ -8263,7 +8263,7 @@ int ins_copychar(linenr_T lnum)
   ptr = ml_get(lnum);
   prev_ptr = ptr;
   validate_virtcol();
-  while ((colnr_T)temp < curwin->w_virtcol && *ptr != NUL) {
+  while ((colnr_T)temp < curwin->w_virtcol && *ptr != '\0') {
     prev_ptr = ptr;
     temp += lbr_chartabsize_adv(&ptr, (colnr_T)temp);
   }
@@ -8271,7 +8271,7 @@ int ins_copychar(linenr_T lnum)
     ptr = prev_ptr;
 
   c = (*mb_ptr2char)(ptr);
-  if (c == NUL)
+  if (c == '\0')
     vim_beep();
   return c;
 }
@@ -8291,7 +8291,7 @@ static int ins_ctrl_ey(int tc)
     redraw_later(VALID);
   } else {
     c = ins_copychar(curwin->w_cursor.lnum + (c == Ctrl_Y ? -1 : 1));
-    if (c != NUL) {
+    if (c != '\0') {
       long tw_save;
 
       /* The character must be taken literally, insert like it
@@ -8352,7 +8352,7 @@ static void ins_try_si(int c)
       i = get_indent();
       curwin->w_cursor = old_pos;
       if (State & VREPLACE_FLAG)
-        change_indent(INDENT_SET, i, FALSE, NUL, TRUE);
+        change_indent(INDENT_SET, i, FALSE, '\0', TRUE);
       else
         (void)set_indent(i, SIN_CHANGED);
     } else if (curwin->w_cursor.col > 0) {
@@ -8368,7 +8368,7 @@ static void ins_try_si(int c)
           ptr = skipwhite(ml_get(--(curwin->w_cursor.lnum)));
 
           /* ignore empty lines and lines starting with '#'. */
-          if (*ptr != '#' && *ptr != NUL)
+          if (*ptr != '#' && *ptr != '\0')
             break;
         }
         if (get_indent() >= i)
@@ -8422,10 +8422,10 @@ static char_u *do_insert_char_pre(int c)
     return NULL;
 
   if (has_mbyte)
-    buf[(*mb_char2bytes)(c, buf)] = NUL;
+    buf[(*mb_char2bytes)(c, buf)] = '\0';
   else {
     buf[0] = c;
-    buf[1] = NUL;
+    buf[1] = '\0';
   }
 
   /* Lock the text to avoid weird things from happening. */
diff --git a/src/eval.c b/src/eval.c
index 93d33a2c96..c237a90b74 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -1051,9 +1051,9 @@ var_redir_start (
   redir_endp = get_lval(redir_varname, NULL, redir_lval, FALSE, FALSE, 0,
       FNE_CHECK_START);
   if (redir_endp == NULL || redir_lval->ll_name == NULL || *redir_endp !=
-      NUL) {
+      '\0') {
     clear_lval(redir_lval);
-    if (redir_endp != NULL && *redir_endp != NUL)
+    if (redir_endp != NULL && *redir_endp != '\0')
       /* Trailing characters are present after the variable name */
       EMSG(_(e_trailing));
     else
@@ -1122,7 +1122,7 @@ void var_redir_stop(void)
   if (redir_lval != NULL) {
     /* If there was no error: assign the text to the variable. */
     if (redir_endp != NULL) {
-      ga_append(&redir_ga, NUL);        /* Append the trailing NUL. */
+      ga_append(&redir_ga, '\0');        /* Append the trailing NUL. */
       tv.v_type = VAR_STRING;
       tv.vval.v_string = redir_ga.ga_data;
       /* Call get_lval() again, if it's inside a Dict or List it may
@@ -1306,7 +1306,7 @@ char_u *eval_to_string(char_u *arg, char_u **nextcmd, int convert)
         if (tv.vval.v_list->lv_len > 0)
           ga_append(&ga, NL);
       }
-      ga_append(&ga, NUL);
+      ga_append(&ga, '\0');
       retval = (char_u *)ga.ga_data;
     } else if (convert && tv.v_type == VAR_FLOAT) {
       vim_snprintf((char *)numbuf, NUMBUFLEN, "%g", tv.vval.v_float);
@@ -1498,7 +1498,7 @@ call_vim_function (
 
   for (i = 0; i < argc; i++) {
     /* Pass a NULL or empty argument as an empty string */
-    if (argv[i] == NULL || *argv[i] == NUL) {
+    if (argv[i] == NULL || *argv[i] == '\0') {
       argvars[i].v_type = VAR_STRING;
       argvars[i].vval.v_string = (char_u *)"";
       continue;
@@ -1689,7 +1689,7 @@ int eval_foldexpr(char_u *arg, int *cp)
   if (use_sandbox)
     ++sandbox;
   ++textlock;
-  *cp = NUL;
+  *cp = '\0';
   if (eval0(arg, &tv, NULL, TRUE) == FAIL)
     retval = 0;
   else {
@@ -1764,7 +1764,7 @@ void ex_let(exarg_T *eap)
     eap->nextcmd = check_nextcmd(arg);
   } else {
     op[0] = '=';
-    op[1] = NUL;
+    op[1] = '\0';
     if (*expr != '=') {
       if (vim_strchr((char_u *)"+-.", *expr) != NULL) {
         op[0] = *expr;  // +=, -=, .=
@@ -1928,7 +1928,7 @@ static char_u *skip_var_list(char_u *arg, int *var_count, int *semicolon)
  */
 static char_u *skip_var_one(char_u *arg)
 {
-  if (*arg == '@' && arg[1] != NUL)
+  if (*arg == '@' && arg[1] != '\0')
     return arg + 2;
   return find_name_end(*arg == '$' || *arg == '&' ? arg + 1 : arg,
       NULL, NULL, FNE_INCL_BR | FNE_CHECK_START);
@@ -2090,7 +2090,7 @@ static char_u *list_arg_vars(exarg_T *eap, char_u *arg, int *first)
 
               s = echo_string(&tv, &tf, numbuf, 0);
               c = *arg;
-              *arg = NUL;
+              *arg = '\0';
               list_one_var_a((char_u *)"",
                   arg == arg_subsc ? name : name_start,
                   tv.v_type,
@@ -2153,7 +2153,7 @@ ex_let_one (
         EMSG(_(e_letunexp));
       else if (!check_secure()) {
         c1 = name[len];
-        name[len] = NUL;
+        name[len] = '\0';
         p = get_tv_string_chk(tv);
         if (p != NULL && op != NULL && *op == '.') {
           int mustfree = FALSE;
@@ -2200,7 +2200,7 @@ ex_let_one (
       char_u      *s;
 
       c1 = *p;
-      *p = NUL;
+      *p = '\0';
 
       n = get_tv_number(tv);
       s = get_tv_string_chk(tv);            /* != NULL if number or string */
@@ -2376,7 +2376,7 @@ get_lval (
     return p;
 
   cc = *p;
-  *p = NUL;
+  *p = '\0';
   v = find_var(lp->ll_name, &ht, flags & GLV_NO_AUTOLOAD);
   if (v == NULL && !quiet)
     EMSG2(_(e_undefvar), lp->ll_name);
@@ -2486,7 +2486,7 @@ get_lval (
       if (len == -1) {
         /* "[key]": get key from "var1" */
         key = get_tv_string(&var1);             /* is number or string */
-        if (*key == NUL) {
+        if (*key == '\0') {
           if (!quiet)
             EMSG(_(e_emptykey));
           clear_tv(&var1);
@@ -2506,7 +2506,7 @@ get_lval (
 
         if (len != -1) {
           prevval = key[len];
-          key[len] = NUL;
+          key[len] = '\0';
         } else
           prevval = 0;           /* avoid compiler warning */
         wrong = (lp->ll_dict->dv_scope == VAR_DEF_SCOPE
@@ -2637,7 +2637,7 @@ static void set_var_lval(lval_T *lp, char_u *endp, typval_T *rettv, int copy, ch
   if (lp->ll_tv == NULL) {
     if (!check_changedtick(lp->ll_name)) {
       cc = *endp;
-      *endp = NUL;
+      *endp = '\0';
       if (op != NULL && *op != '=') {
         typval_T tv;
 
@@ -2986,13 +2986,13 @@ void set_context_for_expression(expand_T *xp, char_u *arg, cmdidx_T cmdidx)
       break;
     } else if (cmdidx != CMD_let || got_eq) {
       if (c == '"') {               /* string */
-        while ((c = *++xp->xp_pattern) != NUL && c != '"')
-          if (c == '\\' && xp->xp_pattern[1] != NUL)
+        while ((c = *++xp->xp_pattern) != '\0' && c != '"')
+          if (c == '\\' && xp->xp_pattern[1] != '\0')
             ++xp->xp_pattern;
         xp->xp_context = EXPAND_NOTHING;
       } else if (c == '\'') {     /* literal string */
         /* Trick: '' is like stopping and starting a literal string. */
-        while ((c = *++xp->xp_pattern) != NUL && c != '\'')
+        while ((c = *++xp->xp_pattern) != '\0' && c != '\'')
           /* skip */;
         xp->xp_context = EXPAND_NOTHING;
       } else if (c == '|') {
@@ -3008,8 +3008,8 @@ void set_context_for_expression(expand_T *xp, char_u *arg, cmdidx_T cmdidx)
        * anyway. */
       xp->xp_context = EXPAND_EXPRESSION;
     arg = xp->xp_pattern;
-    if (*arg != NUL)
-      while ((c = *++arg) != NUL && (c == ' ' || c == '\t'))
+    if (*arg != '\0')
+      while ((c = *++arg) != '\0' && (c == ' ' || c == '\t'))
         /* skip */;
   }
   xp->xp_pattern = arg;
@@ -3210,7 +3210,7 @@ static int do_unlet_var(lval_T *lp, char_u *name_end, int forceit)
 
   if (lp->ll_tv == NULL) {
     cc = *name_end;
-    *name_end = NUL;
+    *name_end = '\0';
 
     /* Normal name or expanded name. */
     if (check_changedtick(lp->ll_name))
@@ -3254,7 +3254,7 @@ int do_unlet(char_u *name, int forceit)
   dictitem_T  *di;
 
   ht = find_var_ht(name, &varname);
-  if (ht != NULL && *varname != NUL) {
+  if (ht != NULL && *varname != '\0') {
     hi = hash_find(ht, varname);
     if (!HASHITEM_EMPTY(hi)) {
       di = HI2DI(hi);
@@ -3287,7 +3287,7 @@ static int do_lock_var(lval_T *lp, char_u *name_end, int deep, int lock)
 
   if (lp->ll_tv == NULL) {
     cc = *name_end;
-    *name_end = NUL;
+    *name_end = '\0';
 
     /* Normal name or expanded name. */
     if (check_changedtick(lp->ll_name))
@@ -4471,7 +4471,7 @@ eval7 (
       rettv->v_type = VAR_STRING;
       rettv->vval.v_string = get_reg_contents(**arg, TRUE, TRUE);
     }
-    if (**arg != NUL)
+    if (**arg != '\0')
       ++*arg;
     break;
 
@@ -4788,7 +4788,7 @@ eval_index (
 
         if (len == -1) {
           key = get_tv_string(&var1);
-          if (*key == NUL) {
+          if (*key == '\0') {
             if (verbose)
               EMSG(_(e_emptykey));
             clear_tv(&var1);
@@ -4854,7 +4854,7 @@ get_option_tv (
   }
 
   c = *option_end;
-  *option_end = NUL;
+  *option_end = '\0';
   opt_type = get_option_value(*arg, &numval,
       rettv == NULL ? NULL : &stringval, opt_flags);
 
@@ -4898,8 +4898,8 @@ static int get_string_tv(char_u **arg, typval_T *rettv, int evaluate)
   /*
    * Find the end of the string, skipping backslashed characters.
    */
-  for (p = *arg + 1; *p != NUL && *p != '"'; mb_ptr_adv(p)) {
-    if (*p == '\\' && p[1] != NUL) {
+  for (p = *arg + 1; *p != '\0' && *p != '"'; mb_ptr_adv(p)) {
+    if (*p == '\\' && p[1] != '\0') {
       ++p;
       /* A "\<x>" form occupies at least 4 characters, and produces up
        * to 6 characters: reserve space for 2 extra */
@@ -4929,7 +4929,7 @@ static int get_string_tv(char_u **arg, typval_T *rettv, int evaluate)
   rettv->v_type = VAR_STRING;
   rettv->vval.v_string = name;
 
-  for (p = *arg + 1; *p != NUL && *p != '"'; ) {
+  for (p = *arg + 1; *p != '\0' && *p != '"'; ) {
     if (*p == '\\') {
       switch (*++p) {
       case 'b': *name++ = BS; ++p; break;
@@ -4998,7 +4998,7 @@ static int get_string_tv(char_u **arg, typval_T *rettv, int evaluate)
       MB_COPY_CHAR(p, name);
 
   }
-  *name = NUL;
+  *name = '\0';
   *arg = p + 1;
 
   return OK;
@@ -5017,7 +5017,7 @@ static int get_lit_string_tv(char_u **arg, typval_T *rettv, int evaluate)
   /*
    * Find the end of the string, skipping ''.
    */
-  for (p = *arg + 1; *p != NUL; mb_ptr_adv(p)) {
+  for (p = *arg + 1; *p != '\0'; mb_ptr_adv(p)) {
     if (*p == '\'') {
       if (p[1] != '\'')
         break;
@@ -5046,7 +5046,7 @@ static int get_lit_string_tv(char_u **arg, typval_T *rettv, int evaluate)
   rettv->v_type = VAR_STRING;
   rettv->vval.v_string = str;
 
-  for (p = *arg + 1; *p != NUL; ) {
+  for (p = *arg + 1; *p != '\0'; ) {
     if (*p == '\'') {
       if (p[1] != '\'')
         break;
@@ -5054,7 +5054,7 @@ static int get_lit_string_tv(char_u **arg, typval_T *rettv, int evaluate)
     }
     MB_COPY_CHAR(p, str);
   }
-  *str = NUL;
+  *str = '\0';
   *arg = p + 1;
 
   return OK;
@@ -5077,7 +5077,7 @@ static int get_list_tv(char_u **arg, typval_T *rettv, int evaluate)
   }
 
   *arg = skipwhite(*arg + 1);
-  while (**arg != ']' && **arg != NUL) {
+  while (**arg != ']' && **arg != '\0') {
     if (eval1(arg, &tv, evaluate) == FAIL)      /* recursive! */
       goto failret;
     if (evaluate) {
@@ -5730,7 +5730,7 @@ static char_u *list2string(typval_T *tv, int copyID)
     return NULL;
   }
   ga_append(&ga, ']');
-  ga_append(&ga, NUL);
+  ga_append(&ga, '\0');
   return (char_u *)ga.ga_data;
 }
 
@@ -6444,7 +6444,7 @@ static char_u *dict2string(typval_T *tv, int copyID)
   }
 
   ga_append(&ga, '}');
-  ga_append(&ga, NUL);
+  ga_append(&ga, '\0');
   return (char_u *)ga.ga_data;
 }
 
@@ -6485,7 +6485,7 @@ static int get_dict_tv(char_u **arg, typval_T *rettv, int evaluate)
   tv.v_type = VAR_UNKNOWN;
 
   *arg = skipwhite(*arg + 1);
-  while (**arg != '}' && **arg != NUL) {
+  while (**arg != '}' && **arg != '\0') {
     if (eval1(arg, &tvkey, evaluate) == FAIL)           /* recursive! */
       goto failret;
     if (**arg != ':') {
@@ -6495,7 +6495,7 @@ static int get_dict_tv(char_u **arg, typval_T *rettv, int evaluate)
     }
     if (evaluate) {
       key = get_tv_string_buf_chk(&tvkey, buf);
-      if (key == NULL || *key == NUL) {
+      if (key == NULL || *key == '\0') {
         /* "key" is NULL when get_tv_string_buf_chk() gave an errmsg */
         if (key != NULL)
           EMSG(_(e_emptykey));
@@ -6673,7 +6673,7 @@ static char_u *string_quote(char_u *str, int function)
   len = (function ? 13 : 3);
   if (str != NULL) {
     len += (unsigned)STRLEN(str);
-    for (p = str; *p != NUL; mb_ptr_adv(p))
+    for (p = str; *p != '\0'; mb_ptr_adv(p))
       if (*p == '\'')
         ++len;
   }
@@ -6685,7 +6685,7 @@ static char_u *string_quote(char_u *str, int function)
     } else
       *r++ = '\'';
     if (str != NULL)
-      for (p = str; *p != NUL; ) {
+      for (p = str; *p != '\0'; ) {
         if (*p == '\'')
           *r++ = '\'';
         MB_COPY_CHAR(p, r);
@@ -6693,7 +6693,7 @@ static char_u *string_quote(char_u *str, int function)
     *r++ = '\'';
     if (function)
       *r++ = ')';
-    *r++ = NUL;
+    *r++ = '\0';
   }
   return s;
 }
@@ -6738,10 +6738,10 @@ static int get_env_tv(char_u **arg, typval_T *rettv, int evaluate)
   if (evaluate) {
     if (len != 0) {
       cc = name[len];
-      name[len] = NUL;
+      name[len] = '\0';
       /* first try vim_getenv(), fast for normal environment vars */
       string = vim_getenv(name, &mustfree);
-      if (string != NULL && *string != NUL) {
+      if (string != NULL && *string != '\0') {
         if (!mustfree)
           string = vim_strsave(string);
       } else {
@@ -7136,7 +7136,7 @@ static char_u *deref_func_name(char_u *name, int *lenp, int no_autoload)
   int cc;
 
   cc = name[*lenp];
-  name[*lenp] = NUL;
+  name[*lenp] = '\0';
   v = find_var(name, NULL, no_autoload);
   name[*lenp] = cc;
   if (v != NULL && v->di_tv.v_type == VAR_FUNC) {
@@ -7179,7 +7179,7 @@ get_func_tv (
   argp = *arg;
   while (argcount < MAX_FUNC_ARGS) {
     argp = skipwhite(argp + 1);             /* skip the '(' or ',' */
-    if (*argp == ')' || *argp == ',' || *argp == NUL)
+    if (*argp == ')' || *argp == ',' || *argp == '\0')
       break;
     if (eval1(&argp, &argvars[argcount], evaluate) == FAIL) {
       ret = FAIL;
@@ -7447,7 +7447,7 @@ static int non_zero_arg(typval_T *argvars)
           && argvars[0].vval.v_number != 0)
          || (argvars[0].v_type == VAR_STRING
              && argvars[0].vval.v_string != NULL
-             && *argvars[0].vval.v_string != NUL);
+             && *argvars[0].vval.v_string != '\0');
 }
 
 /*********************************************
@@ -7765,9 +7765,9 @@ static buf_T *get_buf_tv(typval_T *tv, int curtab_only)
     return buflist_findnr((int)tv->vval.v_number);
   if (tv->v_type != VAR_STRING)
     return NULL;
-  if (name == NULL || *name == NUL)
+  if (name == NULL || *name == '\0')
     return curbuf;
-  if (name[0] == '$' && name[1] == NUL)
+  if (name[0] == '$' && name[1] == '\0')
     return lastbuf;
 
   /* Ignore 'magic' and 'cpoptions' here to make scripts portable */
@@ -7887,7 +7887,7 @@ static void byteidx(typval_T *argvars, typval_T *rettv, int comp)
 
   t = str;
   for (; idx > 0; idx--) {
-    if (*t == NUL)              /* EOL reached */
+    if (*t == '\0')              /* EOL reached */
       return;
     if (enc_utf8 && comp)
       t += utf_ptr2len(t);
@@ -7964,7 +7964,7 @@ static void f_call(typval_T *argvars, typval_T *rettv)
     func = argvars[0].vval.v_string;
   else
     func = get_tv_string(&argvars[0]);
-  if (*func == NUL)
+  if (*func == '\0')
     return;             /* type error or empty name */
 
   if (argvars[2].v_type != VAR_UNKNOWN) {
@@ -8073,7 +8073,7 @@ static void f_col(typval_T *argvars, typval_T *rettv)
                 curwin->w_virtcol - curwin->w_cursor.coladd)) {
           int l;
 
-          if (*p != NUL && p[(l = (*mb_ptr2len)(p))] == NUL)
+          if (*p != '\0' && p[(l = (*mb_ptr2len)(p))] == '\0')
             col += l;
         }
       }
@@ -8172,7 +8172,7 @@ static void f_confirm(typval_T *argvars, typval_T *rettv)
     }
   }
 
-  if (buttons == NULL || *buttons == NUL)
+  if (buttons == NULL || *buttons == '\0')
     buttons = (char_u *)_("&Ok");
 
   if (!error)
@@ -8452,7 +8452,7 @@ static void f_empty(typval_T *argvars, typval_T *rettv)
   case VAR_STRING:
   case VAR_FUNC:
     n = argvars[0].vval.v_string == NULL
-        || *argvars[0].vval.v_string == NUL;
+        || *argvars[0].vval.v_string == '\0';
     break;
   case VAR_NUMBER:
     n = argvars[0].vval.v_number == 0;
@@ -8502,7 +8502,7 @@ static void f_eval(typval_T *argvars, typval_T *rettv)
   if (s == NULL || eval1(&s, rettv, TRUE) == FAIL) {
     rettv->v_type = VAR_NUMBER;
     rettv->vval.v_number = 0;
-  } else if (*s != NUL)
+  } else if (*s != '\0')
     EMSG(_(e_trailing));
 }
 
@@ -8546,7 +8546,7 @@ static void f_exists(typval_T *argvars, typval_T *rettv)
     }
   } else if (*p == '&' || *p == '+') {                /* option */
     n = (get_option_tv(&p, NULL, TRUE) == OK);
-    if (*skipwhite(p) != NUL)
+    if (*skipwhite(p) != '\0')
       n = FALSE;                        /* trailing garbage */
   } else if (*p == '*') {             /* internal or user defined function */
     n = function_exists(p + 1);
@@ -8575,7 +8575,7 @@ static void f_exists(typval_T *argvars, typval_T *rettv)
           clear_tv(&tv);
       }
     }
-    if (*p != NUL)
+    if (*p != '\0')
       n = FALSE;
 
     vim_free(tofree);
@@ -8794,10 +8794,10 @@ static void f_feedkeys(typval_T *argvars, typval_T *rettv)
     return;
 
   keys = get_tv_string(&argvars[0]);
-  if (*keys != NUL) {
+  if (*keys != '\0') {
     if (argvars[1].v_type != VAR_UNKNOWN) {
       flags = get_tv_string_buf(&argvars[1], nbuf);
-      for (; *flags != NUL; ++flags) {
+      for (; *flags != '\0'; ++flags) {
         switch (*flags) {
         case 'n': remap = FALSE; break;
         case 'm': remap = TRUE; break;
@@ -8859,7 +8859,7 @@ static void findfilendir(typval_T *argvars, typval_T *rettv, int find_what)
 {
   char_u      *fname;
   char_u      *fresult = NULL;
-  char_u      *path = *curbuf->b_p_path == NUL ? p_path : curbuf->b_p_path;
+  char_u      *path = *curbuf->b_p_path == '\0' ? p_path : curbuf->b_p_path;
   char_u      *p;
   char_u pathbuf[NUMBUFLEN];
   int count = 1;
@@ -8876,7 +8876,7 @@ static void findfilendir(typval_T *argvars, typval_T *rettv, int find_what)
     if (p == NULL)
       error = TRUE;
     else {
-      if (*p != NUL)
+      if (*p != '\0')
         path = p;
 
       if (argvars[2].v_type != VAR_UNKNOWN)
@@ -8888,7 +8888,7 @@ static void findfilendir(typval_T *argvars, typval_T *rettv, int find_what)
     rettv_list_alloc(rettv);
   }
 
-  if (*fname != NUL && !error) {
+  if (*fname != '\0' && !error) {
     do {
       if (rettv->v_type == VAR_STRING || rettv->v_type == VAR_LIST)
         vim_free(fresult);
@@ -9023,7 +9023,7 @@ static int filter_map_one(typval_T *tv, char_u *expr, int map, int *remp)
   s = expr;
   if (eval1(&s, &rettv, TRUE) == FAIL)
     goto theend;
-  if (*s != NUL) {  /* check for trailing chars after expr */
+  if (*s != '\0') {  /* check for trailing chars after expr */
     EMSG2(_(e_invexpr2), s);
     goto theend;
   }
@@ -9239,7 +9239,7 @@ static void f_foldtext(typval_T *argvars, typval_T *rettv)
     /* skip C comment-start */
     if (s[0] == '/' && (s[1] == '*' || s[1] == '/')) {
       s = skipwhite(s + 2);
-      if (*skipwhite(s) == NUL
+      if (*skipwhite(s) == '\0'
           && lnum + 1 < (linenr_T)vimvars[VV_FOLDEND].vv_nr) {
         s = skipwhite(ml_get(lnum + 1));
         if (*s == '*')
@@ -9306,7 +9306,7 @@ static void f_function(typval_T *argvars, typval_T *rettv)
   char_u      *s;
 
   s = get_tv_string(&argvars[0]);
-  if (s == NULL || *s == NUL || VIM_ISDIGIT(*s))
+  if (s == NULL || *s == '\0' || VIM_ISDIGIT(*s))
     EMSG2(_(e_invarg2), s);
   /* Don't check an autoload name for existence here. */
   else if (vim_strchr(s, AUTOLOAD_CHAR) == NULL && !function_exists(s))
@@ -9521,7 +9521,7 @@ static void f_getchar(typval_T *argvars, typval_T *rettv)
     else if (get_tv_number_chk(&argvars[0], &error) == 1)
       /* getchar(1): only check if char avail */
       n = vpeekc();
-    else if (error || vpeekc() == NUL)
+    else if (error || vpeekc() == '\0')
       /* illegal argument or getchar(0) and no char avail: return zero */
       n = 0;
     else
@@ -9557,7 +9557,7 @@ static void f_getchar(typval_T *argvars, typval_T *rettv)
       i += (*mb_char2bytes)(n, temp + i);
     else
       temp[i++] = n;
-    temp[i++] = NUL;
+    temp[i++] = '\0';
     rettv->v_type = VAR_STRING;
     rettv->vval.v_string = vim_strsave(temp);
 
@@ -9618,7 +9618,7 @@ static void f_getcmdtype(typval_T *argvars, typval_T *rettv)
   rettv->vval.v_string = alloc(2);
   if (rettv->vval.v_string != NULL) {
     rettv->vval.v_string[0] = get_cmdline_type();
-    rettv->vval.v_string[1] = NUL;
+    rettv->vval.v_string[1] = '\0';
   }
 }
 
@@ -9933,8 +9933,8 @@ static void f_getregtype(typval_T *argvars, typval_T *rettv)
   if (regname == 0)
     regname = '"';
 
-  buf[0] = NUL;
-  buf[1] = NUL;
+  buf[0] = '\0';
+  buf[1] = '\0';
   switch (get_reg_type(regname, &reglen)) {
   case MLINE: buf[0] = 'V'; break;
   case MCHAR: buf[0] = 'v'; break;
@@ -10408,9 +10408,9 @@ static void f_histadd(typval_T *argvars, typval_T *rettv)
   histype = str != NULL ? get_histtype(str) : -1;
   if (histype >= 0) {
     str = get_tv_string_buf(&argvars[1], buf);
-    if (*str != NUL) {
+    if (*str != '\0') {
       init_history();
-      add_to_history(histype, str, FALSE, NUL);
+      add_to_history(histype, str, FALSE, '\0');
       rettv->vval.v_number = TRUE;
       return;
     }
@@ -10636,7 +10636,7 @@ static void get_user_input(typval_T *argvars, typval_T *rettv, int inputdialog)
     else {
       ++p;
       c = *p;
-      *p = NUL;
+      *p = '\0';
       msg_start();
       msg_clr_eos();
       msg_puts_attr(prompt, echo_attr);
@@ -10675,7 +10675,7 @@ static void get_user_input(typval_T *argvars, typval_T *rettv, int inputdialog)
       int save_ex_normal_busy = ex_normal_busy;
       ex_normal_busy = 0;
       rettv->vval.v_string =
-        getcmdline_prompt(inputsecret_flag ? NUL : '@', p, echo_attr,
+        getcmdline_prompt(inputsecret_flag ? '\0' : '@', p, echo_attr,
             xp_type, xp_arg);
       ex_normal_busy = save_ex_normal_busy;
     }
@@ -10856,7 +10856,7 @@ static void f_islocked(typval_T *argvars, typval_T *rettv)
   end = get_lval(get_tv_string(&argvars[0]), NULL, &lv, FALSE, FALSE,
       GLV_NO_AUTOLOAD, FNE_CHECK_START);
   if (end != NULL && lv.ll_name != NULL) {
-    if (*end != NUL)
+    if (*end != '\0')
       EMSG(_(e_trailing));
     else {
       if (lv.ll_tv == NULL) {
@@ -11127,7 +11127,7 @@ static void f_join(typval_T *argvars, typval_T *rettv)
   if (sep != NULL) {
     ga_init(&ga, (int)sizeof(char), 80);
     list_join(&ga, argvars[0].vval.v_list, sep, TRUE, 0);
-    ga_append(&ga, NUL);
+    ga_append(&ga, '\0');
     rettv->vval.v_string = (char_u *)ga.ga_data;
   } else
     rettv->vval.v_string = NULL;
@@ -11311,7 +11311,7 @@ static void get_maparg(typval_T *argvars, typval_T *rettv, int exact)
   rettv->vval.v_string = NULL;
 
   keys = get_tv_string(&argvars[0]);
-  if (*keys == NUL)
+  if (*keys == '\0')
     return;
 
   if (argvars[1].v_type != VAR_UNKNOWN) {
@@ -11776,12 +11776,12 @@ static void f_mkdir(typval_T *argvars, typval_T *rettv)
     return;
 
   dir = get_tv_string_buf(&argvars[0], buf);
-  if (*dir == NUL)
+  if (*dir == '\0')
     rettv->vval.v_number = FAIL;
   else {
-    if (*path_tail(dir) == NUL)
+    if (*path_tail(dir) == '\0')
       /* remove trailing slashes */
-      *path_tail_with_sep(dir) = NUL;
+      *path_tail_with_sep(dir) = '\0';
 
     if (argvars[1].v_type != VAR_UNKNOWN) {
       if (argvars[2].v_type != VAR_UNKNOWN)
@@ -11800,8 +11800,8 @@ static void f_mode(typval_T *argvars, typval_T *rettv)
 {
   char_u buf[3];
 
-  buf[1] = NUL;
-  buf[2] = NUL;
+  buf[1] = '\0';
+  buf[2] = '\0';
 
   if (VIsual_active) {
     if (VIsual_select)
@@ -11841,7 +11841,7 @@ static void f_mode(typval_T *argvars, typval_T *rettv)
   /* Clear out the minor mode when the argument is not a non-zero number or
    * non-empty string.  */
   if (!non_zero_arg(&argvars[0]))
-    buf[1] = NUL;
+    buf[1] = '\0';
 
   rettv->vval.v_string = vim_strsave(buf);
   rettv->v_type = VAR_STRING;
@@ -11860,7 +11860,7 @@ static void f_nextnonblank(typval_T *argvars, typval_T *rettv)
       lnum = 0;
       break;
     }
-    if (*skipwhite(ml_get(lnum)) != NUL)
+    if (*skipwhite(ml_get(lnum)) != '\0')
       break;
   }
   rettv->vval.v_number = lnum;
@@ -11879,12 +11879,12 @@ static void f_nr2char(typval_T *argvars, typval_T *rettv)
     if (argvars[1].v_type != VAR_UNKNOWN)
       utf8 = get_tv_number_chk(&argvars[1], NULL);
     if (utf8)
-      buf[(*utf_char2bytes)((int)get_tv_number(&argvars[0]), buf)] = NUL;
+      buf[(*utf_char2bytes)((int)get_tv_number(&argvars[0]), buf)] = '\0';
     else
-      buf[(*mb_char2bytes)((int)get_tv_number(&argvars[0]), buf)] = NUL;
+      buf[(*mb_char2bytes)((int)get_tv_number(&argvars[0]), buf)] = '\0';
   } else {
     buf[0] = (char_u)get_tv_number(&argvars[0]);
-    buf[1] = NUL;
+    buf[1] = '\0';
   }
   rettv->v_type = VAR_STRING;
   rettv->vval.v_string = vim_strsave(buf);
@@ -11944,7 +11944,7 @@ static void f_prevnonblank(typval_T *argvars, typval_T *rettv)
   if (lnum < 1 || lnum > curbuf->b_ml.ml_line_count)
     lnum = 0;
   else
-    while (lnum >= 1 && *skipwhite(ml_get(lnum)) == NUL)
+    while (lnum >= 1 && *skipwhite(ml_get(lnum)) == '\0')
       --lnum;
   rettv->vval.v_number = lnum;
 }
@@ -12062,8 +12062,8 @@ static void f_readfile(typval_T *argvars, typval_T *rettv)
   /* Always open the file in binary mode, library functions have a mind of
    * their own about CR-LF conversion. */
   fname = get_tv_string(&argvars[0]);
-  if (*fname == NUL || (fd = mch_fopen((char *)fname, READBIN)) == NULL) {
-    EMSG2(_(e_notopen), *fname == NUL ? (char_u *)_("<empty>") : fname);
+  if (*fname == '\0' || (fd = mch_fopen((char *)fname, READBIN)) == NULL) {
+    EMSG2(_(e_notopen), *fname == '\0' ? (char_u *)_("<empty>") : fname);
     return;
   }
 
@@ -12100,7 +12100,7 @@ static void f_readfile(typval_T *argvars, typval_T *rettv)
            * allocated only once.  */
           if ((s = xrealloc(prev, prevlen + len + 1)) != NULL) {
             memmove(s + prevlen, start, len);
-            s[prevlen + len] = NUL;
+            s[prevlen + len] = '\0';
             prev = NULL;             /* the list will own the string */
             prevlen = prevsize = 0;
           }
@@ -12124,7 +12124,7 @@ static void f_readfile(typval_T *argvars, typval_T *rettv)
         start = p + 1;         /* step over newline */
         if ((++cnt >= maxline && maxline >= 0) || readlen <= 0)
           break;
-      } else if (*p == NUL)
+      } else if (*p == '\0')
         *p = '\n';
       /* Check for utf8 "bom"; U+FEFF is encoded as EF BB BF.  Do this
        * when finding the BF and check the previous two bytes. */
@@ -12132,10 +12132,10 @@ static void f_readfile(typval_T *argvars, typval_T *rettv)
         /* Find the two bytes before the 0xbf.	If p is at buf, or buf
          * + 1, these may be in the "prev" string. */
         char_u back1 = p >= buf + 1 ? p[-1]
-                       : prevlen >= 1 ? prev[prevlen - 1] : NUL;
+                       : prevlen >= 1 ? prev[prevlen - 1] : '\0';
         char_u back2 = p >= buf + 2 ? p[-2]
                        : p == buf + 1 && prevlen >= 1 ? prev[prevlen - 1]
-                       : prevlen >= 2 ? prev[prevlen - 2] : NUL;
+                       : prevlen >= 2 ? prev[prevlen - 2] : '\0';
 
         if (back2 == 0xef && back1 == 0xbb) {
           char_u *dest = p - 2;
@@ -12456,7 +12456,7 @@ static void f_repeat(typval_T *argvars, typval_T *rettv)
     if (r != NULL) {
       for (i = 0; i < n; i++)
         memmove(r + i * slen, p, (size_t)slen);
-      r[len] = NUL;
+      r[len] = '\0';
     }
 
     rettv->vval.v_string = r;
@@ -12504,15 +12504,15 @@ static void f_resolve(typval_T *argvars, typval_T *rettv)
     len = STRLEN(p);
     if (len > 0 && after_pathsep(p, p + len)) {
       has_trailing_pathsep = TRUE;
-      p[len - 1] = NUL;       /* the trailing slash breaks readlink() */
+      p[len - 1] = '\0';       /* the trailing slash breaks readlink() */
     }
 
     q = path_next_component(p);
-    if (*q != NUL) {
+    if (*q != '\0') {
       /* Separate the first path component in "p", and keep the
        * remainder (beginning with the path separator). */
       remain = vim_strsave(q - 1);
-      q[-1] = NUL;
+      q[-1] = '\0';
     }
 
     buf = alloc(MAXPATHL + 1);
@@ -12524,7 +12524,7 @@ static void f_resolve(typval_T *argvars, typval_T *rettv)
         len = readlink((char *)p, (char *)buf, MAXPATHL);
         if (len <= 0)
           break;
-        buf[len] = NUL;
+        buf[len] = '\0';
 
         if (limit-- == 0) {
           vim_free(p);
@@ -12542,7 +12542,7 @@ static void f_resolve(typval_T *argvars, typval_T *rettv)
         /* Separate the first path component in the link value and
          * concatenate the remainders. */
         q = path_next_component(vim_ispathsep(*buf) ? buf + 1 : buf);
-        if (*q != NUL) {
+        if (*q != '\0') {
           if (remain == NULL)
             remain = vim_strsave(q - 1);
           else {
@@ -12552,13 +12552,13 @@ static void f_resolve(typval_T *argvars, typval_T *rettv)
               remain = cpy;
             }
           }
-          q[-1] = NUL;
+          q[-1] = '\0';
         }
 
         q = path_tail(p);
-        if (q > p && *q == NUL) {
+        if (q > p && *q == '\0') {
           /* Ignore trailing path separator. */
-          q[-1] = NUL;
+          q[-1] = '\0';
           q = path_tail(p);
         }
         if (q > p && !path_is_absolute_path(buf)) {
@@ -12581,7 +12581,7 @@ static void f_resolve(typval_T *argvars, typval_T *rettv)
 
       /* Append the first path component of "remain" to "p". */
       q = path_next_component(remain + 1);
-      len = q - remain - (*q != NUL);
+      len = q - remain - (*q != '\0');
       cpy = vim_strnsave(p, STRLEN(p) + len);
       if (cpy != NULL) {
         STRNCAT(cpy, remain, len);
@@ -12589,7 +12589,7 @@ static void f_resolve(typval_T *argvars, typval_T *rettv)
         p = cpy;
       }
       /* Shorten "remain". */
-      if (*q != NUL)
+      if (*q != '\0')
         STRMOVE(remain, q - 1);
       else {
         vim_free(remain);
@@ -12601,12 +12601,12 @@ static void f_resolve(typval_T *argvars, typval_T *rettv)
      * the current directory if and only if the argument had this form. */
     if (!vim_ispathsep(*p)) {
       if (is_relative_to_current
-          && *p != NUL
+          && *p != '\0'
           && !(p[0] == '.'
-               && (p[1] == NUL
+               && (p[1] == '\0'
                    || vim_ispathsep(p[1])
                    || (p[1] == '.'
-                       && (p[2] == NUL
+                       && (p[2] == '\0'
                            || vim_ispathsep(p[2])))))) {
         /* Prepend "./". */
         cpy = concat_str((char_u *)"./", p);
@@ -12629,7 +12629,7 @@ static void f_resolve(typval_T *argvars, typval_T *rettv)
     if (!has_trailing_pathsep) {
       q = p + STRLEN(p);
       if (after_pathsep(p, q))
-        *path_tail_with_sep(p) = NUL;
+        *path_tail_with_sep(p) = '\0';
     }
 
     rettv->vval.v_string = p;
@@ -12701,7 +12701,7 @@ static int get_search_arg(typval_T *varp, int *flagsp)
     flags = get_tv_string_buf_chk(varp, nbuf);
     if (flags == NULL)
       return 0;                 /* type error; errmsg already given */
-    while (*flags != NUL) {
+    while (*flags != '\0') {
       switch (*flags) {
       case 'b': dir = BACKWARD; break;
       case 'w': p_ws = TRUE; break;
@@ -13082,7 +13082,7 @@ do_searchpair (
   if (pat2 == NULL || pat3 == NULL)
     goto theend;
   sprintf((char *)pat2, "\\(%s\\m\\)\\|\\(%s\\m\\)", spat, epat);
-  if (*mpat == NUL)
+  if (*mpat == '\0')
     STRCPY(pat3, pat2);
   else
     sprintf((char *)pat3, "\\(%s\\m\\)\\|\\(%s\\m\\)\\|\\(%s\\m\\)",
@@ -13119,7 +13119,7 @@ do_searchpair (
     options &= ~SEARCH_START;
 
     /* If the skip pattern matches, ignore this match. */
-    if (*skip != NUL) {
+    if (*skip != '\0') {
       save_pos = curwin->w_cursor;
       curwin->w_cursor = pos;
       r = eval_to_bool(skip, &err, NULL, FALSE);
@@ -13455,7 +13455,7 @@ static void f_setpos(typval_T *argvars, typval_T *rettv)
     if (list2fpos(&argvars[1], &pos, &fnum) == OK) {
       if (--pos.col < 0)
         pos.col = 0;
-      if (name[0] == '.' && name[1] == NUL) {
+      if (name[0] == '.' && name[1] == '\0') {
         /* set cursor */
         if (fnum == curbuf->b_fnum) {
           curwin->w_cursor = pos;
@@ -13463,7 +13463,7 @@ static void f_setpos(typval_T *argvars, typval_T *rettv)
           rettv->vval.v_number = 0;
         } else
           EMSG(_(e_invarg));
-      } else if (name[0] == '\'' && name[1] != NUL && name[2] == NUL)   {
+      } else if (name[0] == '\'' && name[1] != '\0' && name[2] == '\0')   {
         /* set mark */
         if (setmark_pos(name[1], &pos, fnum) == OK)
           rettv->vval.v_number = 0;
@@ -13513,7 +13513,7 @@ static void f_setreg(typval_T *argvars, typval_T *rettv)
     stropt = get_tv_string_chk(&argvars[2]);
     if (stropt == NULL)
       return;                   /* type error */
-    for (; *stropt != NUL; ++stropt)
+    for (; *stropt != '\0'; ++stropt)
       switch (*stropt) {
       case 'a': case 'A':               /* append */
         append = TRUE;
@@ -13959,13 +13959,13 @@ static void f_spellbadword(typval_T *argvars, typval_T *rettv)
     len = spell_move_to(curwin, FORWARD, TRUE, TRUE, &attr);
     if (len != 0)
       word = ml_get_cursor();
-  } else if (curwin->w_p_spell && *curbuf->b_s.b_p_spl != NUL) {
+  } else if (curwin->w_p_spell && *curbuf->b_s.b_p_spl != '\0') {
     char_u  *str = get_tv_string_chk(&argvars[0]);
     int capcol = -1;
 
     if (str != NULL) {
       /* Check the argument for spelling. */
-      while (*str != NUL) {
+      while (*str != '\0') {
         len = spell_check(curwin, str, &attr, &capcol, FALSE);
         if (attr != HLF_COUNT) {
           word = str;
@@ -14000,7 +14000,7 @@ static void f_spellsuggest(typval_T *argvars, typval_T *rettv)
 
   rettv_list_alloc(rettv);
 
-  if (curwin->w_p_spell && *curwin->w_s->b_p_spl != NUL) {
+  if (curwin->w_p_spell && *curwin->w_s->b_p_spl != '\0') {
     str = get_tv_string(&argvars[0]);
     if (argvars[1].v_type != VAR_UNKNOWN) {
       maxcount = get_tv_number_chk(&argvars[1], &typeerr);
@@ -14058,7 +14058,7 @@ static void f_split(typval_T *argvars, typval_T *rettv)
     if (argvars[2].v_type != VAR_UNKNOWN)
       keepempty = get_tv_number_chk(&argvars[2], &typeerr);
   }
-  if (pat == NULL || *pat == NUL)
+  if (pat == NULL || *pat == '\0')
     pat = (char_u *)"[\\x01- ]\\+";
 
   rettv_list_alloc(rettv);
@@ -14069,8 +14069,8 @@ static void f_split(typval_T *argvars, typval_T *rettv)
   regmatch.regprog = vim_regcomp(pat, RE_MAGIC + RE_STRING);
   if (regmatch.regprog != NULL) {
     regmatch.rm_ic = FALSE;
-    while (*str != NUL || keepempty) {
-      if (*str == NUL)
+    while (*str != '\0' || keepempty) {
+      if (*str == '\0')
         match = FALSE;          /* empty item at the end */
       else
         match = vim_regexec_nl(&regmatch, str, col);
@@ -14079,7 +14079,7 @@ static void f_split(typval_T *argvars, typval_T *rettv)
       else
         end = str + STRLEN(str);
       if (keepempty || end > str || (rettv->vval.v_list->lv_len > 0
-                                     && *str != NUL && match && end <
+                                     && *str != '\0' && match && end <
                                      regmatch.endp[0])) {
         list_append_string(rettv->vval.v_list, str, (int)(end - str));
       }
@@ -14186,7 +14186,7 @@ static void f_strftime(typval_T *argvars, typval_T *rettv)
       (void)strftime((char *)result_buf, sizeof(result_buf),
           (char *)p, curtime);
     else
-      result_buf[0] = NUL;
+      result_buf[0] = '\0';
 
     if (conv.vc_type != CONV_NONE)
       vim_free(p);
@@ -14267,7 +14267,7 @@ static void f_strchars(typval_T *argvars, typval_T *rettv)
   char_u              *s = get_tv_string(&argvars[0]);
   varnumber_T len = 0;
 
-  while (*s != NUL) {
+  while (*s != '\0') {
     mb_cptr2char_adv(&s);
     ++len;
   }
@@ -14368,7 +14368,7 @@ static void f_strridx(typval_T *argvars, typval_T *rettv)
   } else
     end_idx = haystack_len;
 
-  if (*needle == NUL) {
+  if (*needle == '\0') {
     /* Empty string matches past the end. */
     lastmatch = haystack + end_idx;
   } else {
@@ -14579,7 +14579,7 @@ static void f_synconcealed(typval_T *argvars, typval_T *rettv)
   lnum = get_tv_lnum(argvars);                  /* -1 on type error */
   col = get_tv_number(&argvars[1]) - 1;         /* -1 on type error */
 
-  memset(str, NUL, sizeof(str));
+  memset(str, '\0', sizeof(str));
 
   rettv_list_alloc(rettv);
   if (lnum >= 1 && lnum <= curbuf->b_ml.ml_line_count && col >= 0
@@ -14590,10 +14590,10 @@ static void f_synconcealed(typval_T *argvars, typval_T *rettv)
     // get the conceal character
     if ((syntax_flags & HL_CONCEAL) && curwin->w_p_cole < 3) {
       cchar = syn_get_sub_char();
-      if (cchar == NUL && curwin->w_p_cole == 1 && lcs_conceal != NUL) {
+      if (cchar == '\0' && curwin->w_p_cole == 1 && lcs_conceal != '\0') {
         cchar = lcs_conceal;
       }
-      if (cchar != NUL) {
+      if (cchar != '\0') {
         if (has_mbyte)
           (*mb_char2bytes)(cchar, str);
         else
@@ -14707,7 +14707,7 @@ static void f_system(typval_T *argvars, typval_T *rettv)
         ++s;
       *d++ = *s;
     }
-    *d = NUL;
+    *d = '\0';
   }
 # endif
 #endif
@@ -14859,7 +14859,7 @@ static void f_taglist(typval_T *argvars, typval_T *rettv)
   tag_pattern = get_tv_string(&argvars[0]);
 
   rettv->vval.v_number = FALSE;
-  if (*tag_pattern == NUL)
+  if (*tag_pattern == '\0')
     return;
 
   rettv_list_alloc(rettv);
@@ -14937,7 +14937,7 @@ static void f_tolower(typval_T *argvars, typval_T *rettv)
   rettv->vval.v_string = p;
 
   if (p != NULL)
-    while (*p != NUL) {
+    while (*p != '\0') {
       int l;
 
       if (enc_utf8) {
@@ -15009,16 +15009,16 @@ error:
     }
 
   /* fromstr and tostr have to contain the same number of chars */
-  while (*in_str != NUL) {
+  while (*in_str != '\0') {
     if (has_mbyte) {
       inlen = (*mb_ptr2len)(in_str);
       cpstr = in_str;
       cplen = inlen;
       idx = 0;
-      for (p = fromstr; *p != NUL; p += fromlen) {
+      for (p = fromstr; *p != '\0'; p += fromlen) {
         fromlen = (*mb_ptr2len)(p);
         if (fromlen == inlen && STRNCMP(in_str, p, inlen) == 0) {
-          for (p = tostr; *p != NUL; p += tolen) {
+          for (p = tostr; *p != '\0'; p += tolen) {
             tolen = (*mb_ptr2len)(p);
             if (idx-- == 0) {
               cplen = tolen;
@@ -15026,7 +15026,7 @@ error:
               break;
             }
           }
-          if (*p == NUL)                /* tostr is shorter than fromstr */
+          if (*p == '\0')                /* tostr is shorter than fromstr */
             goto error;
           break;
         }
@@ -15038,7 +15038,7 @@ error:
          * (multi-byte) characters.  Done only once when a character
          * of in_str doesn't appear in fromstr. */
         first = FALSE;
-        for (p = tostr; *p != NUL; p += tolen) {
+        for (p = tostr; *p != '\0'; p += tolen) {
           tolen = (*mb_ptr2len)(p);
           --idx;
         }
@@ -15064,7 +15064,7 @@ error:
 
   /* add a terminating NUL */
   ga_grow(&ga, 1);
-  ga_append(&ga, NUL);
+  ga_append(&ga, '\0');
 
   rettv->vval.v_string = ga.ga_data;
 }
@@ -15112,7 +15112,7 @@ static void f_undofile(typval_T *argvars, typval_T *rettv)
   {
     char_u *fname = get_tv_string(&argvars[0]);
 
-    if (*fname == NUL) {
+    if (*fname == '\0') {
       /* If there is no file name there will be no undo file. */
       rettv->vval.v_string = NULL;
     } else {
@@ -15186,12 +15186,12 @@ static void f_visualmode(typval_T *argvars, typval_T *rettv)
 
   rettv->v_type = VAR_STRING;
   str[0] = curbuf->b_visual_mode_eval;
-  str[1] = NUL;
+  str[1] = '\0';
   rettv->vval.v_string = vim_strsave(str);
 
   /* A non-zero number or non-empty string argument: reset mode. */
   if (non_zero_arg(&argvars[0]))
-    curbuf->b_visual_mode_eval = NUL;
+    curbuf->b_visual_mode_eval = '\0';
 }
 
 /*
@@ -15278,7 +15278,7 @@ static void f_winrestcmd(typval_T *argvars, typval_T *rettv)
     ga_concat(&ga, buf);
     ++winnr;
   }
-  ga_append(&ga, NUL);
+  ga_append(&ga, '\0');
 
   rettv->vval.v_string = ga.ga_data;
   rettv->v_type = VAR_STRING;
@@ -15386,15 +15386,15 @@ static void f_writefile(typval_T *argvars, typval_T *rettv)
   /* Always open the file in binary mode, library functions have a mind of
    * their own about CR-LF conversion. */
   fname = get_tv_string(&argvars[1]);
-  if (*fname == NUL || (fd = mch_fopen((char *)fname, WRITEBIN)) == NULL) {
-    EMSG2(_(e_notcreate), *fname == NUL ? (char_u *)_("<empty>") : fname);
+  if (*fname == '\0' || (fd = mch_fopen((char *)fname, WRITEBIN)) == NULL) {
+    EMSG2(_(e_notcreate), *fname == '\0' ? (char_u *)_("<empty>") : fname);
     ret = -1;
   } else {
     for (li = argvars[0].vval.v_list->lv_first; li != NULL;
          li = li->li_next) {
-      for (s = get_tv_string(&li->li_tv); *s != NUL; ++s) {
+      for (s = get_tv_string(&li->li_tv); *s != '\0'; ++s) {
         if (*s == '\n')
-          c = putc(NUL, fd);
+          c = putc('\0', fd);
         else
           c = putc(*s, fd);
         if (c == EOF) {
@@ -15490,7 +15490,7 @@ var2fpos (
     return NULL;
   if (name[0] == '.')                           /* cursor */
     return &curwin->w_cursor;
-  if (name[0] == 'v' && name[1] == NUL) {       /* Visual start */
+  if (name[0] == 'v' && name[1] == '\0') {       /* Visual start */
     if (VIsual_active)
       return &VIsual;
     return &curwin->w_cursor;
@@ -15706,7 +15706,7 @@ static char_u *find_name_end(char_u *arg, char_u **expr_start, char_u **expr_end
   if ((flags & FNE_CHECK_START) && !eval_isnamec1(*arg) && *arg != '{')
     return arg;
 
-  for (p = arg; *p != NUL
+  for (p = arg; *p != '\0'
        && (eval_isnamec(*p)
            || *p == '{'
            || ((flags & FNE_INCL_BR) && (*p == '[' || *p == '.'))
@@ -15714,16 +15714,16 @@ static char_u *find_name_end(char_u *arg, char_u **expr_start, char_u **expr_end
            || br_nest != 0); mb_ptr_adv(p)) {
     if (*p == '\'') {
       /* skip over 'string' to avoid counting [ and ] inside it. */
-      for (p = p + 1; *p != NUL && *p != '\''; mb_ptr_adv(p))
+      for (p = p + 1; *p != '\0' && *p != '\''; mb_ptr_adv(p))
         ;
-      if (*p == NUL)
+      if (*p == '\0')
         break;
     } else if (*p == '"') {
       /* skip over "str\"ing" to avoid counting [ and ] inside it. */
-      for (p = p + 1; *p != NUL && *p != '"'; mb_ptr_adv(p))
-        if (*p == '\\' && p[1] != NUL)
+      for (p = p + 1; *p != '\0' && *p != '"'; mb_ptr_adv(p))
+        if (*p == '\\' && p[1] != '\0')
           ++p;
-      if (*p == NUL)
+      if (*p == '\0')
         break;
     }
 
@@ -15772,10 +15772,10 @@ static char_u *make_expanded_name(char_u *in_start, char_u *expr_start, char_u *
 
   if (expr_end == NULL || in_end == NULL)
     return NULL;
-  *expr_start = NUL;
-  *expr_end = NUL;
+  *expr_start = '\0';
+  *expr_end = '\0';
   c1 = *in_end;
-  *in_end = NUL;
+  *in_end = '\0';
 
   temp_result = eval_to_string(expr_start + 1, &nextcmd, FALSE);
   if (temp_result != NULL && nextcmd == NULL) {
@@ -15866,10 +15866,10 @@ void set_vim_var_char(int c)
   char_u buf[MB_MAXBYTES + 1];
 
   if (has_mbyte)
-    buf[(*mb_char2bytes)(c, buf)] = NUL;
+    buf[(*mb_char2bytes)(c, buf)] = '\0';
   else {
     buf[0] = c;
-    buf[1] = NUL;
+    buf[1] = '\0';
   }
   set_vim_var_string(VV_CHAR, buf, -1);
 }
@@ -16011,7 +16011,7 @@ char_u *set_cmdarg(exarg_T *eap, char_u *oldarg)
   else if (eap->force_bin == FORCE_NOBIN)
     sprintf((char *)newval, " ++nobin");
   else
-    *newval = NUL;
+    *newval = '\0';
 
   if (eap->read_edit)
     STRCAT(newval, " ++edit");
@@ -16053,7 +16053,7 @@ get_var_tv (
 
   /* truncate the name, so that we can use strcmp() */
   cc = name[len];
-  name[len] = NUL;
+  name[len] = '\0';
 
   /*
    * Check for "b:changedtick".
@@ -16405,7 +16405,7 @@ static dictitem_T *find_var_in_ht(hashtab_T *ht, int htname, char_u *varname, in
 {
   hashitem_T  *hi;
 
-  if (*varname == NUL) {
+  if (*varname == '\0') {
     /* Must be something like "s:", otherwise "ht" would be NULL. */
     switch (htname) {
     case 's': return &SCRIPT_SV(current_SID)->sv_var;
@@ -16551,7 +16551,7 @@ void init_var_dict(dict_T *dict, dictitem_T *dict_var, int scope)
   dict_var->di_tv.v_type = VAR_DICT;
   dict_var->di_tv.v_lock = VAR_FIXED;
   dict_var->di_flags = DI_FLAGS_RO | DI_FLAGS_FIX;
-  dict_var->di_key[0] = NUL;
+  dict_var->di_key[0] = '\0';
 }
 
 /*
@@ -16691,7 +16691,7 @@ set_var (
   hashtab_T   *ht;
 
   ht = find_var_ht(name, &varname);
-  if (ht == NULL || *varname == NUL) {
+  if (ht == NULL || *varname == '\0') {
     EMSG2(_(e_illvar), name);
     return;
   }
@@ -16821,7 +16821,7 @@ var_check_func_name (
 )
 {
   if (!(vim_strchr((char_u *)"wbs", name[0]) != NULL && name[1] == ':')
-      && !ASCII_ISUPPER((name[0] != NUL && name[1] == ':')
+      && !ASCII_ISUPPER((name[0] != '\0' && name[1] == ':')
           ? name[2] : name[0])) {
     EMSG2(_("E704: Funcref variable name must start with a capital: %s"),
         name);
@@ -16846,7 +16846,7 @@ static int valid_varname(char_u *varname)
 {
   char_u *p;
 
-  for (p = varname; *p != NUL; ++p)
+  for (p = varname; *p != '\0'; ++p)
     if (!eval_isnamec1(*p) && (p == varname || !VIM_ISDIGIT(*p))
         && *p != AUTOLOAD_CHAR) {
       EMSG2(_(e_illvar), varname);
@@ -17002,7 +17002,7 @@ void ex_echo(exarg_T *eap)
 
   if (eap->skip)
     ++emsg_skip;
-  while (*arg != NUL && *arg != '|' && *arg != '\n' && !got_int) {
+  while (*arg != '\0' && *arg != '|' && *arg != '\n' && !got_int) {
     /* If eval1() causes an error message the text from the command may
      * still need to be cleared. E.g., "echo 22,44". */
     need_clr_eos = needclr;
@@ -17038,7 +17038,7 @@ void ex_echo(exarg_T *eap)
       current_copyID += COPYID_INC;
       p = echo_string(&rettv, &tofree, numbuf, current_copyID);
       if (p != NULL)
-        for (; *p != NUL && !got_int; ++p) {
+        for (; *p != '\0' && !got_int; ++p) {
           if (*p == '\n' || *p == '\r' || *p == TAB) {
             if (*p != TAB && needclr) {
               /* remove any text still there from the command */
@@ -17109,7 +17109,7 @@ void ex_execute(exarg_T *eap)
 
   if (eap->skip)
     ++emsg_skip;
-  while (*arg != NUL && *arg != '|' && *arg != '\n') {
+  while (*arg != '\0' && *arg != '|' && *arg != '\n') {
     p = arg;
     if (eval1(&arg, &rettv, !eap->skip) == FAIL) {
       /*
@@ -17184,7 +17184,7 @@ static char_u *find_option_end(char_u **arg, int *opt_flags)
     return NULL;
   *arg = p;
 
-  if (p[0] == 't' && p[1] == '_' && p[2] != NUL && p[3] != NUL)
+  if (p[0] == 't' && p[1] == '_' && p[2] != '\0' && p[3] != '\0')
     p += 4;         /* termcap option */
   else
     while (ASCII_ISALPHA(*p))
@@ -17253,7 +17253,7 @@ void ex_function(exarg_T *eap)
       regmatch_T regmatch;
 
       c = *p;
-      *p = NUL;
+      *p = '\0';
       regmatch.regprog = vim_regcomp(eap->arg + 1, RE_MAGIC);
       *p = c;
       if (regmatch.regprog != NULL) {
@@ -17325,7 +17325,7 @@ void ex_function(exarg_T *eap)
     }
     eap->nextcmd = check_nextcmd(p);
     if (eap->nextcmd != NULL)
-      *p = NUL;
+      *p = '\0';
     if (!eap->skip && !got_int) {
       fp = find_func(name);
       if (fp != NULL) {
@@ -17384,10 +17384,10 @@ void ex_function(exarg_T *eap)
         j = 3;
       else
         j = 0;
-      while (arg[j] != NUL && (j == 0 ? eval_isnamec1(arg[j])
+      while (arg[j] != '\0' && (j == 0 ? eval_isnamec1(arg[j])
                                : eval_isnamec(arg[j])))
         ++j;
-      if (arg[j] != NUL)
+      if (arg[j] != '\0')
         emsg_funcname((char *)e_invarg2, arg);
     }
     /* Disallow using the g: dict. */
@@ -17416,7 +17416,7 @@ void ex_function(exarg_T *eap)
       }
       ga_grow(&newargs, 1);
       c = *p;
-      *p = NUL;
+      *p = '\0';
       arg = vim_strsave(arg);
       if (arg == NULL)
         goto erret;
@@ -17466,7 +17466,7 @@ void ex_function(exarg_T *eap)
    * Makes 'exe "func Test()\n...\nendfunc"' work. */
   if (*p == '\n')
     line_arg = p + 1;
-  else if (*p != NUL && *p != '"' && !eap->skip && !did_emsg)
+  else if (*p != '\0' && *p != '"' && !eap->skip && !did_emsg)
     EMSG(_(e_trailing));
 
   /*
@@ -17507,7 +17507,7 @@ void ex_function(exarg_T *eap)
       if (p == NULL)
         line_arg += STRLEN(line_arg);
       else {
-        *p = NUL;
+        *p = '\0';
         line_arg = p + 1;
       }
     } else if (eap->getline == NULL)
@@ -17597,7 +17597,7 @@ void ex_function(exarg_T *eap)
               )) {
         /* ":python <<" continues until a dot, like ":append" */
         p = skipwhite(arg + 2);
-        if (*p == NUL)
+        if (*p == '\0')
           skip_until = vim_strsave((char_u *)".");
         else
           skip_until = vim_strsave(p);
@@ -17625,7 +17625,7 @@ void ex_function(exarg_T *eap)
       ((char_u **)(newlines.ga_data))[newlines.ga_len++] = NULL;
 
     /* Check for end of eap->arg. */
-    if (line_arg != NULL && *line_arg == NUL)
+    if (line_arg != NULL && *line_arg == '\0')
       line_arg = NULL;
   }
 
@@ -17955,7 +17955,7 @@ trans_function_name (
         STRCPY(name + 3, sid_buf);
     }
     memmove(name + lead, lv.ll_name, (size_t)len);
-    name[lead + len] = NUL;
+    name[lead + len] = '\0';
   }
   *pp = end;
 
@@ -18081,7 +18081,7 @@ static int function_exists(char_u *name)
 
   /* Only accept "funcname", "funcname ", "funcname (..." and
    * "funcname(...", not "funcname!...". */
-  if (p != NULL && (*nm == NUL || *nm == '('))
+  if (p != NULL && (*nm == '\0' || *nm == '('))
     n = translated_function_exists(p);
   vim_free(p);
   return n;
@@ -18094,7 +18094,7 @@ char_u *get_expanded_name(char_u *name, int check)
 
   p = trans_function_name(&nm, FALSE, TFN_INT|TFN_QUIET, NULL);
 
-  if (p != NULL && *nm == NUL)
+  if (p != NULL && *nm == '\0')
     if (!check || translated_function_exists(p))
       return p;
 
@@ -18349,7 +18349,7 @@ static char_u *autoload_name(char_u *name)
     return FALSE;
   STRCPY(scriptname, "autoload/");
   STRCAT(scriptname, name);
-  *vim_strrchr(scriptname, AUTOLOAD_CHAR) = NUL;
+  *vim_strrchr(scriptname, AUTOLOAD_CHAR) = '\0';
   STRCAT(scriptname, ".vim");
   while ((p = vim_strchr(scriptname, AUTOLOAD_CHAR)) != NULL)
     *p = '/';
@@ -18435,7 +18435,7 @@ void ex_delfunction(exarg_T *eap)
   }
   eap->nextcmd = check_nextcmd(p);
   if (eap->nextcmd != NULL)
-    *p = NUL;
+    *p = '\0';
 
   if (!eap->skip)
     fp = find_func(name);
@@ -18918,7 +18918,7 @@ void ex_return(exarg_T *eap)
     ++emsg_skip;
 
   eap->nextcmd = NULL;
-  if ((*arg != NUL && *arg != '|' && *arg != '\n')
+  if ((*arg != '\0' && *arg != '|' && *arg != '\n')
       && eval0(arg, &rettv, &eap->nextcmd, !eap->skip) != FAIL) {
     if (!eap->skip)
       returning = do_return(eap, FALSE, TRUE, &rettv);
@@ -19321,7 +19321,7 @@ int store_session_globals(FILE *fd)
             (char_u *)"\\\"\n\r");
         if (p == NULL)              /* out of memory */
           break;
-        for (t = p; *t != NUL; ++t)
+        for (t = p; *t != '\0'; ++t)
           if (*t == '\n')
             *t = 'n';
           else if (*t == '\r')
@@ -19447,7 +19447,7 @@ repeat:
 # ifdef BACKSLASH_IN_FILENAME
             || (*fnamep)[1] == '\\'
 # endif
-            || (*fnamep)[1] == NUL)
+            || (*fnamep)[1] == '\0')
 
 #endif
         ) {
@@ -19459,19 +19459,19 @@ repeat:
     }
 
     /* When "/." or "/.." is used: force expansion to get rid of it. */
-    for (p = *fnamep; *p != NUL; mb_ptr_adv(p)) {
+    for (p = *fnamep; *p != '\0'; mb_ptr_adv(p)) {
       if (vim_ispathsep(*p)
           && p[1] == '.'
-          && (p[2] == NUL
+          && (p[2] == '\0'
               || vim_ispathsep(p[2])
               || (p[2] == '.'
-                  && (p[3] == NUL || vim_ispathsep(p[3])))))
+                  && (p[3] == '\0' || vim_ispathsep(p[3])))))
         break;
     }
 
     /* FullName_save() is slow, don't use it when not needed. */
-    if (*p != NUL || !vim_isAbsName(*fnamep)) {
-      *fnamep = FullName_save(*fnamep, *p != NUL);
+    if (*p != '\0' || !vim_isAbsName(*fnamep)) {
+      *fnamep = FullName_save(*fnamep, *p != '\0');
       vim_free(*bufp);          /* free any allocated file name */
       *bufp = *fnamep;
       if (*fnamep == NULL)
@@ -19732,7 +19732,7 @@ char_u *do_string_sub(char_u *str, char_u *pat, char_u *sub, char_u *flags)
           + ga.ga_len + i, TRUE, TRUE, FALSE);
       ga.ga_len += i + sublen - 1;
       tail = regmatch.endp[0];
-      if (*tail == NUL)
+      if (*tail == '\0')
         break;
       if (!do_all)
         break;
@@ -19781,7 +19781,7 @@ static void on_job_data(RStream *rstream, void *data, bool eof, char *type)
   char *str = xmalloc(read_count + 1);
 
   rstream_read(rstream, str, read_count);
-  str[read_count] = NUL;
+  str[read_count] = '\0';
   apply_job_autocmds(job, job_data(job), type, str);
 }
 
diff --git a/src/ex_cmds.c b/src/ex_cmds.c
index 0cbd4faccb..2986ab6dc5 100644
--- a/src/ex_cmds.c
+++ b/src/ex_cmds.c
@@ -92,15 +92,15 @@ void do_ascii(exarg_T *eap)
     c = utfc_ptr2char(ml_get_cursor(), cc);
   else
     c = gchar_cursor();
-  if (c == NUL) {
-    MSG("NUL");
+  if (c == '\0') {
+    MSG("'\0'");
     return;
   }
 
-  IObuff[0] = NUL;
+  IObuff[0] = '\0';
   if (!has_mbyte || (enc_dbcs != 0 && c < 0x100) || c < 0x80) {
-    if (c == NL)            /* NUL is stored as NL */
-      c = NUL;
+    if (c == NL)            /* '\0' is stored as NL */
+      c = '\0';
     if (c == CAR && get_fileformat(curbuf) == EOL_MAC)
       cval = NL;            /* NL is stored as CR */
     else
@@ -111,12 +111,12 @@ void do_ascii(exarg_T *eap)
       transchar_nonprint(buf3, c);
       vim_snprintf(buf1, sizeof(buf1), "  <%s>", (char *)buf3);
     } else
-      buf1[0] = NUL;
+      buf1[0] = '\0';
     if (c >= 0x80)
       vim_snprintf(buf2, sizeof(buf2), "  <M-%s>",
           (char *)transchar(c & 0x7f));
     else
-      buf2[0] = NUL;
+      buf2[0] = '\0';
     vim_snprintf((char *)IObuff, IOSIZE,
         _("<%s>%s%s  %d,  Hex %02x,  Octal %03o"),
         transchar(c), buf1, buf2, cval, cval, cval);
@@ -264,7 +264,7 @@ static int linelen(int *has_tab)
        last > first && vim_iswhite(last[-1]); --last)
     ;
   save = *last;
-  *last = NUL;
+  *last = '\0';
   len = linetabsize(line);              /* get line length */
   if (has_tab != NULL)                  /* check for embedded TAB */
     *has_tab = (vim_strrchr(first, TAB) != NULL);
@@ -373,7 +373,7 @@ void ex_sort(exarg_T *eap)
 
   sort_abort = sort_ic = sort_rx = sort_nr = sort_oct = sort_hex = 0;
 
-  for (p = eap->arg; *p != NUL; ++p) {
+  for (p = eap->arg; *p != '\0'; ++p) {
     if (vim_iswhite(*p))
       ;
     else if (*p == 'i')
@@ -399,7 +399,7 @@ void ex_sort(exarg_T *eap)
         EMSG(_(e_invalpat));
         goto sortend;
       }
-      *s = NUL;
+      *s = '\0';
       /* Use last search pattern if sort pattern is empty. */
       if (s == p + 1) {
         if (last_search_pat() == NULL) {
@@ -458,7 +458,7 @@ void ex_sort(exarg_T *eap)
        * of the match, by temporarily terminating the string there */
       s2 = s + end_col;
       c = *s2;
-      *s2 = NUL;
+      *s2 = '\0';
       /* Sorting on number: Store the number itself. */
       p = s + start_col;
       if (sort_hex)
@@ -467,7 +467,7 @@ void ex_sort(exarg_T *eap)
         s = skiptodigit(p);
       if (s > p && s[-1] == '-')
         --s;          /* include preceding negative sign */
-      if (*s == NUL)
+      if (*s == '\0')
         /* empty line should sort before any number */
         nrs[lnum - eap->line1].start_col_nr = -MAXLNUM;
       else
@@ -647,7 +647,7 @@ void ex_retab(exarg_T *eap)
         got_tab = FALSE;
         num_spaces = 0;
       }
-      if (ptr[col] == NUL)
+      if (ptr[col] == '\0')
         break;
       vcol += chartabsize(ptr + col, (colnr_T)vcol);
       if (has_mbyte)
@@ -896,7 +896,7 @@ void do_bang(int addr_count, exarg_T *eap, int forceit, int do_in, int do_out)
       vim_free(newcmd);
       return;
     }
-    *t = NUL;
+    *t = '\0';
     if (newcmd != NULL)
       STRCAT(t, newcmd);
     if (ins_prevcmd)
@@ -917,7 +917,7 @@ void do_bang(int addr_count, exarg_T *eap, int forceit, int do_in, int do_out)
           STRMOVE(p - 1, p);
         else {
           trailarg = p;
-          *trailarg++ = NUL;
+          *trailarg++ = '\0';
           ins_prevcmd = TRUE;
           break;
         }
@@ -943,7 +943,7 @@ void do_bang(int addr_count, exarg_T *eap, int forceit, int do_in, int do_out)
   /*
    * Add quotes around the command, for shells that need them.
    */
-  if (*p_shq != NUL) {
+  if (*p_shq != '\0') {
     newcmd = alloc((unsigned)(STRLEN(prevcmd) + 2 * STRLEN(p_shq) + 1));
     if (newcmd == NULL)
       return;
@@ -1006,7 +1006,7 @@ do_filter (
   buf_T       *old_curbuf = curbuf;
   int shell_flags = 0;
 
-  if (*cmd == NUL)          /* no filter command */
+  if (*cmd == '\0')          /* no filter command */
     return;
 
 
@@ -1376,14 +1376,14 @@ make_filter_cmd (
      * Don't do this when 'shellquote' is not empty, otherwise the
      * redirection would be inside the quotes.
      */
-    if (*p_shq == NUL) {
+    if (*p_shq == '\0') {
       p = vim_strchr(buf, '|');
       if (p != NULL)
-        *p = NUL;
+        *p = '\0';
     }
     STRCAT(buf, " < ");
     STRCAT(buf, itmp);
-    if (*p_shq == NUL) {
+    if (*p_shq == '\0') {
       p = vim_strchr(cmd, '|');
       if (p != NULL) {
         STRCAT(buf, " ");           /* insert a space before the '|' for DOS */
@@ -1448,7 +1448,7 @@ int viminfo_error(char *errnum, char *message, char_u *line)
       errnum, message);
   STRNCAT(IObuff, line, IOSIZE - STRLEN(IObuff) - 1);
   if (IObuff[STRLEN(IObuff) - 1] == '\n')
-    IObuff[STRLEN(IObuff) - 1] = NUL;
+    IObuff[STRLEN(IObuff) - 1] = '\0';
   emsg(IObuff);
   if (++viminfo_errcnt >= 10) {
     EMSG(_("E136: viminfo: Too many errors, skipping rest of file"));
@@ -1737,10 +1737,10 @@ end:
  */
 static char_u *viminfo_filename(char_u *file)
 {
-  if (file == NULL || *file == NUL) {
+  if (file == NULL || *file == '\0') {
     if (use_viminfo != NULL)
       file = use_viminfo;
-    else if ((file = find_viminfo_parameter('n')) == NULL || *file == NUL) {
+    else if ((file = find_viminfo_parameter('n')) == NULL || *file == '\0') {
 #ifdef VIMINFO_FILE2
       /* don't use $HOME when not defined (turned into "c:/"!). */
       if (os_getenv((char_u *)"HOME") == NULL) {
@@ -1831,7 +1831,7 @@ static int read_viminfo_up_to_marks(vir_T *virp, int forceit, int writing)
     case '^':         /* to be defined */
     case '<':         /* long line - ignored */
     /* A comment or empty line. */
-    case NUL:
+    case '\0':
     case '\r':
     case '\n':
     case '#':
@@ -1905,7 +1905,7 @@ static int viminfo_encoding(vir_T *virp)
       ++p;
       for (i = 0; vim_isprintc(p[i]); ++i)
         ;
-      p[i] = NUL;
+      p[i] = '\0';
 
       convert_setup(&virp->vir_conv, p, p_enc);
     }
@@ -1962,8 +1962,8 @@ viminfo_readstring (
 
   /* Change CTRL-V CTRL-V to CTRL-V and CTRL-V n to \n in-place. */
   d = retval;
-  while (*s != NUL && *s != '\n') {
-    if (s[0] == Ctrl_V && s[1] != NUL) {
+  while (*s != '\0' && *s != '\n') {
+    if (s[0] == Ctrl_V && s[1] != '\0') {
       if (s[1] == 'n')
         *d++ = '\n';
       else
@@ -1972,9 +1972,9 @@ viminfo_readstring (
     } else
       *d++ = *s++;
   }
-  *d = NUL;
+  *d = '\0';
 
-  if (convert && virp->vir_conv.vc_type != CONV_NONE && *retval != NUL) {
+  if (convert && virp->vir_conv.vc_type != CONV_NONE && *retval != '\0') {
     d = string_convert(&virp->vir_conv, retval, NULL);
     if (d != NULL) {
       vim_free(retval);
@@ -2001,7 +2001,7 @@ void viminfo_writestring(FILE *fd, char_u *p)
   char_u      *s;
   int len = 0;
 
-  for (s = p; *s != NUL; ++s) {
+  for (s = p; *s != '\0'; ++s) {
     if (*s == Ctrl_V || *s == '\n')
       ++len;
     ++len;
@@ -2014,7 +2014,7 @@ void viminfo_writestring(FILE *fd, char_u *p)
   if (len > LSIZE / 2)
     fprintf(fd, IF_EB("\026%d\n<", CTRL_V_STR "%d\n<"), len + 3);
 
-  while ((c = *p++) != NUL) {
+  while ((c = *p++) != '\0') {
     if (c == Ctrl_V || c == '\n') {
       putc(Ctrl_V, fd);
       if (c == '\n')
@@ -2102,7 +2102,7 @@ int rename_buffer(char_u *new_fname)
     return FAIL;
   }
   curbuf->b_flags |= BF_NOTEDITED;
-  if (xfname != NULL && *xfname != NUL) {
+  if (xfname != NULL && *xfname != '\0') {
     buf = buflist_new(fname, xfname, curwin->w_cursor.lnum, 0);
     if (buf != NULL && !cmdmod.keepalt)
       curwin->w_alt_fnum = buf->b_fnum;
@@ -2123,14 +2123,14 @@ void ex_file(exarg_T *eap)
   /* ":0file" removes the file name.  Check for illegal uses ":3file",
    * "0file name", etc. */
   if (eap->addr_count > 0
-      && (*eap->arg != NUL
+      && (*eap->arg != '\0'
           || eap->line2 > 0
           || eap->addr_count > 1)) {
     EMSG(_(e_invarg));
     return;
   }
 
-  if (*eap->arg != NUL || eap->addr_count == 1) {
+  if (*eap->arg != '\0' || eap->addr_count == 1) {
     if (rename_buffer(eap->arg) == FAIL)
       return;
   }
@@ -2179,7 +2179,7 @@ int do_write(exarg_T *eap)
     return FAIL;
 
   ffname = eap->arg;
-  if (*ffname == NUL) {
+  if (*ffname == '\0') {
     if (eap->cmdidx == CMD_saveas) {
       EMSG(_(e_argreq));
       goto theend;
@@ -2286,7 +2286,7 @@ int do_write(exarg_T *eap)
       }
 
       /* If 'filetype' was empty try detecting it now. */
-      if (*curbuf->b_p_ft == NUL) {
+      if (*curbuf->b_p_ft == '\0') {
         if (au_has_group((char_u *)"filetypedetect"))
           (void)do_doautocmd((char_u *)"filetypedetect BufRead",
               TRUE);
@@ -2377,7 +2377,7 @@ check_overwrite (
        * will probably fail anyway.
        * Use 'shortname' of the current buffer, since there is no buffer
        * for the written file. */
-      if (*p_dir == NUL) {
+      if (*p_dir == '\0') {
         dir = alloc(5);
         if (dir == NULL)
           return FAIL;
@@ -2680,16 +2680,16 @@ do_ecmd (
       fname_case(sfname, 0);             /* set correct case for sfname */
 #endif
 
-    if ((flags & ECMD_ADDBUF) && (ffname == NULL || *ffname == NUL))
+    if ((flags & ECMD_ADDBUF) && (ffname == NULL || *ffname == '\0'))
       goto theend;
 
     if (ffname == NULL)
       other_file = TRUE;
     /* there is no file name */
-    else if (*ffname == NUL && curbuf->b_ffname == NULL)
+    else if (*ffname == '\0' && curbuf->b_ffname == NULL)
       other_file = FALSE;
     else {
-      if (*ffname == NUL) {                 /* re-edit with same file name */
+      if (*ffname == '\0') {                 /* re-edit with same file name */
         ffname = curbuf->b_ffname;
         sfname = curbuf->b_fname;
       }
@@ -2724,7 +2724,7 @@ do_ecmd (
   reset_VIsual();
 
   if ((command != NULL || newlnum > (linenr_T)0)
-      && *get_vim_var_str(VV_SWAPCOMMAND) == NUL) {
+      && *get_vim_var_str(VV_SWAPCOMMAND) == '\0') {
     int len;
     char_u  *p;
 
@@ -3121,7 +3121,7 @@ do_ecmd (
 
   /* If the window options were changed may need to set the spell language.
    * Can only do this after the buffer has been properly setup. */
-  if (did_get_winopts && curwin->w_p_spell && *curwin->w_s->b_p_spl != NUL)
+  if (did_get_winopts && curwin->w_p_spell && *curwin->w_s->b_p_spl != '\0')
     (void)did_set_spelllang(curwin);
 
   if (command == NULL) {
@@ -3261,19 +3261,19 @@ void ex_append(exarg_T *eap)
     if (eap->getline == NULL) {
       /* No getline() function, use the lines that follow. This ends
        * when there is no more. */
-      if (eap->nextcmd == NULL || *eap->nextcmd == NUL)
+      if (eap->nextcmd == NULL || *eap->nextcmd == '\0')
         break;
       p = vim_strchr(eap->nextcmd, NL);
       if (p == NULL)
         p = eap->nextcmd + STRLEN(eap->nextcmd);
       theline = vim_strnsave(eap->nextcmd, (int)(p - eap->nextcmd));
-      if (*p != NUL)
+      if (*p != '\0')
         ++p;
       eap->nextcmd = p;
     } else
       theline = eap->getline(
           eap->cstack->cs_looplevel > 0 ? -1 :
-          NUL, eap->cookie, indent);
+          '\0', eap->cookie, indent);
     lines_left = Rows - 1;
     if (theline == NULL)
       break;
@@ -3292,7 +3292,7 @@ void ex_append(exarg_T *eap)
       else
         break;
     }
-    if ((p[0] == '.' && p[1] == NUL)
+    if ((p[0] == '.' && p[1] == '\0')
         || (!did_undo && u_save(lnum, lnum + 1 + (empty ? 1 : 0))
             == FAIL)) {
       vim_free(theline);
@@ -3300,8 +3300,8 @@ void ex_append(exarg_T *eap)
     }
 
     /* don't use autoindent if nothing was typed. */
-    if (p[0] == NUL)
-      theline[0] = NUL;
+    if (p[0] == '\0')
+      theline[0] = '\0';
 
     did_undo = TRUE;
     ml_append(lnum, theline, (colnr_T)0, FALSE);
@@ -3581,7 +3581,7 @@ void do_sub(exarg_T *eap)
     which_pat = RE_SUBST;       /* use last substitute regexp */
 
   /* new pattern and substitution */
-  if (eap->cmd[0] == 's' && *cmd != NUL && !vim_iswhite(*cmd)
+  if (eap->cmd[0] == 's' && *cmd != '\0' && !vim_iswhite(*cmd)
       && vim_strchr((char_u *)"0123456789cegriIp|\"", *cmd) == NULL) {
     /* don't accept alphanumeric for separator */
     if (isalpha(*cmd)) {
@@ -3611,7 +3611,7 @@ void do_sub(exarg_T *eap)
       pat = cmd;                            /* remember start of search pat */
       cmd = skip_regexp(cmd, delimiter, p_magic, &eap->arg);
       if (cmd[0] == delimiter)              /* end delimiter found */
-        *cmd++ = NUL;                       /* replace it with a NUL */
+        *cmd++ = '\0';                       /* replace it with a NUL */
     }
 
     /*
@@ -3622,7 +3622,7 @@ void do_sub(exarg_T *eap)
 
     while (cmd[0]) {
       if (cmd[0] == delimiter) {                /* end delimiter found */
-        *cmd++ = NUL;                           /* replace it with a NUL */
+        *cmd++ = '\0';                           /* replace it with a NUL */
         break;
       }
       if (cmd[0] == '\\' && cmd[1] != 0)        /* skip escaped characters */
@@ -3663,8 +3663,8 @@ void do_sub(exarg_T *eap)
   // efficient, avoid allocating a string that grows in size.
   if (pat != NULL
       && strcmp((const char *)pat, "\\n") == 0
-      && *sub == NUL
-      && (*cmd == NUL || (cmd[1] == NUL
+      && *sub == '\0'
+      && (*cmd == '\0' || (cmd[1] == '\0'
                           && (*cmd == 'g'
                               || *cmd == 'l'
                               || *cmd == 'p'
@@ -3915,7 +3915,7 @@ void do_sub(exarg_T *eap)
         if (matchcol == prev_matchcol
             && regmatch.endpos[0].lnum == 0
             && matchcol == regmatch.endpos[0].col) {
-          if (sub_firstline[matchcol] == NUL)
+          if (sub_firstline[matchcol] == '\0')
             /* We already were at the end of the line.  Don't look
              * for a match in this line again. */
             skip_match = TRUE;
@@ -4182,7 +4182,7 @@ void do_sub(exarg_T *eap)
            */
           new_start_len = needed_len + 50;
           new_start = (char_u *)xmalloc((size_t)new_start_len);
-          *new_start = NUL;
+          *new_start = '\0';
           new_end = new_start;
         } else {
           /*
@@ -4251,11 +4251,11 @@ void do_sub(exarg_T *eap)
          * That is Vi compatible.
          */
         for (p1 = new_end; *p1; ++p1) {
-          if (p1[0] == '\\' && p1[1] != NUL)            /* remove backslash */
+          if (p1[0] == '\\' && p1[1] != '\0')            /* remove backslash */
             STRMOVE(p1, p1 + 1);
           else if (*p1 == CAR) {
             if (u_inssub(lnum) == OK) {             /* prepare for undo */
-              *p1 = NUL;                            /* truncate up to the CR */
+              *p1 = '\0';                            /* truncate up to the CR */
               ml_append(lnum - 1, new_start,
                   (colnr_T)(p1 - new_start + 1), FALSE);
               mark_adjust(lnum + 1, (linenr_T)MAXLNUM, 1L, 0L);
@@ -4297,7 +4297,7 @@ skip:
                    || got_quit
                    || lnum > line2
                    || !(do_all || do_again)
-                   || (sub_firstline[matchcol] == NUL && nmatch <= 1
+                   || (sub_firstline[matchcol] == '\0' && nmatch <= 1
                        && !re_multiline(regmatch.regprog)));
         nmatch = -1;
 
@@ -4474,7 +4474,7 @@ do_sub_msg (
     if (got_int)
       STRCPY(msg_buf, _("(Interrupted) "));
     else
-      *msg_buf = NUL;
+      *msg_buf = '\0';
     if (sub_nsubs == 1)
       vim_snprintf_add((char *)msg_buf, sizeof(msg_buf),
           "%s", count_only ? _("1 match") : _("1 substitution"));
@@ -4559,7 +4559,7 @@ void ex_global(exarg_T *eap)
       which_pat = RE_SEARCH;            /* use previous search pattern */
     ++cmd;
     pat = (char_u *)"";
-  } else if (*cmd == NUL) {
+  } else if (*cmd == '\0') {
     EMSG(_("E148: Regular expression missing from global"));
     return;
   } else {
@@ -4569,7 +4569,7 @@ void ex_global(exarg_T *eap)
     pat = cmd;                  /* remember start of pattern */
     cmd = skip_regexp(cmd, delim, p_magic, &eap->arg);
     if (cmd[0] == delim)                    /* end delimiter found */
-      *cmd++ = NUL;                         /* replace it with a NUL */
+      *cmd++ = '\0';                         /* replace it with a NUL */
   }
 
   if (p_altkeymap && curwin->w_p_rl)
@@ -4638,7 +4638,7 @@ void global_exe(char_u *cmd)
   while (!got_int && (lnum = ml_firstmarked()) != 0 && global_busy == 1) {
     curwin->w_cursor.lnum = lnum;
     curwin->w_cursor.col = 0;
-    if (*cmd == NUL || *cmd == '\n')
+    if (*cmd == '\0' || *cmd == '\n')
       do_cmdline((char_u *)"p", NULL, NULL, DOCMD_NOWAIT);
     else
       do_cmdline(cmd, NULL, NULL, DOCMD_NOWAIT);
@@ -4763,15 +4763,15 @@ void ex_help(exarg_T *eap)
      */
     for (arg = eap->arg; *arg; ++arg) {
       if (*arg == '\n' || *arg == '\r'
-          || (*arg == '|' && arg[1] != NUL && arg[1] != '|')) {
-        *arg++ = NUL;
+          || (*arg == '|' && arg[1] != '\0' && arg[1] != '|')) {
+        *arg++ = '\0';
         eap->nextcmd = arg;
         break;
       }
     }
     arg = eap->arg;
 
-    if (eap->forceit && *arg == NUL && !curbuf->b_help) {
+    if (eap->forceit && *arg == '\0' && !curbuf->b_help) {
       EMSG(_("E478: Don't panic!"));
       return;
     }
@@ -4784,13 +4784,13 @@ void ex_help(exarg_T *eap)
   /* remove trailing blanks */
   p = arg + STRLEN(arg) - 1;
   while (p > arg && vim_iswhite(*p) && p[-1] != '\\')
-    *p-- = NUL;
+    *p-- = '\0';
 
   /* Check for a specified language */
   lang = check_help_lang(arg);
 
   /* When no argument given go to the index. */
-  if (*arg == NUL)
+  if (*arg == '\0')
     arg = (char_u *)"help.txt";
 
   /*
@@ -4917,7 +4917,7 @@ char_u *check_help_lang(char_u *arg)
 
   if (len >= 3 && arg[len - 3] == '@' && ASCII_ISALPHA(arg[len - 2])
       && ASCII_ISALPHA(arg[len - 1])) {
-    arg[len - 3] = NUL;                 /* remove the '@' */
+    arg[len - 3] = '\0';                 /* remove the '@' */
     return arg + len - 2;
   }
   return NULL;
@@ -4967,7 +4967,7 @@ help_heuristic (
     offset += 5000;
   /* Features are less interesting than the subjects themselves, but "+"
    * alone is not a feature. */
-  if (matched_string[0] == '+' && matched_string[1] != NUL)
+  if (matched_string[0] == '+' && matched_string[1] != '\0')
     offset += 100;
   return (int)(100 * num_letters + STRLEN(matched_string) + offset);
 }
@@ -5035,9 +5035,9 @@ int find_help_tags(char_u *arg, int *num_matches, char_u ***matches, int keep_la
      * Also "\@<", "\@=", "\@<=", etc.
      * And also "\_$" and "\_^". */
     if (arg[0] == '\\'
-        && ((arg[1] != NUL && arg[2] == NUL)
+        && ((arg[1] != '\0' && arg[2] == '\0')
             || (vim_strchr((char_u *)"%_z@", arg[1]) != NULL
-                && arg[2] != NUL))) {
+                && arg[2] != '\0'))) {
       STRCPY(d, "/\\\\");
       STRCPY(d + 3, arg + 1);
       /* Check for "/\\_$", should be "/\\_\$" */
@@ -5101,7 +5101,7 @@ int find_help_tags(char_u *arg, int *num_matches, char_u ***matches, int keep_la
               *d++ = '\\';              /* double a backslash */
           } else
             *d++ = *++s;
-          if (s[1] != NUL && s[1] != '_')
+          if (s[1] != '\0' && s[1] != '_')
             *d++ = '_';                 /* append a '_' */
           continue;
         } else if (*s == '^')           /* "^" or "CTRL-^" or "^_" */
@@ -5132,22 +5132,22 @@ int find_help_tags(char_u *arg, int *num_matches, char_u ***matches, int keep_la
         if (*s == '\'' && s > arg && *arg == '\'')
           break;
       }
-      *d = NUL;
+      *d = '\0';
 
       if (*IObuff == '`') {
         if (d > IObuff + 2 && d[-1] == '`') {
           /* remove the backticks from `command` */
           memmove(IObuff, IObuff + 1, STRLEN(IObuff));
-          d[-2] = NUL;
+          d[-2] = '\0';
         } else if (d > IObuff + 3 && d[-2] == '`' && d[-1] == ',') {
           /* remove the backticks and comma from `command`, */
           memmove(IObuff, IObuff + 1, STRLEN(IObuff));
-          d[-3] = NUL;
+          d[-3] = '\0';
         } else if (d > IObuff + 4 && d[-3] == '`'
                    && d[-2] == '\\' && d[-1] == '.') {
           /* remove the backticks and dot from `command`\. */
           memmove(IObuff, IObuff + 1, STRLEN(IObuff));
-          d[-4] = NUL;
+          d[-4] = '\0';
         }
       }
     }
@@ -5227,7 +5227,7 @@ void fix_help_buffer(void)
           && ASCII_ISALPHA(fname[5])
           && ASCII_ISALPHA(fname[6])
           && TOLOWER_ASC(fname[7]) == 'x'
-          && fname[8] == NUL)
+          && fname[8] == '\0')
       ) {
     for (lnum = 1; lnum < curbuf->b_ml.ml_line_count; ++lnum) {
       line = ml_get_buf(curbuf, lnum, FALSE);
@@ -5237,7 +5237,7 @@ void fix_help_buffer(void)
       /* Go through all directories in 'runtimepath', skipping
        * $VIMRUNTIME. */
       p = p_rtp;
-      while (*p != NUL) {
+      while (*p != '\0') {
         copy_option_part(&p, NameBuff, MAXPATHL, ",");
         mustfree = FALSE;
         rt = vim_getenv((char_u *)"VIMRUNTIME", &mustfree);
@@ -5279,7 +5279,7 @@ void fix_help_buffer(void)
                   continue;
                 e1 = vim_strrchr(t1, '.');
                 e2 = vim_strrchr(path_tail(f2), '.');
-                if (e1 == NUL || e2 == NUL)
+                if (e1 == '\0' || e2 == '\0')
                   continue;
                 if (fnamecmp(e1, ".txt") != 0
                     && fnamecmp(e1, fname + 4) != 0) {
@@ -5312,9 +5312,9 @@ void fix_help_buffer(void)
                    * reference and remove <CR>/<NL>. */
                   IObuff[0] = '|';
                   *s = '|';
-                  while (*s != NUL) {
+                  while (*s != '\0') {
                     if (*s == '\r' || *s == '\n')
-                      *s = NUL;
+                      *s = '\0';
                     /* The text is utf-8 when a byte
                      * above 127 is found and no
                      * illegal byte sequence is found.
@@ -5474,7 +5474,7 @@ void ex_helptags(exarg_T *eap)
     fname[6] = ((char_u *)ga.ga_data)[j + 1];
     if (fname[5] == 'e' && fname[6] == 'n') {
       /* English is an exception: use ".txt" and "tags". */
-      fname[4] = NUL;
+      fname[4] = '\0';
       STRCPY(ext, ".txt");
     } else {
       /* Language "ab" uses ".abx" and "tags-ab". */
@@ -5574,7 +5574,7 @@ helptags_one (
       if (firstline) {
         /* Detect utf-8 file by a non-ASCII char in the first line. */
         this_utf8 = MAYBE;
-        for (s = IObuff; *s != NUL; ++s)
+        for (s = IObuff; *s != '\0'; ++s)
           if (*s >= 0x80) {
             int l;
 
@@ -5660,7 +5660,7 @@ helptags_one (
       p2 = ((char_u **)ga.ga_data)[i];
       while (*p1 == *p2) {
         if (*p2 == '\t') {
-          *p2 = NUL;
+          *p2 = '\0';
           vim_snprintf((char *)NameBuff, MAXPATHL,
               _("E154: Duplicate tag \"%s\" in file %s/%s"),
               ((char_u **)ga.ga_data)[i], dir, p2 + 1);
@@ -5757,7 +5757,7 @@ static int sign_cmd_idx(
     int  idx;
     char save = *end_cmd;
 
-    *end_cmd = NUL;
+    *end_cmd = '\0';
     for (idx = 0; ; ++idx) {
         if (cmds[idx] == NULL || STRCMP(begin_cmd, cmds[idx]) == 0) {
             break;
@@ -5794,22 +5794,22 @@ void ex_sign(exarg_T *eap)
 	/*
 	 * Define, undefine or list signs.
 	 */
-	if (idx == SIGNCMD_LIST && *arg == NUL)
+	if (idx == SIGNCMD_LIST && *arg == '\0')
 	{
 	    /* ":sign list": list all defined signs */
 	    for (sp = first_sign; sp != NULL && !got_int; sp = sp->sn_next)
 		sign_list_defined(sp);
 	}
-	else if (*arg == NUL)
+	else if (*arg == '\0')
 	    EMSG(_("E156: Missing sign name"));
 	else
 	{
 	    /* Isolate the sign name.  If it's a number skip leading zeroes,
 	     * so that "099" and "99" are the same sign.  But keep "0". */
 	    p = skiptowhite(arg);
-	    if (*p != NUL)
-		*p++ = NUL;
-	    while (arg[0] == '0' && arg[1] != NUL)
+	    if (*p != '\0')
+		*p++ = '\0';
+	    while (arg[0] == '0' && arg[1] != '\0')
 		++arg;
 
 	    sp_prev = NULL;
@@ -5874,7 +5874,7 @@ void ex_sign(exarg_T *eap)
 		for (;;)
 		{
 		    arg = skipwhite(p);
-		    if (*arg == NUL)
+		    if (*arg == '\0')
 			break;
 		    p = skiptowhite_esc(arg);
 		    if (STRNCMP(arg, "icon=", 5) == 0)
@@ -5914,7 +5914,7 @@ void ex_sign(exarg_T *eap)
 			/* Currently must be one or two display cells */
 			if (s != p || cells < 1 || cells > 2)
 			{
-			    *p = NUL;
+			    *p = '\0';
 			    EMSG2(_("E239: Invalid sign text: %s"), arg);
 			    return;
 			}
@@ -5962,7 +5962,7 @@ void ex_sign(exarg_T *eap)
 	char_u		*sign_name = NULL;
 	char_u		*arg1;
 
-	if (*arg == NUL)
+	if (*arg == '\0')
 	{
 	    if (idx == SIGNCMD_PLACE)
 	    {
@@ -5986,7 +5986,7 @@ void ex_sign(exarg_T *eap)
 	    return;
 	}
 
-	if (idx == SIGNCMD_UNPLACE && arg[0] == '*' && arg[1] == NUL)
+	if (idx == SIGNCMD_UNPLACE && arg[0] == '*' && arg[1] == '\0')
 	{
 	    /* ":sign unplace *": remove all placed signs */
 	    buf_delete_all_signs();
@@ -5998,7 +5998,7 @@ void ex_sign(exarg_T *eap)
 	if (VIM_ISDIGIT(*arg))
 	{
 	    id = getdigits(&arg);
-	    if (!vim_iswhite(*arg) && *arg != NUL)
+	    if (!vim_iswhite(*arg) && *arg != '\0')
 	    {
 		id = -1;
 		arg = arg1;
@@ -6006,7 +6006,7 @@ void ex_sign(exarg_T *eap)
 	    else
 	    {
 		arg = skipwhite(arg);
-		if (idx == SIGNCMD_UNPLACE && *arg == NUL)
+		if (idx == SIGNCMD_UNPLACE && *arg == '\0')
 		{
 		    /* ":sign unplace {id}": remove placed sign by number */
 		    for (buf = firstbuf; buf != NULL; buf = buf->b_next)
@@ -6044,9 +6044,9 @@ void ex_sign(exarg_T *eap)
 		arg += 5;
 		sign_name = arg;
 		arg = skiptowhite(arg);
-		if (*arg != NUL)
-		    *arg++ = NUL;
-		while (sign_name[0] == '0' && sign_name[1] != NUL)
+		if (*arg != '\0')
+		    *arg++ = '\0';
+		while (sign_name[0] == '0' && sign_name[1] != '\0')
 		    ++sign_name;
 	    }
 	    else if (STRNCMP(arg, "file=", 5) == 0)
@@ -6059,7 +6059,7 @@ void ex_sign(exarg_T *eap)
 	    {
 		arg += 7;
 		buf = buflist_findnr((int)getdigits(&arg));
-		if (*skipwhite(arg) != NUL)
+		if (*skipwhite(arg) != '\0')
 		    EMSG(_(e_trailing));
 		break;
 	    }
@@ -6363,7 +6363,7 @@ set_context_in_sign_cmd(xp, arg)
     xp->xp_pattern = arg;
 
     end_subcmd = skiptowhite(arg);
-    if (*end_subcmd == NUL)
+    if (*end_subcmd == '\0')
 	/* expand subcmd name
 	 * :sign {subcmd}<CTRL-D>*/
 	return;
@@ -6375,7 +6375,7 @@ set_context_in_sign_cmd(xp, arg)
      *		      begin_subcmd_args */
     begin_subcmd_args = skipwhite(end_subcmd);
     p = skiptowhite(begin_subcmd_args);
-    if (*p == NUL)
+    if (*p == '\0')
     {
 	/*
 	 * Expand first argument of subcmd when possible.
@@ -6409,14 +6409,14 @@ set_context_in_sign_cmd(xp, arg)
 	p = skipwhite(p);
 	last = p;
 	p = skiptowhite(p);
-    } while (*p != NUL);
+    } while (*p != '\0');
 
     p = vim_strchr(last, '=');
 
     /* :sign define {name} {args}... {last}=
      *				     |	   |
      *				  last	   p */
-    if (p == NUL)
+    if (p == '\0')
     {
         /* Expand last argument name (before equal sign). */
         xp->xp_pattern = last;
diff --git a/src/ex_cmds2.c b/src/ex_cmds2.c
index 0148a7b6b2..c071b7b021 100644
--- a/src/ex_cmds2.c
+++ b/src/ex_cmds2.c
@@ -179,7 +179,7 @@ void do_debug(char_u *cmd)
        * If not, reset "last_cmd".
        * For a blank line use previous command. */
       p = skipwhite(cmdline);
-      if (*p != NUL) {
+      if (*p != '\0') {
         switch (*p) {
         case 'c': last_cmd = CMD_CONT;
           tail = "ont";
@@ -204,7 +204,7 @@ void do_debug(char_u *cmd)
         if (last_cmd != 0) {
           /* Check that the tail matches. */
           ++p;
-          while (*p != NUL && *p == *tail) {
+          while (*p != '\0' && *p == *tail) {
             ++p;
             ++tail;
           }
@@ -322,7 +322,7 @@ void dbg_check_breakpoint(exarg_T *eap)
         p = (char_u *)"";
       smsg((char_u *)_("Breakpoint in \"%s%s\" line %" PRId64),
           p,
-          debug_breakpoint_name + (*p == NUL ? 0 : 3),
+          debug_breakpoint_name + (*p == '\0' ? 0 : 3),
           (int64_t)debug_breakpoint_lnum);
       debug_breakpoint_name = NULL;
       do_debug(eap->cmd);
@@ -448,8 +448,8 @@ dbg_parsearg (
     bp->dbg_lnum = 0;
 
   /* Find the function or file name.  Don't accept a function name with (). */
-  if ((!here && *p == NUL)
-      || (here && *p != NUL)
+  if ((!here && *p == '\0')
+      || (here && *p != '\0')
       || (bp->dbg_type == DBG_FUNC && strstr((char *)p, "()") != NULL)) {
     EMSG2(_(e_invarg2), arg);
     return FAIL;
@@ -937,7 +937,7 @@ void ex_profile(exarg_T *eap)
   len = (int)(e - eap->arg);
   e = skipwhite(e);
 
-  if (len == 5 && STRNCMP(eap->arg, "start", 5) == 0 && *e != NUL) {
+  if (len == 5 && STRNCMP(eap->arg, "start", 5) == 0 && *e != '\0') {
     vim_free(profile_fname);
     profile_fname = vim_strsave(e);
     do_profiling = PROF_YES;
@@ -1010,7 +1010,7 @@ void set_context_in_profile_cmd(expand_T *xp, char_u *arg)
   xp->xp_pattern = arg;
 
   end_subcmd = skiptowhite(arg);
-  if (*end_subcmd == NUL)
+  if (*end_subcmd == '\0')
     return;
 
   if (end_subcmd - arg == 5 && STRNCMP(arg, "start", 5) == 0) {
@@ -1529,7 +1529,7 @@ static char_u *do_one_arg(char_u *str)
     }
   }
   str = skipwhite(str);
-  *p = NUL;
+  *p = '\0';
 
   return str;
 }
@@ -1541,7 +1541,7 @@ static char_u *do_one_arg(char_u *str)
 void get_arglist(garray_T *gap, char_u *str)
 {
   ga_init(gap, (int)sizeof(char_u *), 20);
-  while (*str != NUL) {
+  while (*str != '\0') {
     ga_grow(gap, 1);
     ((char_u **)gap->ga_data)[gap->ga_len++] = str;
 
@@ -1898,7 +1898,7 @@ void ex_next(exarg_T *eap)
              || !check_changed(curbuf, CCGD_AW
                  | (eap->forceit ? CCGD_FORCEIT : 0)
                  | CCGD_EXCMD)) {
-    if (*eap->arg != NUL) {                 /* redefine file list */
+    if (*eap->arg != '\0') {                 /* redefine file list */
       if (do_arglist(eap->arg, AL_SET, 0) == FAIL)
         return;
       i = 0;
@@ -1963,7 +1963,7 @@ void ex_argdelete(exarg_T *eap)
     if (eap->line2 > ARGCOUNT)
       eap->line2 = ARGCOUNT;
     n = eap->line2 - eap->line1 + 1;
-    if (*eap->arg != NUL || n <= 0)
+    if (*eap->arg != '\0' || n <= 0)
       EMSG(_(e_invarg));
     else {
       for (i = eap->line1; i <= eap->line2; ++i)
@@ -1976,7 +1976,7 @@ void ex_argdelete(exarg_T *eap)
       else if (curwin->w_arg_idx > eap->line1)
         curwin->w_arg_idx = eap->line1;
     }
-  } else if (*eap->arg == NUL)
+  } else if (*eap->arg == '\0')
     EMSG(_(e_argreq));
   else
     do_arglist(eap->arg, AL_DEL, 0);
@@ -2153,7 +2153,7 @@ void ex_compiler(exarg_T *eap)
   char_u      *old_cur_comp = NULL;
   char_u      *p;
 
-  if (*eap->arg == NUL) {
+  if (*eap->arg == '\0') {
     /* List all compiler scripts. */
     do_cmdline_cmd((char_u *)"echo globpath(&rtp, 'compiler/*.vim')");
     /* ) keep the indenter happy... */
@@ -2272,7 +2272,7 @@ void        *cookie;
 
     /* Loop over all entries in 'runtimepath'. */
     rtp = rtp_copy;
-    while (*rtp != NUL && (all || !did_one)) {
+    while (*rtp != '\0' && (all || !did_one)) {
       /* Copy the path from 'runtimepath' to buf[]. */
       copy_option_part(&rtp, buf, MAXPATHL, ",");
       if (name == NULL) {
@@ -2285,7 +2285,7 @@ void        *cookie;
 
         /* Loop over all patterns in "name" */
         np = name;
-        while (*np != NUL && (all || !did_one)) {
+        while (*np != '\0' && (all || !did_one)) {
           /* Append the pattern from "name" to buf[]. */
           copy_option_part(&np, tail, (int)(MAXPATHL - (tail - buf)),
               "\t ");
@@ -2341,7 +2341,7 @@ void ex_source(exarg_T *eap)
 
 static void cmd_source(char_u *fname, exarg_T *eap)
 {
-  if (*fname == NUL)
+  if (*fname == '\0')
     EMSG(_(e_argreq));
 
   else if (eap != NULL && eap->forceit)
@@ -2560,7 +2560,7 @@ do_source (
 
 #ifdef USE_CRNL
   /* If no automatic file format: Set default to CR-NL. */
-  if (*p_ffs == NUL)
+  if (*p_ffs == '\0')
     cookie.fileformat = EOL_DOS;
   else
     cookie.fileformat = EOL_UNKNOWN;
@@ -2569,7 +2569,7 @@ do_source (
 
 #ifdef USE_CR
   /* If no automatic file format: Set default to CR. */
-  if (*p_ffs == NUL)
+  if (*p_ffs == '\0')
     cookie.fileformat = EOL_MAC;
   else
     cookie.fileformat = EOL_UNKNOWN;
@@ -2914,7 +2914,7 @@ char_u *getsourceline(int c, void *cookie, int indent)
         }
         ga_concat(&ga, p + 1);
       }
-      ga_append(&ga, NUL);
+      ga_append(&ga, '\0');
       vim_free(line);
       line = ga.ga_data;
     }
@@ -2985,7 +2985,7 @@ static char_u *get_one_sourceline(struct source_cookie *sp)
     if (       (len == 1 || (len >= 2 && buf[len - 2] == '\n'))
                && sp->fileformat == EOL_DOS
                && buf[len - 1] == Ctrl_Z) {
-      buf[len - 1] = NUL;
+      buf[len - 1] = '\0';
       break;
     }
 #endif
@@ -3056,7 +3056,7 @@ static char_u *get_one_sourceline(struct source_cookie *sp)
         continue;
       }
 
-      buf[len - 1] = NUL;               /* remove the NL */
+      buf[len - 1] = '\0';               /* remove the NL */
     }
 
     /*
@@ -3162,7 +3162,7 @@ void ex_scriptencoding(exarg_T *eap)
     return;
   }
 
-  if (*eap->arg != NUL) {
+  if (*eap->arg != '\0') {
     name = enc_canonize(eap->arg);
     if (name == NULL)           /* out of memory */
       return;
@@ -3290,9 +3290,9 @@ char_u *get_mess_lang(void)
 #  endif
 # else
   p = os_getenv((char_u *)"LC_ALL");
-  if (p == NULL || *p == NUL) {
+  if (p == NULL || *p == '\0') {
     p = os_getenv((char_u *)"LC_MESSAGES");
-    if (p == NULL || *p == NUL)
+    if (p == NULL || *p == '\0')
       p = os_getenv((char_u *)"LANG");
   }
 # endif
@@ -3311,14 +3311,14 @@ static char_u *get_mess_env(void)
   char_u      *p;
 
   p = (char_u *)os_getenv("LC_ALL");
-  if (p == NULL || *p == NUL) {
+  if (p == NULL || *p == '\0') {
     p = (char_u *)os_getenv("LC_MESSAGES");
-    if (p == NULL || *p == NUL) {
+    if (p == NULL || *p == '\0') {
       p = (char_u *)os_getenv("LANG");
       if (p != NULL && VIM_ISDIGIT(*p))
         p = NULL;                       /* ignore something like "1043" */
 # ifdef HAVE_GET_LOCALE_VAL
-      if (p == NULL || *p == NUL)
+      if (p == NULL || *p == '\0')
         p = (char_u *)get_locale_val(LC_CTYPE);
 # endif
     }
@@ -3383,7 +3383,7 @@ void ex_language(exarg_T *eap)
    * Allow abbreviation, but require at least 3 characters to avoid
    * confusion with a two letter language name "me" or "ct". */
   p = skiptowhite(eap->arg);
-  if ((*p == NUL || vim_iswhite(*p)) && p - eap->arg >= 3) {
+  if ((*p == '\0' || vim_iswhite(*p)) && p - eap->arg >= 3) {
     if (STRNICMP(eap->arg, "messages", p - eap->arg) == 0) {
       what = VIM_LC_MESSAGES;
       name = skipwhite(p);
@@ -3399,14 +3399,14 @@ void ex_language(exarg_T *eap)
     }
   }
 
-  if (*name == NUL) {
+  if (*name == '\0') {
 #ifdef HAVE_WORKING_LIBINTL
     if (what == VIM_LC_MESSAGES)
       p = get_mess_env();
     else
 #endif
     p = (char_u *)setlocale(what, NULL);
-    if (p == NULL || *p == NUL)
+    if (p == NULL || *p == '\0')
       p = (char_u *)"Unknown";
     smsg((char_u *)_("Current %slanguage: \"%s\""), whatstr, p);
   } else {
diff --git a/src/ex_docmd.c b/src/ex_docmd.c
index ff1ebeb2f5..12efbe2582 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -1317,7 +1317,7 @@ void                *cookie;                    /*argument for fgetline() */
       ++ea.cmd;
 
     /* in ex mode, an empty line works like :+ */
-    if (*ea.cmd == NUL && exmode_active
+    if (*ea.cmd == '\0' && exmode_active
         && (getline_equal(fgetline, cookie, getexmodeline)
             || getline_equal(fgetline, cookie, getexline))
         && curwin->w_cursor.lnum < curbuf->b_ml.ml_line_count) {
@@ -1328,7 +1328,7 @@ void                *cookie;                    /*argument for fgetline() */
     /* ignore comment and empty lines */
     if (*ea.cmd == '"')
       goto doend;
-    if (*ea.cmd == NUL) {
+    if (*ea.cmd == '\0') {
       ex_pressedreturn = TRUE;
       goto doend;
     }
@@ -1382,7 +1382,7 @@ void                *cookie;                    /*argument for fgetline() */
 
     /* ":hide" and ":hide | cmd" are not modifiers */
     case 'h':   if (p != ea.cmd || !checkforcmd(&p, "hide", 3)
-                    || *p == NUL || ends_excmd(*p))
+                    || *p == '\0' || ends_excmd(*p))
         break;
       ea.cmd = p;
       cmdmod.hide = TRUE;
@@ -1587,7 +1587,7 @@ void                *cookie;                    /*argument for fgetline() */
    * If we got a line, but no command, then go to the line.
    * If we find a '|' or '\n' we set ea.nextcmd.
    */
-  if (*ea.cmd == NUL || *ea.cmd == '"' ||
+  if (*ea.cmd == '\0' || *ea.cmd == '"' ||
       (ea.nextcmd = check_nextcmd(ea.cmd)) != NULL) {
     /*
      * strange vi behaviour:
@@ -1855,7 +1855,7 @@ void                *cookie;                    /*argument for fgetline() */
         STRMOVE(p, p + 1);
       else if (*p == '\n') {
         ea.nextcmd = p + 1;
-        *p = NUL;
+        *p = '\0';
         break;
       }
     }
@@ -1868,7 +1868,7 @@ void                *cookie;                    /*argument for fgetline() */
 
   /* accept numbered register only when no count allowed (:put) */
   if (       (ea.argt & REGSTR)
-             && *ea.arg != NUL
+             && *ea.arg != '\0'
              /* Do not allow register = for user commands */
              && (!USER_CMDIDX(ea.cmdidx) || *ea.arg != '=')
              && !((ea.argt & COUNT) && VIM_ISDIGIT(*ea.arg))) {
@@ -1883,7 +1883,7 @@ void                *cookie;                    /*argument for fgetline() */
       ) {
       ea.regname = *ea.arg++;
       /* for '=' register: accept the rest of the line as an expression */
-      if (ea.arg[-1] == '=' && ea.arg[0] != NUL) {
+      if (ea.arg[-1] == '=' && ea.arg[0] != '\0') {
         set_expr_line(vim_strsave(ea.arg));
         ea.arg += STRLEN(ea.arg);
       }
@@ -1896,7 +1896,7 @@ void                *cookie;                    /*argument for fgetline() */
    * count, it's a buffer name.
    */
   if ((ea.argt & COUNT) && VIM_ISDIGIT(*ea.arg)
-      && (!(ea.argt & BUFNAME) || *(p = skipdigits(ea.arg)) == NUL
+      && (!(ea.argt & BUFNAME) || *(p = skipdigits(ea.arg)) == '\0'
           || vim_iswhite(*p))) {
     n = getdigits(&ea.arg);
     ea.arg = skipwhite(ea.arg);
@@ -1926,13 +1926,13 @@ void                *cookie;                    /*argument for fgetline() */
   if (ea.argt & EXFLAGS)
     get_flags(&ea);
   /* no arguments allowed */
-  if (!ni && !(ea.argt & EXTRA) && *ea.arg != NUL
+  if (!ni && !(ea.argt & EXTRA) && *ea.arg != '\0'
       && *ea.arg != '"' && (*ea.arg != '|' || (ea.argt & TRLBAR) == 0)) {
     errormsg = (char_u *)_(e_trailing);
     goto doend;
   }
 
-  if (!ni && (ea.argt & NEEDARG) && *ea.arg == NUL) {
+  if (!ni && (ea.argt & NEEDARG) && *ea.arg == '\0') {
     errormsg = (char_u *)_(e_argreq);
     goto doend;
   }
@@ -2028,7 +2028,7 @@ void                *cookie;                    /*argument for fgetline() */
    * Accept buffer name.  Cannot be used at the same time with a buffer
    * number.  Don't do this for a user command.
    */
-  if ((ea.argt & BUFNAME) && *ea.arg != NUL && ea.addr_count == 0
+  if ((ea.argt & BUFNAME) && *ea.arg != '\0' && ea.addr_count == 0
       && !USER_CMDIDX(ea.cmdidx)
       ) {
     /*
@@ -2099,7 +2099,7 @@ doend:
   if (curwin->w_cursor.lnum == 0)       /* can happen with zero line number */
     curwin->w_cursor.lnum = 1;
 
-  if (errormsg != NULL && *errormsg != NUL && !did_emsg) {
+  if (errormsg != NULL && *errormsg != '\0' && !did_emsg) {
     if (sourcing) {
       if (errormsg != IObuff) {
         STRCPY(IObuff, errormsg);
@@ -2148,7 +2148,7 @@ doend:
     --sandbox;
 #endif
 
-  if (ea.nextcmd && *ea.nextcmd == NUL)         /* not really a next command */
+  if (ea.nextcmd && *ea.nextcmd == '\0')         /* not really a next command */
     ea.nextcmd = NULL;
 
   --ex_nesting_level;
@@ -2169,7 +2169,7 @@ checkforcmd (
 {
   int i;
 
-  for (i = 0; cmd[i] != NUL; ++i)
+  for (i = 0; cmd[i] != '\0'; ++i)
     if (((char_u *)cmd)[i] != (*pp)[i])
       break;
   if (i >= len && !isalpha((*pp)[i])) {
@@ -2191,7 +2191,7 @@ static void append_command(char_u *cmd)
 
   STRCAT(IObuff, ": ");
   d = IObuff + STRLEN(IObuff);
-  while (*s != NUL && d - IObuff < IOSIZE - 7) {
+  while (*s != '\0' && d - IObuff < IOSIZE - 7) {
     if (
       enc_utf8 ? (s[0] == 0xc2 && s[1] == 0xa0) :
       *s == 0xa0) {
@@ -2203,7 +2203,7 @@ static void append_command(char_u *cmd)
     } else
       MB_COPY_CHAR(s, d);
   }
-  *d = NUL;
+  *d = '\0';
 }
 
 /*
@@ -2278,7 +2278,7 @@ static char_u *find_command(exarg_T *eap, int *full)
       if (STRNCMP(cmdnames[(int)eap->cmdidx].cmd_name, (char *)eap->cmd,
               (size_t)len) == 0) {
         if (full != NULL
-            && cmdnames[(int)eap->cmdidx].cmd_name[len] == NUL)
+            && cmdnames[(int)eap->cmdidx].cmd_name[len] == '\0')
           *full = TRUE;
         break;
       }
@@ -2334,19 +2334,19 @@ find_ucmd (
       cp = eap->cmd;
       np = uc->uc_name;
       k = 0;
-      while (k < len && *np != NUL && *cp++ == *np++)
+      while (k < len && *np != '\0' && *cp++ == *np++)
         k++;
-      if (k == len || (*np == NUL && vim_isdigit(eap->cmd[k]))) {
+      if (k == len || (*np == '\0' && vim_isdigit(eap->cmd[k]))) {
         /* If finding a second match, the command is ambiguous.  But
          * not if a buffer-local command wasn't a full match and a
          * global command is a full match. */
-        if (k == len && found && *np != NUL) {
+        if (k == len && found && *np != '\0') {
           if (gap == &ucmds)
             return NULL;
           amb_local = TRUE;
         }
 
-        if (!found || (k == len && *np == NUL)) {
+        if (!found || (k == len && *np == '\0')) {
           /* If we matched up to a digit, then there could
            * be another command including the digit that we
            * should use instead.
@@ -2372,7 +2372,7 @@ find_ucmd (
           /* Do not search for further abbreviations
            * if this is an exact match. */
           matchlen = k;
-          if (k == len && *np == NUL) {
+          if (k == len && *np == '\0') {
             if (full != NULL)
               *full = TRUE;
             amb_local = FALSE;
@@ -2443,7 +2443,7 @@ int modifier_len(char_u *cmd)
   if (VIM_ISDIGIT(*cmd))
     p = skipwhite(skipdigits(cmd));
   for (i = 0; i < (int)(sizeof(cmdmods) / sizeof(struct cmdmod)); ++i) {
-    for (j = 0; p[j] != NUL; ++j)
+    for (j = 0; p[j] != '\0'; ++j)
       if (p[j] != cmdmods[i].name[j])
         break;
     if (!ASCII_ISALPHA(p[j]) && j >= cmdmods[i].minlen
@@ -2468,11 +2468,11 @@ int cmd_exists(char_u *name)
 
   /* Check command modifiers. */
   for (i = 0; i < (int)(sizeof(cmdmods) / sizeof(struct cmdmod)); ++i) {
-    for (j = 0; name[j] != NUL; ++j)
+    for (j = 0; name[j] != '\0'; ++j)
       if (name[j] != cmdmods[i].name[j])
         break;
-    if (name[j] == NUL && j >= cmdmods[i].minlen)
-      return cmdmods[i].name[j] == NUL ? 2 : 1;
+    if (name[j] == '\0' && j >= cmdmods[i].minlen)
+      return cmdmods[i].name[j] == '\0' ? 2 : 1;
   }
 
   /* Check built-in commands and user defined commands.
@@ -2484,7 +2484,7 @@ int cmd_exists(char_u *name)
     return 3;
   if (vim_isdigit(*name) && ea.cmdidx != CMD_match)
     return 0;
-  if (*skipwhite(p) != NUL)
+  if (*skipwhite(p) != '\0')
     return 0;           /* trailing garbage */
   return ea.cmdidx == CMD_SIZE ? 0 : (full ? 2 : 1);
 }
@@ -2523,7 +2523,7 @@ set_one_cmd_context (
     ;
   xp->xp_pattern = cmd;
 
-  if (*cmd == NUL)
+  if (*cmd == '\0')
     return NULL;
   if (*cmd == '"') {        /* ignore comment lines */
     xp->xp_context = EXPAND_NOTHING;
@@ -2539,7 +2539,7 @@ set_one_cmd_context (
    * 4. parse command
    */
   xp->xp_pattern = cmd;
-  if (*cmd == NUL)
+  if (*cmd == '\0')
     return NULL;
   if (*cmd == '"') {
     xp->xp_context = EXPAND_NOTHING;
@@ -2593,7 +2593,7 @@ set_one_cmd_context (
    * If the cursor is touching the command, and it ends in an alpha-numeric
    * character, complete the command name.
    */
-  if (*p == NUL && ASCII_ISALNUM(p[-1]))
+  if (*p == '\0' && ASCII_ISALNUM(p[-1]))
     return NULL;
 
   if (ea.cmdidx == CMD_SIZE) {
@@ -2662,7 +2662,7 @@ set_one_cmd_context (
     arg = skip_cmd_arg(arg, FALSE);
 
     /* Still touching the command after '+'? */
-    if (*arg == NUL)
+    if (*arg == '\0')
       return p;
 
     /* Skip space(s) after +command to get to the real argument */
@@ -2680,7 +2680,7 @@ set_one_cmd_context (
       p += 2;
     while (*p) {
       if (*p == Ctrl_V) {
-        if (p[1] != NUL)
+        if (p[1] != '\0')
           ++p;
       } else if ( (*p == '"' && !(ea.argt & NOTRLCOM))
                   || *p == '|' || *p == '\n') {
@@ -2695,7 +2695,7 @@ set_one_cmd_context (
   }
 
   /* no arguments allowed */
-  if (!(ea.argt & EXTRA) && *arg != NUL &&
+  if (!(ea.argt & EXTRA) && *arg != '\0' &&
       vim_strchr((char_u *)"|\"", *arg) == NULL)
     return NULL;
 
@@ -2708,7 +2708,7 @@ set_one_cmd_context (
       /* argument starts after a space */
       xp->xp_pattern = ++p;
     } else {
-      if (*p == '\\' && *(p + 1) != NUL)
+      if (*p == '\\' && *(p + 1) != '\0')
         ++p;         /* skip over escaped character */
       mb_ptr_adv(p);
     }
@@ -2725,12 +2725,12 @@ set_one_cmd_context (
      */
     xp->xp_pattern = skipwhite(arg);
     p = xp->xp_pattern;
-    while (*p != NUL) {
+    while (*p != '\0') {
       if (has_mbyte)
         c = mb_ptr2char(p);
       else
         c = *p;
-      if (c == '\\' && p[1] != NUL)
+      if (c == '\\' && p[1] != '\0')
         ++p;
       else if (c == '`') {
         if (!in_quote) {
@@ -2748,7 +2748,7 @@ set_one_cmd_context (
 #endif
                                                      )) {
         len = 0;          /* avoid getting stuck when space is in 'isfname' */
-        while (*p != NUL) {
+        while (*p != '\0') {
           if (has_mbyte)
             c = mb_ptr2char(p);
           else
@@ -2791,10 +2791,10 @@ set_one_cmd_context (
     /* Check for environment variable */
     if (*xp->xp_pattern == '$'
         ) {
-      for (p = xp->xp_pattern + 1; *p != NUL; ++p)
+      for (p = xp->xp_pattern + 1; *p != '\0'; ++p)
         if (!vim_isIDc(*p))
           break;
-      if (*p == NUL) {
+      if (*p == '\0') {
         xp->xp_context = EXPAND_ENV_VARS;
         ++xp->xp_pattern;
         /* Avoid that the assignment uses EXPAND_FILES again. */
@@ -2804,12 +2804,12 @@ set_one_cmd_context (
     }
     /* Check for user names */
     if (*xp->xp_pattern == '~') {
-      for (p = xp->xp_pattern + 1; *p != NUL && *p != '/'; ++p)
+      for (p = xp->xp_pattern + 1; *p != '\0' && *p != '/'; ++p)
         ;
       /* Complete ~user only if it partially matches a user name.
        * A full match ~user<Tab> will be replaced by user's home
        * directory i.e. something like ~user<Tab> -> /home/user/ */
-      if (*p == NUL && p > xp->xp_pattern + 1
+      if (*p == '\0' && p > xp->xp_pattern + 1
           && match_user(xp->xp_pattern + 1) == 1) {
         xp->xp_context = EXPAND_USER;
         ++xp->xp_pattern;
@@ -2872,11 +2872,11 @@ set_one_cmd_context (
     return arg;
 
   case CMD_match:
-    if (*arg == NUL || !ends_excmd(*arg)) {
+    if (*arg == '\0' || !ends_excmd(*arg)) {
       /* also complete "None" */
       set_context_in_echohl_cmd(xp, arg);
       arg = skipwhite(skiptowhite(arg));
-      if (*arg != NUL) {
+      if (*arg != '\0') {
         xp->xp_context = EXPAND_NOTHING;
         arg = skip_regexp(arg + 1, *arg, p_magic, NULL);
       }
@@ -2892,7 +2892,7 @@ set_one_cmd_context (
     while (*arg == '-') {
       arg++;                /* Skip "-" */
       p = skiptowhite(arg);
-      if (*p == NUL) {
+      if (*p == '\0') {
         /* Cursor is still in the attribute */
         p = vim_strchr(arg, '=');
         if (p == NULL) {
@@ -2921,7 +2921,7 @@ set_one_cmd_context (
 
     /* After the attributes comes the new command name */
     p = skiptowhite(arg);
-    if (*p == NUL) {
+    if (*p == '\0') {
       xp->xp_context = EXPAND_USER_COMMANDS;
       xp->xp_pattern = arg;
       break;
@@ -2941,12 +2941,12 @@ set_one_cmd_context (
     if (delim)
       ++arg;                        /* skip delimiter if there is one */
 
-    while (arg[0] != NUL && arg[0] != delim) {
-      if (arg[0] == '\\' && arg[1] != NUL)
+    while (arg[0] != '\0' && arg[0] != delim) {
+      if (arg[0] == '\\' && arg[1] != '\0')
         ++arg;
       ++arg;
     }
-    if (arg[0] != NUL)
+    if (arg[0] != '\0')
       return arg + 1;
     break;
   case CMD_and:
@@ -2958,16 +2958,16 @@ set_one_cmd_context (
       arg = skip_regexp(arg, delim, p_magic, NULL);
     }
     /* skip "to" part */
-    while (arg[0] != NUL && arg[0] != delim) {
-      if (arg[0] == '\\' && arg[1] != NUL)
+    while (arg[0] != '\0' && arg[0] != delim) {
+      if (arg[0] == '\\' && arg[1] != '\0')
         ++arg;
       ++arg;
     }
-    if (arg[0] != NUL)          /* skip delimiter */
+    if (arg[0] != '\0')          /* skip delimiter */
       ++arg;
     while (arg[0] && vim_strchr((char_u *)"|\"#", arg[0]) == NULL)
       ++arg;
-    if (arg[0] != NUL)
+    if (arg[0] != '\0')
       return arg;
     break;
   case CMD_isearch:
@@ -2982,7 +2982,7 @@ set_one_cmd_context (
     arg = skipwhite(skipdigits(arg));               /* skip count */
     if (*arg == '/') {          /* Match regexp, not just whole words */
       for (++arg; *arg && *arg != '/'; arg++)
-        if (*arg == '\\' && arg[1] != NUL)
+        if (*arg == '\\' && arg[1] != '\0')
           arg++;
       if (*arg) {
         arg = skipwhite(arg + 1);
@@ -3020,7 +3020,7 @@ set_one_cmd_context (
   case CMD_tjump:
   case CMD_stjump:
   case CMD_ptjump:
-    if (*p_wop != NUL)
+    if (*p_wop != '\0')
       xp->xp_context = EXPAND_TAGS_LISTFILES;
     else
       xp->xp_context = EXPAND_TAGS;
@@ -3105,7 +3105,7 @@ set_one_cmd_context (
           if (*p == ' ')
             /* argument starts after a space */
             arg = p + 1;
-          else if (*p == '\\' && *(p + 1) != NUL)
+          else if (*p == '\\' && *(p + 1) != '\0')
             ++p;                 /* skip over escaped character */
           mb_ptr_adv(p);
         }
@@ -3180,7 +3180,7 @@ set_one_cmd_context (
 #ifdef HAVE_WORKING_LIBINTL
   case CMD_language:
     p = skiptowhite(arg);
-    if (*p == NUL) {
+    if (*p == '\0') {
       xp->xp_context = EXPAND_LANGUAGE;
       xp->xp_pattern = arg;
     } else {
@@ -3237,17 +3237,17 @@ skip_range (
 
   while (vim_strchr((char_u *)" \t0123456789.$%'/?-+,;", *cmd) != NULL) {
     if (*cmd == '\'') {
-      if (*++cmd == NUL && ctx != NULL)
+      if (*++cmd == '\0' && ctx != NULL)
         *ctx = EXPAND_NOTHING;
     } else if (*cmd == '/' || *cmd == '?') {
       delim = *cmd++;
-      while (*cmd != NUL && *cmd != delim)
-        if (*cmd++ == '\\' && *cmd != NUL)
+      while (*cmd != '\0' && *cmd != delim)
+        if (*cmd++ == '\\' && *cmd != '\0')
           ++cmd;
-      if (*cmd == NUL && ctx != NULL)
+      if (*cmd == '\0' && ctx != NULL)
         *ctx = EXPAND_NOTHING;
     }
-    if (*cmd != NUL)
+    if (*cmd != '\0')
       ++cmd;
   }
 
@@ -3296,7 +3296,7 @@ get_address (
       break;
 
     case '\'':                              /* ''' - mark */
-      if (*++cmd == NUL) {
+      if (*++cmd == '\0') {
         cmd = NULL;
         goto error;
       }
@@ -3305,7 +3305,7 @@ get_address (
       else {
         /* Only accept a mark in another file when it is
          * used by itself: ":'M". */
-        fp = getmark(*cmd, to_other_file && cmd[1] == NUL);
+        fp = getmark(*cmd, to_other_file && cmd[1] == '\0');
         ++cmd;
         if (fp == (pos_T *)-1)
           /* Jumped to another file. */
@@ -3516,7 +3516,7 @@ static char_u *skip_grep_pat(exarg_T *eap)
 {
   char_u      *p = eap->arg;
 
-  if (*p != NUL && (eap->cmdidx == CMD_vimgrep || eap->cmdidx == CMD_lvimgrep
+  if (*p != '\0' && (eap->cmdidx == CMD_vimgrep || eap->cmdidx == CMD_lvimgrep
                     || eap->cmdidx == CMD_vimgrepadd
                     || eap->cmdidx == CMD_lvimgrepadd
                     || grep_internal(eap->cmdidx))) {
@@ -3550,12 +3550,12 @@ static char_u *replace_makeprg(exarg_T *eap, char_u *p, char_u **cmdlinep)
       && !grep_internal(eap->cmdidx)) {
     if (eap->cmdidx == CMD_grep || eap->cmdidx == CMD_lgrep
         || eap->cmdidx == CMD_grepadd || eap->cmdidx == CMD_lgrepadd) {
-      if (*curbuf->b_p_gp == NUL)
+      if (*curbuf->b_p_gp == '\0')
         program = p_gp;
       else
         program = curbuf->b_p_gp;
     } else {
-      if (*curbuf->b_p_mp == NUL)
+      if (*curbuf->b_p_mp == '\0')
         program = p_mp;
       else
         program = curbuf->b_p_mp;
@@ -3621,7 +3621,7 @@ int expand_filename(exarg_T *eap, char_u **cmdlinep, char_u **errormsgp)
    * (it will be expanded anyway if there is a wildcard before replacing).
    */
   has_wildcards = mch_has_wildcard(p);
-  while (*p != NUL) {
+  while (*p != '\0') {
     /* Skip over `=expr`, wildcards in it are not expanded. */
     if (p[0] == '`' && p[1] == '=') {
       p += 2;
@@ -3876,7 +3876,7 @@ void separate_nextcmd(exarg_T *eap)
       else
         /* remove CTRL-V and skip next char */
         STRMOVE(p, p + 1);
-      if (*p == NUL)                    /* stop at NUL after CTRL-V */
+      if (*p == '\0')                    /* stop at NUL after CTRL-V */
         break;
     }
     /* Skip over `=expr` when wildcards are expanded. */
@@ -3903,7 +3903,7 @@ void separate_nextcmd(exarg_T *eap)
         --p;
       } else {
         eap->nextcmd = check_nextcmd(p);
-        *p = NUL;
+        *p = '\0';
         break;
       }
     }
@@ -3928,8 +3928,8 @@ static char_u *getargcmd(char_u **argp)
     else {
       command = arg;
       arg = skip_cmd_arg(command, TRUE);
-      if (*arg != NUL)
-        *arg++ = NUL;                   /* terminate command with NUL */
+      if (*arg != '\0')
+        *arg++ = '\0';                   /* terminate command with NUL */
     }
 
     arg = skipwhite(arg);       /* skip over spaces */
@@ -3948,7 +3948,7 @@ skip_cmd_arg (
 )
 {
   while (*p && !vim_isspace(*p)) {
-    if (*p == '\\' && p[1] != NUL) {
+    if (*p == '\\' && p[1] != '\0') {
       if (rembs)
         STRMOVE(p, p + 1);
       else
@@ -4014,14 +4014,14 @@ static int getargopt(exarg_T *eap)
   *pp = (int)(arg - eap->cmd);
   arg = skip_cmd_arg(arg, FALSE);
   eap->arg = skipwhite(arg);
-  *arg = NUL;
+  *arg = '\0';
 
   if (pp == &eap->force_ff) {
     if (check_ff_value(eap->cmd + eap->force_ff) == FAIL)
       return FAIL;
   } else if (pp == &eap->force_enc) {
     /* Make 'fileencoding' lower case. */
-    for (p = eap->cmd + eap->force_enc; *p != NUL; ++p)
+    for (p = eap->cmd + eap->force_enc; *p != '\0'; ++p)
       *p = TOLOWER_ASC(*p);
   } else {
     /* Check ++bad= argument.  Must be a single-byte character, "keep" or
@@ -4031,7 +4031,7 @@ static int getargopt(exarg_T *eap)
       eap->bad_char = BAD_KEEP;
     else if (STRICMP(p, "drop") == 0)
       eap->bad_char = BAD_DROP;
-    else if (MB_BYTE2LEN(*p) == 1 && p[1] == NUL)
+    else if (MB_BYTE2LEN(*p) == 1 && p[1] == '\0')
       eap->bad_char = *p;
     else
       return FAIL;
@@ -4198,7 +4198,7 @@ static void ex_blast(exarg_T *eap)
 
 int ends_excmd(int c)
 {
-  return c == NUL || c == '|' || c == '"' || c == '\n';
+  return c == '\0' || c == '|' || c == '"' || c == '\n';
 }
 
 #if defined(FEAT_SYN_HL) || defined(FEAT_SEARCH_EXTRA) || defined(FEAT_EVAL) \
@@ -4210,7 +4210,7 @@ int ends_excmd(int c)
 char_u *find_nextcmd(char_u *p)
 {
   while (*p != '|' && *p != '\n') {
-    if (*p == NUL)
+    if (*p == '\0')
       return NULL;
     ++p;
   }
@@ -4813,7 +4813,7 @@ static char_u *uc_split_args(char_u *arg, size_t *lenp)
       p += 1;
     } else if (vim_iswhite(*p)) {
       p = skipwhite(p);
-      if (*p == NUL)
+      if (*p == '\0')
         break;
       len += 3;       /* "," */
     } else {
@@ -4845,7 +4845,7 @@ static char_u *uc_split_args(char_u *arg, size_t *lenp)
       *q++ = *p++;
     } else if (vim_iswhite(*p)) {
       p = skipwhite(p);
-      if (*p == NUL)
+      if (*p == '\0')
         break;
       *q++ = '"';
       *q++ = ',';
@@ -4915,7 +4915,7 @@ uc_check_code (
   switch (type) {
   case ct_ARGS:
     /* Simple case first */
-    if (*eap->arg == NUL) {
+    if (*eap->arg == '\0') {
       if (quote == 1) {
         result = 2;
         if (buf != NULL)
@@ -5086,7 +5086,7 @@ static void do_ucmd(exarg_T *eap)
       if (start != NULL)
         end = vim_strchr(start + 1, '>');
       if (buf != NULL) {
-        for (ksp = p; *ksp != NUL && *ksp != K_SPECIAL; ++ksp)
+        for (ksp = p; *ksp != '\0' && *ksp != K_SPECIAL; ++ksp)
           ;
         if (*ksp == K_SPECIAL
             && (start == NULL || ksp < start || end == NULL)
@@ -5272,7 +5272,7 @@ int parse_compl_arg(char_u *value, int vallen, int *complp, long *argt, char_u *
 
 static void ex_colorscheme(exarg_T *eap)
 {
-  if (*eap->arg == NUL) {
+  if (*eap->arg == '\0') {
     char_u *expr = vim_strsave((char_u *)"g:colors_name");
     char_u *p = NULL;
 
@@ -5293,7 +5293,7 @@ static void ex_colorscheme(exarg_T *eap)
 
 static void ex_highlight(exarg_T *eap)
 {
-  if (*eap->arg == NUL && eap->cmd[2] == '!')
+  if (*eap->arg == '\0' && eap->cmd[2] == '!')
     MSG(_("Greetings, Vim user!"));
   do_highlight(eap->arg, eap->forceit, FALSE);
 }
@@ -5576,7 +5576,7 @@ void ex_all(exarg_T *eap)
 
 static void ex_hide(exarg_T *eap)
 {
-  if (*eap->arg != NUL && check_nextcmd(eap->arg) == NULL)
+  if (*eap->arg != '\0' && check_nextcmd(eap->arg) == NULL)
     eap->errmsg = e_invarg;
   else {
     /* ":hide" or ":hide | cmd": hide current window */
@@ -5954,7 +5954,7 @@ static void ex_recover(exarg_T *eap)
           | (eap->forceit ? CCGD_FORCEIT : 0)
           | CCGD_EXCMD)
 
-      && (*eap->arg == NUL
+      && (*eap->arg == '\0'
           || setfname(curbuf, eap->arg, NULL, TRUE) == OK))
     ml_recover();
   recoverymode = FALSE;
@@ -6027,7 +6027,7 @@ void ex_splitview(exarg_T *eap)
                  *eap->cmd == 'v' ? WSP_VERT : 0) != FAIL) {
     /* Reset 'scrollbind' when editing another file, but keep it when
      * doing ":split" without arguments. */
-    if (*eap->arg != NUL
+    if (*eap->arg != '\0'
         ) {
       RESET_BINDING(curwin);
     } else
@@ -6084,7 +6084,7 @@ static void ex_tabmove(exarg_T *eap)
 {
   int tab_number = 9999;
 
-  if (eap->arg && *eap->arg != NUL) {
+  if (eap->arg && *eap->arg != '\0') {
     char_u *p = eap->arg;
     int relative = 0;        /* argument +N/-N means: move N places to the
                               * right/left relative to the current position. */
@@ -6160,7 +6160,7 @@ static void ex_tabs(exarg_T *eap)
  */
 static void ex_mode(exarg_T *eap)
 {
-  if (*eap->arg == NUL) {
+  if (*eap->arg == '\0') {
     shell_resized();
   } else {
     EMSG(_(e_screenmode));
@@ -6186,13 +6186,13 @@ static void ex_resize(exarg_T *eap)
   if (cmdmod.split & WSP_VERT) {
     if (*eap->arg == '-' || *eap->arg == '+')
       n += W_WIDTH(curwin);
-    else if (n == 0 && eap->arg[0] == NUL)      /* default is very wide */
+    else if (n == 0 && eap->arg[0] == '\0')      /* default is very wide */
       n = 9999;
     win_setwidth_win((int)n, wp);
   } else {
     if (*eap->arg == '-' || *eap->arg == '+')
       n += curwin->w_height;
-    else if (n == 0 && eap->arg[0] == NUL)      /* default is very wide */
+    else if (n == 0 && eap->arg[0] == '\0')      /* default is very wide */
       n = 9999;
     win_setheight_win((int)n, wp);
   }
@@ -6240,7 +6240,7 @@ static void ex_open(exarg_T *eap)
     /* ":open /pattern/": put cursor in column found with pattern */
     ++eap->arg;
     p = skip_regexp(eap->arg, '/', p_magic, NULL);
-    *p = NUL;
+    *p = '\0';
     regmatch.regprog = vim_regcomp(eap->arg, p_magic ? RE_MAGIC : 0);
     if (regmatch.regprog != NULL) {
       regmatch.rm_ic = p_ic;
@@ -6287,7 +6287,7 @@ do_exedit (
   if (exmode_active && (eap->cmdidx == CMD_visual
                         || eap->cmdidx == CMD_view)) {
     exmode_active = FALSE;
-    if (*eap->arg == NUL) {
+    if (*eap->arg == '\0') {
       /* Special case:  ":global/pat/visual\NLvi-commands" */
       if (global_busy) {
         int rd = RedrawingDisabled;
@@ -6321,7 +6321,7 @@ do_exedit (
        || eap->cmdidx == CMD_tabnew
        || eap->cmdidx == CMD_tabedit
        || eap->cmdidx == CMD_vnew
-       ) && *eap->arg == NUL) {
+       ) && *eap->arg == '\0') {
     /* ":new" or ":tabnew" without argument: edit an new empty buffer */
     setpcmark();
     (void)do_ecmd(0, NULL, NULL, eap, ECMD_ONE,
@@ -6330,11 +6330,11 @@ do_exedit (
   } else if ((eap->cmdidx != CMD_split
               && eap->cmdidx != CMD_vsplit
               )
-             || *eap->arg != NUL
+             || *eap->arg != '\0'
              ) {
     /* Can't edit another file when "curbuf_lock" is set.  Only ":edit"
      * can bring us here, others are stopped earlier. */
-    if (*eap->arg != NUL && curbuf_locked())
+    if (*eap->arg != '\0' && curbuf_locked())
       return;
     n = readonlymode;
     if (eap->cmdidx == CMD_view || eap->cmdidx == CMD_sview)
@@ -6346,7 +6346,7 @@ do_exedit (
     if (do_ecmd(0, (eap->cmdidx == CMD_enew ? NULL : eap->arg),
             NULL, eap,
             /* ":edit" goes to first line if Vi compatible */
-            (*eap->arg == NUL && eap->do_ecmd_lnum == 0
+            (*eap->arg == '\0' && eap->do_ecmd_lnum == 0
              && vim_strchr(p_cpo, CPO_GOTO1) != NULL)
             ? ECMD_ONE : eap->do_ecmd_lnum,
             (P_HID(curbuf) ? ECMD_HIDE : 0)
@@ -6394,7 +6394,7 @@ do_exedit (
    * file
    */
   if (old_curwin != NULL
-      && *eap->arg != NUL
+      && *eap->arg != '\0'
       && curwin != old_curwin
       && win_valid(old_curwin)
       && old_curwin->w_buffer != curbuf
@@ -6502,7 +6502,7 @@ static void ex_read(exarg_T *eap)
     if (u_save(eap->line2, (linenr_T)(eap->line2 + 1)) == FAIL)
       return;
 
-    if (*eap->arg == NUL) {
+    if (*eap->arg == '\0') {
       if (check_fname() == FAIL)        /* check for no file name */
         return;
       i = readfile(curbuf->b_ffname, curbuf->b_fname,
@@ -6525,7 +6525,7 @@ static void ex_read(exarg_T *eap)
           lnum = curbuf->b_ml.ml_line_count;
         else
           lnum = 1;
-        if (*ml_get(lnum) == NUL && u_savedel(lnum, 1L) == OK) {
+        if (*ml_get(lnum) == '\0' && u_savedel(lnum, 1L) == OK) {
           ml_delete(lnum, FALSE);
           if (curwin->w_cursor.lnum > 1
               && curwin->w_cursor.lnum >= lnum)
@@ -6590,7 +6590,7 @@ void ex_cd(exarg_T *eap)
   new_dir = eap->arg;
 #if !defined(UNIX)
   /* for non-UNIX ":cd" means: print current directory */
-  if (*new_dir == NUL)
+  if (*new_dir == '\0')
     ex_pwd(NULL);
   else
 #endif
@@ -6622,7 +6622,7 @@ void ex_cd(exarg_T *eap)
 
 #if defined(UNIX)
     /* for UNIX ":cd" means: go to home directory */
-    if (*new_dir == NUL) {
+    if (*new_dir == '\0') {
       /* use NameBuff for home directory name */
       expand_env((char_u *)"$HOME", NameBuff, MAXPATHL);
       new_dir = NameBuff;
@@ -6678,7 +6678,7 @@ static void ex_sleep(exarg_T *eap)
   len = eap->line2;
   switch (*eap->arg) {
   case 'm': break;
-  case NUL: len *= 1000L; break;
+  case '\0': len *= 1000L; break;
   default: EMSG2(_(e_invarg2), eap->arg); return;
   }
   do_sleep(len);
@@ -6729,7 +6729,7 @@ static void ex_winsize(exarg_T *eap)
   arg = skipwhite(arg);
   p = arg;
   h = getdigits(&arg);
-  if (*p != NUL && *arg == NUL)
+  if (*p != '\0' && *arg == '\0')
     set_shellsize(w, h, TRUE);
   else
     EMSG(_("E465: :winsize requires two number arguments"));
@@ -6737,12 +6737,12 @@ static void ex_winsize(exarg_T *eap)
 
 static void ex_wincmd(exarg_T *eap)
 {
-  int xchar = NUL;
+  int xchar = '\0';
   char_u      *p;
 
   if (*eap->arg == 'g' || *eap->arg == Ctrl_G) {
     /* CTRL-W g and CTRL-W CTRL-G  have an extra command character */
-    if (eap->arg[1] == NUL) {
+    if (eap->arg[1] == '\0') {
       EMSG(_(e_invarg));
       return;
     }
@@ -6753,7 +6753,7 @@ static void ex_wincmd(exarg_T *eap)
 
   eap->nextcmd = check_nextcmd(p);
   p = skipwhite(p);
-  if (*p != NUL && *p != '"' && eap->nextcmd == NULL)
+  if (*p != '\0' && *p != '"' && eap->nextcmd == NULL)
     EMSG(_(e_invarg));
   else if (!eap->skip) {
     /* Pass flags on for ":vertical wincmd ]". */
@@ -6776,14 +6776,14 @@ exarg_T     *eap;
   char_u      *arg = eap->arg;
   char_u      *p;
 
-  if (*arg == NUL) {
+  if (*arg == '\0') {
     EMSG(_("E188: Obtaining window position not implemented for this platform"));
   } else {
     x = getdigits(&arg);
     arg = skipwhite(arg);
     p = arg;
     y = getdigits(&arg);
-    if (*p == NUL || *arg != NUL) {
+    if (*p == '\0' || *arg != '\0') {
       EMSG(_("E466: :winpos requires two number arguments"));
       return;
     }
@@ -6950,7 +6950,7 @@ static void ex_at(exarg_T *eap)
 
   /* get the register name.  No name means to use the previous one */
   c = *eap->arg;
-  if (c == NUL || (c == '*' && *eap->cmd == '*'))
+  if (c == '\0' || (c == '*' && *eap->cmd == '*'))
     c = '@';
   /* Put the register in the typeahead buffer with the "silent" flag. */
   if (do_execreg(c, TRUE, vim_strchr(p_cpo, CPO_EXECBUF) != NULL, TRUE)
@@ -7026,7 +7026,7 @@ static void ex_later(exarg_T *eap)
   int file = FALSE;
   char_u      *p = eap->arg;
 
-  if (*p == NUL)
+  if (*p == '\0')
     count = 1;
   else if (isdigit(*p)) {
     count = getdigits(&p);
@@ -7039,7 +7039,7 @@ static void ex_later(exarg_T *eap)
     }
   }
 
-  if (*p != NUL)
+  if (*p != '\0')
     EMSG2(_(e_invarg2), eap->arg);
   else
     undo_time(eap->cmdidx == CMD_earlier ? -count : count,
@@ -7091,11 +7091,11 @@ static void ex_redir(exarg_T *eap)
             arg++;
           /* Make register empty when not using @A-@Z and the
            * command is valid. */
-          if (*arg == NUL && !isupper(redir_reg))
+          if (*arg == '\0' && !isupper(redir_reg))
             write_reg_contents(redir_reg, (char_u *)"", -1, FALSE);
         }
       }
-      if (*arg != NUL) {
+      if (*arg != '\0') {
         redir_reg = 0;
         EMSG2(_(e_invarg2), eap->arg);
       }
@@ -7220,15 +7220,15 @@ static void ex_mkrc(exarg_T *eap)
 
   /* ":mkview" or ":mkview 9": generate file name with 'viewdir' */
   if (eap->cmdidx == CMD_mkview
-      && (*eap->arg == NUL
-          || (vim_isdigit(*eap->arg) && eap->arg[1] == NUL))) {
+      && (*eap->arg == '\0'
+          || (vim_isdigit(*eap->arg) && eap->arg[1] == '\0'))) {
     eap->forceit = TRUE;
     fname = get_view_file(*eap->arg);
     if (fname == NULL)
       return;
     viewFile = fname;
     using_vdir = TRUE;
-  } else if (*eap->arg != NUL)
+  } else if (*eap->arg != '\0')
     fname = eap->arg;
   else if (eap->cmdidx == CMD_mkvimrc)
     fname = (char_u *)VIMRC_FILE;
@@ -7296,11 +7296,11 @@ static void ex_mkrc(exarg_T *eap)
            */
           if (os_dirname(dirnow, MAXPATHL) == FAIL
               || os_chdir((char *)dirnow) != 0)
-            *dirnow = NUL;
-          if (*dirnow != NUL && (ssop_flags & SSOP_SESDIR)) {
+            *dirnow = '\0';
+          if (*dirnow != '\0' && (ssop_flags & SSOP_SESDIR)) {
             if (vim_chdirfile(fname) == OK)
               shorten_fnames(TRUE);
-          } else if (*dirnow != NUL
+          } else if (*dirnow != '\0'
                      && (ssop_flags & SSOP_CURDIR) && globaldir != NULL) {
             if (os_chdir((char *)globaldir) == 0)
               shorten_fnames(TRUE);
@@ -7309,7 +7309,7 @@ static void ex_mkrc(exarg_T *eap)
           failed |= (makeopens(fd, dirnow) == FAIL);
 
           /* restore original dir */
-          if (*dirnow != NUL && ((ssop_flags & SSOP_SESDIR)
+          if (*dirnow != '\0' && ((ssop_flags & SSOP_SESDIR)
                                  || ((ssop_flags & SSOP_CURDIR) && globaldir !=
                                      NULL))) {
             if (os_chdir((char *)dirnow) != 0)
@@ -7405,9 +7405,9 @@ static void ex_mark(exarg_T *eap)
 {
   pos_T pos;
 
-  if (*eap->arg == NUL)                 /* No argument? */
+  if (*eap->arg == '\0')                 /* No argument? */
     EMSG(_(e_argreq));
-  else if (eap->arg[1] != NUL)          /* more than one character? */
+  else if (eap->arg[1] != '\0')          /* more than one character? */
     EMSG(_(e_trailing));
   else {
     pos = curwin->w_cursor;             /* save curwin->w_cursor */
@@ -7471,7 +7471,7 @@ static void ex_normal(exarg_T *eap)
     int len = 0;
 
     /* Count the number of characters to be escaped. */
-    for (p = eap->arg; *p != NUL; ++p) {
+    for (p = eap->arg; *p != '\0'; ++p) {
       for (l = (*mb_ptr2len)(p) - 1; l > 0; --l)
         if (*++p == K_SPECIAL             /* trailbyte K_SPECIAL or CSI */
             )
@@ -7481,7 +7481,7 @@ static void ex_normal(exarg_T *eap)
       arg = alloc((unsigned)(STRLEN(eap->arg) + len + 1));
       if (arg != NULL) {
         len = 0;
-        for (p = eap->arg; *p != NUL; ++p) {
+        for (p = eap->arg; *p != '\0'; ++p) {
           arg[len++] = *p;
           for (l = (*mb_ptr2len)(p) - 1; l > 0; --l) {
             arg[len++] = *++p;
@@ -7490,7 +7490,7 @@ static void ex_normal(exarg_T *eap)
               arg[len++] = KE_FILLER;
             }
           }
-          arg[len] = NUL;
+          arg[len] = '\0';
         }
       }
     }
@@ -7653,7 +7653,7 @@ static void ex_findpat(exarg_T *eap)
     ++eap->arg;
     p = skip_regexp(eap->arg, '/', p_magic, NULL);
     if (*p) {
-      *p++ = NUL;
+      *p++ = '\0';
       p = skipwhite(p);
 
       /* Check for trailing illegal characters */
@@ -7745,7 +7745,7 @@ static void ex_tag_cmd(exarg_T *eap, char_u *name)
   case 'l': cmd = DT_LAST;              /* ":tlast" */
     break;
   default:                              /* ":tag" */
-    if (p_cst && *eap->arg != NUL) {
+    if (p_cst && *eap->arg != '\0') {
       do_cstag(eap);
       return;
     }
@@ -8063,7 +8063,7 @@ static char_u *arg_all(void)
             retval[len] = ' ';
           ++len;
         }
-        for (; *p != NUL; ++p) {
+        for (; *p != '\0'; ++p) {
           if (*p == ' ' || *p == '\\') {
             /* insert a backslash */
             if (retval != NULL)
@@ -8079,7 +8079,7 @@ static char_u *arg_all(void)
 
     /* second time: break here */
     if (retval != NULL) {
-      retval[len] = NUL;
+      retval[len] = '\0';
       break;
     }
 
@@ -8678,7 +8678,7 @@ put_view (
    */
   if ((*flagp & SSOP_FOLDS)
       && wp->w_buffer->b_ffname != NULL
-      && (*wp->w_buffer->b_p_bt == NUL || wp->w_buffer->b_help)
+      && (*wp->w_buffer->b_p_bt == '\0' || wp->w_buffer->b_help)
       ) {
     if (put_folds(fd, wp) == FAIL)
       return FAIL;
@@ -8839,7 +8839,7 @@ static int ses_put_fname(FILE *fd, char_u *name, unsigned *flagp)
 
   if (*flagp & SSOP_SLASH) {
     /* change all backslashes to forward slashes */
-    for (p = sname; *p != NUL; mb_ptr_adv(p))
+    for (p = sname; *p != '\0'; mb_ptr_adv(p))
       if (*p == '\\')
         *p = '/';
   }
@@ -8968,7 +8968,7 @@ static void ex_viminfo(exarg_T *eap)
   char_u      *save_viminfo;
 
   save_viminfo = p_viminfo;
-  if (*p_viminfo == NUL)
+  if (*p_viminfo == '\0')
     p_viminfo = (char_u *)"'100";
   if (eap->cmdidx == CMD_rviminfo) {
     if (read_viminfo(eap->arg, VIF_WANT_INFO | VIF_WANT_MARKS
@@ -9042,7 +9042,7 @@ static void ex_filetype(exarg_T *eap)
   int plugin = FALSE;
   int indent = FALSE;
 
-  if (*eap->arg == NUL) {
+  if (*eap->arg == '\0') {
     /* Print current status. */
     smsg((char_u *)"filetype detection:%s  plugin:%s  indent:%s",
         filetype_detect ? "ON" : "OFF",
@@ -9111,7 +9111,7 @@ static void ex_setfiletype(exarg_T *eap)
 
 static void ex_digraphs(exarg_T *eap)
 {
-  if (*eap->arg != NUL)
+  if (*eap->arg != '\0')
     putdigraph(eap->arg);
   else
     listdigraphs();
@@ -9171,14 +9171,14 @@ static void ex_match(exarg_T *eap)
     if (!eap->skip)
       g = vim_strnsave(eap->arg, (int)(p - eap->arg));
     p = skipwhite(p);
-    if (*p == NUL) {
+    if (*p == '\0') {
       /* There must be two arguments. */
       EMSG2(_(e_invarg2), eap->arg);
       return;
     }
     end = skip_regexp(p + 1, *p, TRUE, NULL);
     if (!eap->skip) {
-      if (*end != NUL && !ends_excmd(*skipwhite(end + 1))) {
+      if (*end != '\0' && !ends_excmd(*skipwhite(end + 1))) {
         eap->errmsg = e_trailing;
         return;
       }
@@ -9188,7 +9188,7 @@ static void ex_match(exarg_T *eap)
       }
 
       c = *end;
-      *end = NUL;
+      *end = '\0';
       match_add(curwin, g, p + 1, 10, id);
       vim_free(g);
       *end = c;
diff --git a/src/ex_eval.c b/src/ex_eval.c
index 14226438e6..0cecdc505c 100644
--- a/src/ex_eval.c
+++ b/src/ex_eval.c
@@ -397,7 +397,7 @@ char_u *get_exception_string(void *value, int type, char_u *cmdname, int *should
   if (type == ET_ERROR) {
     *should_free = FALSE;
     mesg = ((struct msglist *)value)->throw_msg;
-    if (cmdname != NULL && *cmdname != NUL) {
+    if (cmdname != NULL && *cmdname != '\0') {
       cmdlen = (int)STRLEN(cmdname);
       ret = vim_strnsave((char_u *)"Vim(",
           4 + cmdlen + 2 + (int)STRLEN(mesg));
@@ -417,7 +417,7 @@ char_u *get_exception_string(void *value, int type, char_u *cmdname, int *should
      * name in quotes.  In the exception value, put the file name in
      * parentheses and move it to the end. */
     for (p = mesg;; p++) {
-      if (*p == NUL
+      if (*p == '\0'
           || (*p == 'E'
               && VIM_ISDIGIT(p[1])
               && (p[2] == ':'
@@ -425,7 +425,7 @@ char_u *get_exception_string(void *value, int type, char_u *cmdname, int *should
                       && (p[3] == ':'
                           || (VIM_ISDIGIT(p[3])
                               && p[4] == ':')))))) {
-        if (*p == NUL || p == mesg)
+        if (*p == '\0' || p == mesg)
           STRCAT(val, mesg);            /* 'E123' missing or at beginning */
         else {
           /* '"filename" E123: message text' */
@@ -435,7 +435,7 @@ char_u *get_exception_string(void *value, int type, char_u *cmdname, int *should
             continue;
 
           STRCAT(val, p);
-          p[-2] = NUL;
+          p[-2] = '\0';
           sprintf((char *)(val + STRLEN(p)), " (%s)", &mesg[1]);
           p[-2] = '"';
         }
@@ -469,7 +469,7 @@ static int throw_exception(void *value, int type, char_u *cmdname)
    */
   if (type == ET_USER) {
     if (STRNCMP((char_u *)value, "Vim", 3) == 0
-        && (((char_u *)value)[3] == NUL || ((char_u *)value)[3] == ':'
+        && (((char_u *)value)[3] == '\0' || ((char_u *)value)[3] == ':'
             || ((char_u *)value)[3] == '(')) {
       EMSG(_("E608: Cannot :throw exceptions with 'Vim' prefix"));
       goto fail;
@@ -507,13 +507,13 @@ static int throw_exception(void *value, int type, char_u *cmdname)
     else
       verbose_enter();
     ++no_wait_return;
-    if (debug_break_level > 0 || *p_vfile == NUL)
+    if (debug_break_level > 0 || *p_vfile == '\0')
       msg_scroll = TRUE;            /* always scroll up, don't overwrite */
 
     smsg((char_u *)_("Exception thrown: %s"), excp->value);
     msg_puts((char_u *)"\n");       /* don't overwrite this either */
 
-    if (debug_break_level > 0 || *p_vfile == NUL)
+    if (debug_break_level > 0 || *p_vfile == '\0')
       cmdline_row = msg_row;
     --no_wait_return;
     if (debug_break_level > 0)
@@ -556,14 +556,14 @@ static void discard_exception(except_T *excp, int was_finished)
     else
       verbose_enter();
     ++no_wait_return;
-    if (debug_break_level > 0 || *p_vfile == NUL)
+    if (debug_break_level > 0 || *p_vfile == '\0')
       msg_scroll = TRUE;            /* always scroll up, don't overwrite */
     smsg(was_finished
         ? (char_u *)_("Exception finished: %s")
         : (char_u *)_("Exception discarded: %s"),
         excp->value);
     msg_puts((char_u *)"\n");       /* don't overwrite this either */
-    if (debug_break_level > 0 || *p_vfile == NUL)
+    if (debug_break_level > 0 || *p_vfile == '\0')
       cmdline_row = msg_row;
     --no_wait_return;
     if (debug_break_level > 0)
@@ -600,7 +600,7 @@ static void catch_exception(except_T *excp)
   excp->caught = caught_stack;
   caught_stack = excp;
   set_vim_var_string(VV_EXCEPTION, excp->value, -1);
-  if (*excp->throw_name != NUL) {
+  if (*excp->throw_name != '\0') {
     if (excp->throw_lnum != 0)
       vim_snprintf((char *)IObuff, IOSIZE, _("%s, line %" PRId64),
           excp->throw_name, (int64_t)excp->throw_lnum);
@@ -619,13 +619,13 @@ static void catch_exception(except_T *excp)
     else
       verbose_enter();
     ++no_wait_return;
-    if (debug_break_level > 0 || *p_vfile == NUL)
+    if (debug_break_level > 0 || *p_vfile == '\0')
       msg_scroll = TRUE;            /* always scroll up, don't overwrite */
 
     smsg((char_u *)_("Exception caught: %s"), excp->value);
     msg_puts((char_u *)"\n");       /* don't overwrite this either */
 
-    if (debug_break_level > 0 || *p_vfile == NUL)
+    if (debug_break_level > 0 || *p_vfile == '\0')
       cmdline_row = msg_row;
     --no_wait_return;
     if (debug_break_level > 0)
@@ -645,7 +645,7 @@ static void finish_exception(except_T *excp)
   caught_stack = caught_stack->caught;
   if (caught_stack != NULL) {
     set_vim_var_string(VV_EXCEPTION, caught_stack->value, -1);
-    if (*caught_stack->throw_name != NUL) {
+    if (*caught_stack->throw_name != '\0') {
       if (caught_stack->throw_lnum != 0)
         vim_snprintf((char *)IObuff, IOSIZE,
             _("%s, line %" PRId64), caught_stack->throw_name,
@@ -1173,7 +1173,7 @@ void ex_throw(exarg_T *eap)
   char_u      *arg = eap->arg;
   char_u      *value;
 
-  if (*arg != NUL && *arg != '|' && *arg != '\n')
+  if (*arg != '\0' && *arg != '|' && *arg != '\n')
     value = eval_to_string_skip(arg, &eap->nextcmd, eap->skip);
   else {
     EMSG(_(e_argreq));
@@ -1384,7 +1384,7 @@ void ex_catch(exarg_T *eap)
      */
     if (!skip && (cstack->cs_flags[idx] & CSF_THROWN)
         && !(cstack->cs_flags[idx] & CSF_CAUGHT)) {
-      if (end != NULL && *end != NUL && !ends_excmd(*skipwhite(end + 1))) {
+      if (end != NULL && *end != '\0' && !ends_excmd(*skipwhite(end + 1))) {
         EMSG(_(e_trailing));
         return;
       }
@@ -1401,7 +1401,7 @@ void ex_catch(exarg_T *eap)
          * while compiling it. */
         if (end != NULL) {
           save_char = *end;
-          *end = NUL;
+          *end = '\0';
         }
         save_cpo  = p_cpo;
         p_cpo = (char_u *)"";
diff --git a/src/ex_getln.c b/src/ex_getln.c
index f07b8c1ce0..d64aa18414 100644
--- a/src/ex_getln.c
+++ b/src/ex_getln.c
@@ -208,7 +208,7 @@ getcmdline (
   struct cmdline_info save_ccline;
 
   if (firstc == -1) {
-    firstc = NUL;
+    firstc = '\0';
     break_ctrl_c = TRUE;
   }
   /* start without Hebrew mapping for a command line */
@@ -234,12 +234,12 @@ getcmdline (
   if (ccline.cmdbuff == NULL)
     return NULL;                            /* out of memory */
   ccline.cmdlen = ccline.cmdpos = 0;
-  ccline.cmdbuff[0] = NUL;
+  ccline.cmdbuff[0] = '\0';
 
   /* autoindent for :insert and :append */
   if (firstc <= 0) {
     copy_spaces(ccline.cmdbuff, indent);
-    ccline.cmdbuff[indent] = NUL;
+    ccline.cmdbuff[indent] = '\0';
     ccline.cmdpos = indent;
     ccline.cmdspos = indent;
     ccline.cmdlen = indent;
@@ -492,7 +492,7 @@ getcmdline (
       upseg[1] = '.';
       upseg[2] = '.';
       upseg[3] = PATHSEP;
-      upseg[4] = NUL;
+      upseg[4] = '\0';
 
       if (c == K_DOWN
           && ccline.cmdpos > 0
@@ -779,7 +779,7 @@ getcmdline (
         goto cmdline_changed;
     }
 
-    if (c == NUL || c == K_ZERO)            /* NUL is stored as NL */
+    if (c == '\0' || c == K_ZERO)            /* NUL is stored as NL */
       c = NL;
 
     do_abbr = TRUE;             /* default: check for abbreviation */
@@ -844,7 +844,7 @@ getcmdline (
           ccline.cmdbuff[i++] = ccline.cmdbuff[j++];
 
         /* Truncate at the end, required for multi-byte chars. */
-        ccline.cmdbuff[ccline.cmdlen] = NUL;
+        ccline.cmdbuff[ccline.cmdlen] = '\0';
         redrawcmd();
       } else if (ccline.cmdlen == 0 && c != Ctrl_W
                  && ccline.cmdprompt == NULL && indent == 0) {
@@ -931,7 +931,7 @@ getcmdline (
       while (i < ccline.cmdlen)
         ccline.cmdbuff[i++] = ccline.cmdbuff[j++];
       /* Truncate at the end, required for multi-byte chars. */
-      ccline.cmdbuff[ccline.cmdlen] = NUL;
+      ccline.cmdbuff[ccline.cmdlen] = '\0';
       redrawcmd();
       goto cmdline_changed;
 
@@ -1152,7 +1152,7 @@ getcmdline (
            * the character to lowercase */
           if (p_ic && p_scs && !pat_has_uppercase(ccline.cmdbuff))
             c = vim_tolower(c);
-          if (c != NUL) {
+          if (c != '\0') {
             if (c == firstc || vim_strchr((char_u *)(
                       p_magic ? "\\^$.*[" : "\\^$"), c)
                 != NULL) {
@@ -1188,7 +1188,7 @@ getcmdline (
     case K_KPAGEUP:
     case K_PAGEDOWN:
     case K_KPAGEDOWN:
-      if (hislen == 0 || firstc == NUL)                 /* no history */
+      if (hislen == 0 || firstc == '\0')                 /* no history */
         goto cmdline_not_changed;
 
       i = hiscnt;
@@ -1197,7 +1197,7 @@ getcmdline (
       if (lookfor == NULL) {
         if ((lookfor = vim_strsave(ccline.cmdbuff)) == NULL)
           goto cmdline_not_changed;
-        lookfor[ccline.cmdpos] = NUL;
+        lookfor[ccline.cmdpos] = '\0';
       }
 
       j = (int)STRLEN(lookfor);
@@ -1262,7 +1262,7 @@ getcmdline (
            * Second loop: copy the characters. */
           for (i = 0; i <= 1; ++i) {
             len = 0;
-            for (j = 0; p[j] != NUL; ++j) {
+            for (j = 0; p[j] != '\0'; ++j) {
               /* Replace old sep with new sep, unless it is
                * escaped. */
               if (p[j] == old_firstc
@@ -1289,7 +1289,7 @@ getcmdline (
                 goto returncmd;
             }
           }
-          ccline.cmdbuff[len] = NUL;
+          ccline.cmdbuff[len] = '\0';
         } else {
           alloc_cmdbuff((int)STRLEN(p));
           if (ccline.cmdbuff == NULL)
@@ -1325,7 +1325,7 @@ getcmdline (
       dont_scroll = TRUE;                   /* disallow scrolling here */
 #endif
       c = get_digraph(TRUE);
-      if (c != NUL)
+      if (c != '\0')
         break;
 
       redrawcmd();
@@ -1383,7 +1383,7 @@ getcmdline (
     else {
       if (has_mbyte) {
         j = (*mb_char2bytes)(c, IObuff);
-        IObuff[j] = NUL;                /* exclude composing chars */
+        IObuff[j] = '\0';                /* exclude composing chars */
         put_on_cmdline(IObuff, j, TRUE);
       } else {
         IObuff[0] = c;
@@ -1501,7 +1501,7 @@ cmdline_changed:
        * right-left typing.  Not efficient, but it works.
        * Do it only when there are no characters left to read
        * to avoid useless intermediate redraws. */
-      if (vpeekc() == NUL)
+      if (vpeekc() == '\0')
         redrawcmd();
   }
 
@@ -1530,10 +1530,10 @@ returncmd:
     /*
      * Put line in history buffer (":" and "=" only when it was typed).
      */
-    if (ccline.cmdlen && firstc != NUL
+    if (ccline.cmdlen && firstc != '\0'
         && (some_key_typed || histype == HIST_SEARCH)) {
       add_to_history(histype, ccline.cmdbuff, TRUE,
-          histype == HIST_SEARCH ? firstc : NUL);
+          histype == HIST_SEARCH ? firstc : '\0');
       if (firstc == ':') {
         vim_free(new_last_cmdline);
         new_last_cmdline = vim_strsave(ccline.cmdbuff);
@@ -1679,7 +1679,7 @@ static int cmdline_charsize(int idx)
  */
 static void set_cmdspos(void)
 {
-  if (ccline.cmdfirstc != NUL)
+  if (ccline.cmdfirstc != '\0')
     ccline.cmdspos = 1 + ccline.cmdindent;
   else
     ccline.cmdspos = 0 + ccline.cmdindent;
@@ -1848,7 +1848,7 @@ getexmodeline (
         long sw = get_sw_value(curbuf);
 
         p = (char_u *)line_ga.ga_data;
-        p[line_ga.ga_len] = NUL;
+        p[line_ga.ga_len] = '\0';
         indent = get_indent_str(p, 8);
         indent += sw - indent % sw;
 add_indent:
@@ -1887,9 +1887,9 @@ redraw:
           if (prev_char == '^')
             ex_keep_indent = TRUE;
           indent = 0;
-          p[--line_ga.ga_len] = NUL;
+          p[--line_ga.ga_len] = '\0';
         } else {
-          p[line_ga.ga_len] = NUL;
+          p[line_ga.ga_len] = '\0';
           indent = get_indent_str(p, 8);
           --indent;
           indent -= indent % get_sw_value(curbuf);
@@ -1956,7 +1956,7 @@ redraw:
       if ((bcount & 1) == 0) {
         --line_ga.ga_len;
         --pend;
-        *pend = NUL;
+        *pend = '\0';
         break;
       }
     }
@@ -2033,7 +2033,7 @@ static int realloc_cmdbuff(int len)
   /* There isn't always a NUL after the command, but it may need to be
    * there, thus copy up to the NUL and add a NUL. */
   memmove(ccline.cmdbuff, p, (size_t)ccline.cmdlen);
-  ccline.cmdbuff[ccline.cmdlen] = NUL;
+  ccline.cmdbuff[ccline.cmdlen] = '\0';
   vim_free(p);
 
   if (ccline.xpc != NULL
@@ -2120,13 +2120,13 @@ static void draw_cmdline(int start, int len)
           pc1 = prev_c1;
           prev_c1 = u8cc[0];
           if (j + mb_l >= start + len)
-            nc = NUL;
+            nc = '\0';
           else
             nc = utf_ptr2char(p + mb_l);
         } else {
           /* displaying from left to right */
           if (j + mb_l >= start + len)
-            pc = NUL;
+            pc = '\0';
           else {
             int pcc[MAX_MCO];
 
@@ -2245,7 +2245,7 @@ int put_on_cmdline(char_u *str, int len, int redraw)
         ccline.cmdlen = ccline.cmdpos + len;
     }
     memmove(ccline.cmdbuff + ccline.cmdpos, str, (size_t)len);
-    ccline.cmdbuff[ccline.cmdlen] = NUL;
+    ccline.cmdbuff[ccline.cmdlen] = '\0';
 
     if (enc_utf8) {
       /* When the inserted text starts with a composing character,
@@ -2480,7 +2480,7 @@ void cmdline_paste_str(char_u *s, int literally)
   if (literally)
     put_on_cmdline(s, -1, TRUE);
   else
-    while (*s != NUL) {
+    while (*s != '\0') {
       cv = *s;
       if (cv == Ctrl_V && s[1])
         ++s;
@@ -2531,13 +2531,13 @@ static void redrawcmdprompt(void)
 
   if (cmd_silent)
     return;
-  if (ccline.cmdfirstc != NUL)
+  if (ccline.cmdfirstc != '\0')
     msg_putchar(ccline.cmdfirstc);
   if (ccline.cmdprompt != NULL) {
     msg_puts_attr(ccline.cmdprompt, ccline.cmdattr);
     ccline.cmdindent = msg_col + (msg_row - cmdline_row) * Columns;
     /* do the reverse of set_cmdspos() */
-    if (ccline.cmdfirstc != NUL)
+    if (ccline.cmdfirstc != '\0')
       --ccline.cmdindent;
   } else
     for (i = ccline.cmdindent; i > 0; --i)
@@ -2957,7 +2957,7 @@ ExpandOne (
       len += (long_u)STRLEN(xp->xp_files[i]) + 1;
     ss = lalloc(len, TRUE);
     if (ss != NULL) {
-      *ss = NUL;
+      *ss = '\0';
       for (i = 0; i < xp->xp_numfiles; ++i) {
         STRCAT(ss, xp->xp_files[i]);
         if (i != xp->xp_numfiles - 1)
@@ -3091,10 +3091,10 @@ char_u *vim_strsave_fnameescape(char_u *fname, int shell)
   int j = 0;
 
   /* Don't escape '[', '{' and '!' if they are in 'isfname'. */
-  for (p = PATH_ESC_CHARS; *p != NUL; ++p)
+  for (p = PATH_ESC_CHARS; *p != '\0'; ++p)
     if ((*p != '[' && *p != '{' && *p != '!') || !vim_isfilec(*p))
       buf[j++] = *p;
-  buf[j] = NUL;
+  buf[j] = '\0';
   p = vim_strsave_escaped(fname, buf);
 #else
   p = vim_strsave_escaped(fname, shell ? SHELL_ESC_CHARS : PATH_ESC_CHARS);
@@ -3111,7 +3111,7 @@ char_u *vim_strsave_fnameescape(char_u *fname, int shell)
 
   /* '>' and '+' are special at the start of some commands, e.g. ":edit" and
    * ":write".  "cd -" has a special meaning. */
-  if (p != NULL && (*p == '>' || *p == '+' || (*p == '-' && p[1] == NUL)))
+  if (p != NULL && (*p == '>' || *p == '+' || (*p == '-' && p[1] == '\0')))
     escape_fname(&p);
 
   return p;
@@ -3318,7 +3318,7 @@ char_u *sm_gettail(char_u *s)
   char_u      *t = s;
   int had_sep = FALSE;
 
-  for (p = s; *p != NUL; ) {
+  for (p = s; *p != '\0'; ) {
     if (vim_ispathsep(*p)
 #ifdef BACKSLASH_IN_FILENAME
         && !rem_backslash(p)
@@ -3451,7 +3451,7 @@ addstar (
           }
           retval[j] = fname[i];
         }
-        retval[j] = NUL;
+        retval[j] = '\0';
       }
     }
   } else {
@@ -3483,7 +3483,7 @@ addstar (
         retval[len++] = '*';
       else if (len > 0 && retval[len - 1] == '$')
         --len;
-      retval[len] = NUL;
+      retval[len] = '\0';
     }
   }
   return retval;
@@ -3553,7 +3553,7 @@ set_cmd_context (
     int col                    /* position of cursor */
 )
 {
-  int old_char = NUL;
+  int old_char = '\0';
   char_u      *nextcomm;
 
   /*
@@ -3562,7 +3562,7 @@ set_cmd_context (
    */
   if (col < len)
     old_char = str[col];
-  str[col] = NUL;
+  str[col] = '\0';
   nextcomm = str;
 
   if (ccline.cmdfirstc == '=') {
@@ -3656,7 +3656,7 @@ static void cleanup_help_tags(int num_file, char_u **file)
             && STRNCMP(file[i], file[j], len + 1) == 0)
           break;
       if (j == num_file)
-        file[i][len] = NUL;
+        file[i][len] = '\0';
     }
   }
 }
@@ -3735,7 +3735,7 @@ ExpandFromContext (
   if (xp->xp_context == EXPAND_HELP) {
     /* With an empty argument we would get all the help tags, which is
      * very slow.  Get matches for "help" instead. */
-    if (find_help_tags(*pat == NUL ? (char_u *)"help" : pat,
+    if (find_help_tags(*pat == '\0' ? (char_u *)"help" : pat,
             num_file, file, FALSE) == OK) {
       cleanup_help_tags(*num_file, *file);
       return OK;
@@ -3874,7 +3874,7 @@ int escaped;
       str = (*func)(xp, i);
       if (str == NULL)              /* end of list */
         break;
-      if (*str == NUL)              /* skip empty strings */
+      if (*str == '\0')              /* skip empty strings */
         continue;
 
       if (vim_regexec(regmatch, str, (colnr_T)0)) {
@@ -3978,7 +3978,7 @@ expand_shellcmd (
    * collect them in "ga".
    */
   ga_init(&ga, (int)sizeof(char *), 10);
-  for (s = path; *s != NUL; s = e) {
+  for (s = path; *s != '\0'; s = e) {
     if (*s == ' ')
       ++s;              /* Skip space used for absolute path name. */
 
@@ -4011,7 +4011,7 @@ expand_shellcmd (
         vim_free(*file);
       }
     }
-    if (*e != NUL)
+    if (*e != '\0')
       ++e;
   }
   *file = ga.ga_data;
@@ -4094,7 +4094,7 @@ static int ExpandUserDefined(expand_T *xp, regmatch_T *regmatch, int *num_file,
     return FAIL;
 
   ga_init(&ga, (int)sizeof(char *), 3);
-  for (s = retstr; *s != NUL; s = e) {
+  for (s = retstr; *s != '\0'; s = e) {
     e = vim_strchr(s, '\n');
     if (e == NULL)
       e = s + STRLEN(s);
@@ -4103,7 +4103,7 @@ static int ExpandUserDefined(expand_T *xp, regmatch_T *regmatch, int *num_file,
 
     if (xp->xp_pattern[0] && vim_regexec(regmatch, s, (colnr_T)0) == 0) {
       *e = keep;
-      if (*e != NUL)
+      if (*e != '\0')
         ++e;
       continue;
     }
@@ -4114,7 +4114,7 @@ static int ExpandUserDefined(expand_T *xp, regmatch_T *regmatch, int *num_file,
     ++ga.ga_len;
 
     *e = keep;
-    if (*e != NUL)
+    if (*e != '\0')
       ++e;
   }
   vim_free(retstr);
@@ -4186,7 +4186,7 @@ static int ExpandRTDir(char_u *pat, int *num_file, char_u ***file, char *dirname
     if (matches == NULL)
       continue;
 
-    for (s = matches; *s != NUL; s = e) {
+    for (s = matches; *s != '\0'; s = e) {
       e = vim_strchr(s, '\n');
       if (e == NULL)
         e = s + STRLEN(s);
@@ -4200,7 +4200,7 @@ static int ExpandRTDir(char_u *pat, int *num_file, char_u ***file, char *dirname
           vim_strnsave(s, (int)(e - s - 4));
         ++ga.ga_len;
       }
-      if (*e != NUL)
+      if (*e != '\0')
         ++e;
     }
     vim_free(matches);
@@ -4244,7 +4244,7 @@ char_u *globpath(char_u *path, char_u *file, int expand_options)
   ga_init(&ga, 1, 100);
 
   /* Loop over all entries in {path}. */
-  while (*path != NUL) {
+  while (*path != '\0') {
     /* Copy one item of the path to buf[] and concatenate the file name. */
     copy_option_part(&path, buf, MAXPATHL, ",");
     if (STRLEN(buf) + STRLEN(file) + 2 < MAXPATHL) {
@@ -4321,7 +4321,7 @@ static char *(history_names[]) =
  */
 static char_u *get_history_arg(expand_T *xp, int idx)
 {
-  static char_u compl[2] = { NUL, NUL };
+  static char_u compl[2] = { '\0', '\0' };
   char *short_names = ":=@>?/";
   int short_names_count = (int)STRLEN(short_names);
   int history_name_count = sizeof(history_names) / sizeof(char *) - 1;
@@ -4485,7 +4485,7 @@ int get_histtype(char_u *name)
     if (STRNICMP(name, history_names[i], len) == 0)
       return i;
 
-  if (vim_strchr((char_u *)":=@>?/", name[0]) != NULL && name[1] == NUL)
+  if (vim_strchr((char_u *)":=@>?/", name[0]) != NULL && name[1] == '\0')
     return hist_char2type(name[0]);
 
   return -1;
@@ -4643,8 +4643,8 @@ int get_cmdline_type(void)
   struct cmdline_info *p = get_ccline_ptr();
 
   if (p == NULL)
-    return NUL;
-  if (p->cmdfirstc == NUL)
+    return '\0';
+  if (p->cmdfirstc == '\0')
     return (p->input_fn) ? '@' : '-';
   return p->cmdfirstc;
 }
@@ -4739,7 +4739,7 @@ int del_history_entry(int histype, char_u *str)
   if (hislen != 0
       && histype >= 0
       && histype < HIST_COUNT
-      && *str != NUL
+      && *str != '\0'
       && (idx = hisidx[histype]) >= 0
       && (regmatch.regprog = vim_regcomp(str, RE_MAGIC + RE_STRING))
       != NULL) {
@@ -4889,7 +4889,7 @@ void ex_history(exarg_T *eap)
            || vim_strchr((char_u *)":=@>/?", *end) != NULL)
       end++;
     i = *end;
-    *end = NUL;
+    *end = '\0';
     histype1 = get_histtype(arg);
     if (histype1 == -1) {
       if (STRNICMP(arg, "all", STRLEN(arg)) == 0) {
@@ -4905,7 +4905,7 @@ void ex_history(exarg_T *eap)
     *end = i;
   } else
     end = arg;
-  if (!get_list_range(&end, &hisidx1, &hisidx2) || *end != NUL) {
+  if (!get_list_range(&end, &hisidx1, &hisidx2) || *end != '\0') {
     EMSG(_(e_trailing));
     return;
   }
@@ -5029,8 +5029,8 @@ int read_viminfo_history(vir_T *virp, int writing)
   type = hist_char2type(virp->vir_line[0]);
   if (viminfo_hisidx[type] < viminfo_hislen[type]) {
     val = viminfo_readstring(virp, 1, TRUE);
-    if (val != NULL && *val != NUL) {
-      int sep = (*val == ' ' ? NUL : *val);
+    if (val != NULL && *val != '\0') {
+      int sep = (*val == ' ' ? '\0' : *val);
 
       if (!in_history(type, val + (type == HIST_SEARCH),
               viminfo_add_at_front, sep, writing)) {
@@ -5047,7 +5047,7 @@ int read_viminfo_history(vir_T *virp, int writing)
             /* Not a search entry: No separator in the viminfo
              * file, add a NUL separator. */
             memmove(p, val, (size_t)len + 1);
-            p[len + 1] = NUL;
+            p[len + 1] = '\0';
           }
           viminfo_history[type][viminfo_hisidx[type]++] = p;
         }
@@ -5171,7 +5171,7 @@ void write_viminfo_history(FILE *fp, int merge)
             * second column; use a space if there isn't one. */
             if (type == HIST_SEARCH) {
               c = p[STRLEN(p) + 1];
-              putc(c == NUL ? ' ' : c, fp);
+              putc(c == '\0' ? ' ' : c, fp);
             }
             viminfo_writestring(fp, p);
           }
@@ -5208,14 +5208,14 @@ void cmd_pchar(int c, int offset)
     return;
   }
   ccline.cmdbuff[ccline.cmdpos + offset] = (char_u)c;
-  ccline.cmdbuff[ccline.cmdlen] = NUL;
+  ccline.cmdbuff[ccline.cmdlen] = '\0';
 }
 
 int cmd_gchar(int offset)
 {
   if (ccline.cmdpos + offset >= ccline.cmdlen || ccline.cmdpos + offset < 0) {
     /*  EMSG(_("cmd_gchar beyond the command length")); */
-    return NUL;
+    return '\0';
   }
   return (int)ccline.cmdbuff[ccline.cmdpos + offset];
 }
@@ -5337,7 +5337,7 @@ static int ex_window(void)
 
   /* Trigger CmdwinEnter autocommands. */
   typestr[0] = cmdwin_type;
-  typestr[1] = NUL;
+  typestr[1] = '\0';
   apply_autocmds(EVENT_CMDWINENTER, typestr, typestr, FALSE, curbuf);
   if (restart_edit != 0)        /* autocmd with ":startinsert" */
     stuffcharReadbuff(K_NOP);
@@ -5457,7 +5457,7 @@ char_u *script_get(exarg_T *eap, char_u *cmd)
 
   ga_init(&ga, 1, 0x400);
 
-  if (cmd[2] != NUL)
+  if (cmd[2] != '\0')
     end_pattern = (char *)skipwhite(cmd + 2);
   else
     end_pattern = dot;
@@ -5465,7 +5465,7 @@ char_u *script_get(exarg_T *eap, char_u *cmd)
   for (;; ) {
     theline = eap->getline(
         eap->cstack->cs_looplevel > 0 ? -1 :
-        NUL, eap->cookie, 0);
+        '\0', eap->cookie, 0);
 
     if (theline == NULL || STRCMP(end_pattern, theline) == 0) {
       vim_free(theline);
@@ -5476,7 +5476,7 @@ char_u *script_get(exarg_T *eap, char_u *cmd)
     ga_append(&ga, '\n');
     vim_free(theline);
   }
-  ga_append(&ga, NUL);
+  ga_append(&ga, '\0');
 
   return (char_u *)ga.ga_data;
 }
diff --git a/src/farsi.c b/src/farsi.c
index ea9432ac8b..a68de6ca25 100644
--- a/src/farsi.c
+++ b/src/farsi.c
@@ -952,7 +952,7 @@ int fkmap(int c)
                     || gchar_cursor() == F_DIVIDE
                     || gchar_cursor() == F_PERCENT
                     || gchar_cursor() == F_EQUALS))
-                && gchar_cursor() != NUL) {
+                && gchar_cursor() != '\0') {
           curwin->w_cursor.col++;
         }
       } else {
@@ -2413,7 +2413,7 @@ static void lrswapbuf(char_u *buf, int len)
 /// @return The buffer with the characters swapped.
 char_u* lrswap(char_u *ibuf)
 {
-  if ((ibuf != NULL) && (*ibuf != NUL)) {
+  if ((ibuf != NULL) && (*ibuf != '\0')) {
     lrswapbuf(ibuf, (int)STRLEN(ibuf));
   }
   return ibuf;
diff --git a/src/file_search.c b/src/file_search.c
index 1bcb9fa519..76e69269a2 100644
--- a/src/file_search.c
+++ b/src/file_search.c
@@ -312,7 +312,7 @@ vim_findfile_init (
   /* Store information on starting dir now if path is relative.
    * If path is absolute, we do that later.  */
   if (path[0] == '.'
-      && (vim_ispathsep(path[1]) || path[1] == NUL)
+      && (vim_ispathsep(path[1]) || path[1] == '\0')
       && (!tagfile || vim_strchr(p_cpo, CPO_DOTTAG) == NULL)
       && rel_fname != NULL) {
     int len = (int)(path_tail(rel_fname) - rel_fname);
@@ -325,17 +325,17 @@ vim_findfile_init (
       search_ctx->ffsc_start_dir = vim_strnsave(rel_fname, len);
     if (search_ctx->ffsc_start_dir == NULL)
       goto error_return;
-    if (*++path != NUL)
+    if (*++path != '\0')
       ++path;
-  } else if (*path == NUL || !vim_isAbsName(path)) {
+  } else if (*path == '\0' || !vim_isAbsName(path)) {
 #ifdef BACKSLASH_IN_FILENAME
     /* "c:dir" needs "c:" to be expanded, otherwise use current dir */
-    if (*path != NUL && path[1] == ':') {
+    if (*path != '\0' && path[1] == ':') {
       char_u drive[3];
 
       drive[0] = path[0];
       drive[1] = ':';
-      drive[2] = NUL;
+      drive[2] = '\0';
       if (vim_FullName(drive, ff_expand_buffer, MAXPATHL, TRUE) == FAIL)
         goto error_return;
       path += 2;
@@ -354,7 +354,7 @@ vim_findfile_init (
     if ((*path == '/' || *path == '\\')
         && path[1] != path[0]
         && search_ctx->ffsc_start_dir[1] == ':')
-      search_ctx->ffsc_start_dir[2] = NUL;
+      search_ctx->ffsc_start_dir[2] = '\0';
 #endif
   }
 
@@ -430,7 +430,7 @@ vim_findfile_init (
      * string.
      */
     len = 0;
-    while (*wc_part != NUL) {
+    while (*wc_part != '\0') {
       if (len + 5 >= MAXPATHL) {
         EMSG(_(e_pathtoolong));
         break;
@@ -448,7 +448,7 @@ vim_findfile_init (
         else
           ff_expand_buffer[len++] = FF_MAX_STAR_STAR_EXPAND;
         wc_part = (char_u *)errpt;
-        if (*wc_part != NUL && !vim_ispathsep(*wc_part)) {
+        if (*wc_part != '\0' && !vim_ispathsep(*wc_part)) {
           EMSG2(_(
                   "E343: Invalid path: '**[number]' must be at the end of the path or be followed by '%s'."),
               PATHSEPSTR);
@@ -457,7 +457,7 @@ vim_findfile_init (
       } else
         ff_expand_buffer[len++] = *wc_part++;
     }
-    ff_expand_buffer[len] = NUL;
+    ff_expand_buffer[len] = '\0';
     search_ctx->ffsc_wc_path = vim_strsave(ff_expand_buffer);
 
     if (search_ctx->ffsc_wc_path == NULL)
@@ -472,7 +472,7 @@ vim_findfile_init (
     search_ctx->ffsc_start_dir = vim_strsave(search_ctx->ffsc_fix_path);
     if (search_ctx->ffsc_start_dir == NULL)
       goto error_return;
-    search_ctx->ffsc_fix_path[0] = NUL;
+    search_ctx->ffsc_fix_path[0] = '\0';
   }
 
   /* create an absolute path */
@@ -561,7 +561,7 @@ char_u *vim_findfile_stopdir(char_u *buf)
 {
   char_u      *r_ptr = buf;
 
-  while (*r_ptr != NUL && *r_ptr != ';') {
+  while (*r_ptr != '\0' && *r_ptr != ';') {
     if (r_ptr[0] == '\\' && r_ptr[1] == ';') {
       /* Overwrite the escape char,
        * use STRLEN(r_ptr) to move the trailing '\0'. */
@@ -573,7 +573,7 @@ char_u *vim_findfile_stopdir(char_u *buf)
   if (*r_ptr == ';') {
     *r_ptr = 0;
     r_ptr++;
-  } else if (*r_ptr == NUL)
+  } else if (*r_ptr == '\0')
     r_ptr = NULL;
   return r_ptr;
 }
@@ -701,7 +701,7 @@ char_u *vim_findfile(void *search_ctx_arg)
         continue;
       }
 
-      file_path[0] = NUL;
+      file_path[0] = '\0';
 
       /*
        * If no filearray till now expand wildcards
@@ -730,7 +730,7 @@ char_u *vim_findfile(void *search_ctx_arg)
         add_pathsep(file_path);
 
         rest_of_wildcards = stackp->ffs_wc_path;
-        if (*rest_of_wildcards != NUL) {
+        if (*rest_of_wildcards != '\0') {
           len = (int)STRLEN(file_path);
           if (STRNCMP(rest_of_wildcards, "**", 2) == 0) {
             /* pointer to the restrict byte
@@ -767,7 +767,7 @@ char_u *vim_findfile(void *search_ctx_arg)
                  && !vim_ispathsep(*rest_of_wildcards))
             file_path[len++] = *rest_of_wildcards++;
 
-          file_path[len] = NUL;
+          file_path[len] = '\0';
           if (vim_ispathsep(*rest_of_wildcards))
             rest_of_wildcards++;
         }
@@ -800,7 +800,7 @@ char_u *vim_findfile(void *search_ctx_arg)
 
       if (stackp->ffs_stage == 0) {
         /* this is the first time we work on this directory */
-        if (*rest_of_wildcards == NUL) {
+        if (*rest_of_wildcards == '\0') {
           /*
            * We don't have further wildcards to expand, so we have to
            * check for the final file now.
@@ -887,7 +887,7 @@ char_u *vim_findfile(void *search_ctx_arg)
               }
 
               /* Not found or found already, try next suffix. */
-              if (*suf == NUL)
+              if (*suf == '\0')
                 break;
               copy_option_part(&suf, file_path + len,
                   MAXPATHL - len, ",");
@@ -1093,8 +1093,8 @@ static ff_visited_list_hdr_T *ff_get_visited_list(char_u *filename, ff_visited_l
 static int ff_wc_equal(char_u *s1, char_u *s2)
 {
   int i;
-  int prev1 = NUL;
-  int prev2 = NUL;
+  int prev1 = '\0';
+  int prev2 = '\0';
 
   if (s1 == s2)
     return TRUE;
@@ -1105,7 +1105,7 @@ static int ff_wc_equal(char_u *s1, char_u *s2)
   if (STRLEN(s1) != STRLEN(s2))
     return FAIL;
 
-  for (i = 0; s1[i] != NUL && s2[i] != NUL; i += MB_PTR2LEN(s1 + i)) {
+  for (i = 0; s1[i] != '\0' && s2[i] != '\0'; i += MB_PTR2LEN(s1 + i)) {
     int c1 = PTR2CHAR(s1 + i);
     int c2 = PTR2CHAR(s2 + i);
 
@@ -1139,7 +1139,7 @@ static int ff_check_visited(ff_visited_T **visited_list, char_u *fname, char_u *
     url = TRUE;
 #endif
   } else {
-    ff_expand_buffer[0] = NUL;
+    ff_expand_buffer[0] = '\0';
 #ifdef UNIX
     if (mch_stat((char *)fname, &st) < 0)
 #else
@@ -1176,7 +1176,7 @@ static int ff_check_visited(ff_visited_T **visited_list, char_u *fname, char_u *
     vp->ffv_dev_valid = TRUE;
     vp->ffv_ino = st.st_ino;
     vp->ffv_dev = st.st_dev;
-    vp->ffv_fname[0] = NUL;
+    vp->ffv_fname[0] = '\0';
   } else {
     vp->ffv_dev_valid = FALSE;
     STRCPY(vp->ffv_fname, ff_expand_buffer);
@@ -1378,7 +1378,7 @@ find_file_in_path (
 )
 {
   return find_file_in_path_option(ptr, len, options, first,
-      *curbuf->b_p_path == NUL ? p_path : curbuf->b_p_path,
+      *curbuf->b_p_path == '\0' ? p_path : curbuf->b_p_path,
       FINDFILE_BOTH, rel_fname, curbuf->b_p_sua);
 }
 
@@ -1439,7 +1439,7 @@ find_file_in_path_option (
   if (first == TRUE) {
     /* copy file name into NameBuff, expanding environment variables */
     save_char = ptr[len];
-    ptr[len] = NUL;
+    ptr[len] = '\0';
     expand_env(ptr, NameBuff, MAXPATHL);
     ptr[len] = save_char;
 
@@ -1452,10 +1452,10 @@ find_file_in_path_option (
   }
 
   rel_to_curdir = (ff_file_to_find[0] == '.'
-                   && (ff_file_to_find[1] == NUL
+                   && (ff_file_to_find[1] == '\0'
                        || vim_ispathsep(ff_file_to_find[1])
                        || (ff_file_to_find[1] == '.'
-                           && (ff_file_to_find[2] == NUL
+                           && (ff_file_to_find[2] == '\0'
                                || vim_ispathsep(ff_file_to_find[2])))));
   if (vim_isAbsName(ff_file_to_find)
       /* "..", "../path", "." and "./path": don't use the path_option */
@@ -1504,7 +1504,7 @@ find_file_in_path_option (
             file_name = vim_strsave(NameBuff);
             goto theend;
           }
-          if (*buf == NUL)
+          if (*buf == '\0')
             break;
           copy_option_part(&buf, NameBuff + l, MAXPATHL - l, ",");
         }
@@ -1533,7 +1533,7 @@ find_file_in_path_option (
       } else {
         char_u  *r_ptr;
 
-        if (dir == NULL || *dir == NUL) {
+        if (dir == NULL || *dir == '\0') {
           /* We searched all paths of the option, now we can
            * free the search context. */
           vim_findfile_cleanup(fdip_search_ctx);
diff --git a/src/fileio.c b/src/fileio.c
index 48172eca03..6c3a7b4f26 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -165,7 +165,7 @@ void filemess(buf_T *buf, char_u *name, char_u *s, int attr)
   msg_add_fname(buf, name);         /* put file name in IObuff with quotes */
   /* If it's extremely long, truncate it. */
   if (STRLEN(IObuff) > IOSIZE - 80)
-    IObuff[IOSIZE - 80] = NUL;
+    IObuff[IOSIZE - 80] = '\0';
   STRCAT(IObuff, s);
   /*
    * For the first message may have to start a new line.
@@ -386,7 +386,7 @@ readfile (
    * file may destroy it!  Reported on MS-DOS and Win 95.
    * If the name is too long we might crash further on, quit here.
    */
-  if (fname != NULL && *fname != NUL) {
+  if (fname != NULL && *fname != '\0') {
     p = fname + STRLEN(fname);
     if (after_pathsep(fname, p) || STRLEN(fname) >= MAXPATHL) {
       filemess(curbuf, fname, (char_u *)_("Illegal file name"), 0);
@@ -761,7 +761,7 @@ readfile (
         keep_dest_enc = TRUE;
     }
     fenc_alloced = FALSE;
-  } else if (*p_fencs == NUL) {
+  } else if (*p_fencs == '\0') {
     fenc = curbuf->b_p_fenc;            /* use format from buffer */
     fenc_alloced = FALSE;
   } else {
@@ -828,7 +828,7 @@ retry:
       try_unix = try_dos = try_mac = FALSE;
     } else if (curbuf->b_p_bin)
       fileformat = EOL_UNIX;                    /* binary: use Unix format */
-    else if (*p_ffs == NUL)
+    else if (*p_ffs == '\0')
       fileformat = get_fileformat(curbuf);      /* use format from buffer */
     else
       fileformat = EOL_UNKNOWN;                 /* detect from file */
@@ -917,7 +917,7 @@ retry:
      * Use the 'charconvert' expression when conversion is required
      * and we can't do it internally or with iconv().
      */
-    if (fio_flags == 0 && !read_stdin && !read_buffer && *p_ccv != NUL
+    if (fio_flags == 0 && !read_stdin && !read_buffer && *p_ccv != '\0'
 #  ifdef USE_ICONV
         && iconv_fd == (iconv_t)-1
 #  endif
@@ -958,7 +958,7 @@ retry:
   /* Set "can_retry" when it's possible to rewind the file and try with
    * another "fenc" value.  It's FALSE when no other "fenc" to try, reading
    * stdin or fixed at a specific encoding. */
-  can_retry = (*fenc != NUL && !read_stdin && !keep_dest_enc);
+  can_retry = (*fenc != '\0' && !read_stdin && !keep_dest_enc);
 
   if (!skip_read) {
     linerest = 0;
@@ -1058,7 +1058,7 @@ retry:
                 n = (int)(size - tlen);
                 for (ni = 0; ni < n; ++ni) {
                   if (p[ni] == NL)
-                    ptr[tlen++] = NUL;
+                    ptr[tlen++] = '\0';
                   else
                     ptr[tlen++] = p[ni];
                 }
@@ -1069,7 +1069,7 @@ retry:
                 * to NUL to reverse the effect done below. */
                 for (ni = 0; ni < n; ++ni) {
                   if (p[ni] == NL)
-                    ptr[tlen++] = NUL;
+                    ptr[tlen++] = '\0';
                   else
                     ptr[tlen++] = p[ni];
                 }
@@ -1119,7 +1119,7 @@ retry:
               illegal_byte = curbuf->b_ml.ml_line_count
                              - linecnt + 1;
             if (bad_char_behavior == BAD_DROP) {
-              *(ptr - conv_restlen) = NUL;
+              *(ptr - conv_restlen) = '\0';
               conv_restlen = 0;
             } else {
               /* Replace the trailing bytes with the replacement
@@ -1177,7 +1177,7 @@ retry:
           && (fio_flags == FIO_UCSBOM
               || (!curbuf->b_p_bomb
                   && tmpname == NULL
-                  && (*fenc == 'u' || (*fenc == NUL && enc_utf8))))) {
+                  && (*fenc == 'u' || (*fenc == '\0' && enc_utf8))))) {
         char_u  *ccname;
         int blen;
 
@@ -1538,7 +1538,7 @@ retry:
 rewind_retry:
           /* Retry reading with another conversion. */
 # if defined(FEAT_EVAL) && defined(USE_ICONV)
-          if (*p_ccv != NUL && iconv_fd != (iconv_t)-1)
+          if (*p_ccv != '\0' && iconv_fd != (iconv_t)-1)
             /* iconv() failed, try 'charconvert' */
             did_iconv = TRUE;
           else
@@ -1613,15 +1613,15 @@ rewind_retry:
       --ptr;
       while (++ptr, --size >= 0) {
         /* catch most common case first */
-        if ((c = *ptr) != NUL && c != CAR && c != NL)
+        if ((c = *ptr) != '\0' && c != CAR && c != NL)
           continue;
-        if (c == NUL)
+        if (c == '\0')
           *ptr = NL;            /* NULs are replaced by newlines! */
         else if (c == NL)
           *ptr = CAR;           /* NLs are replaced by CRs! */
         else {
           if (skip_count == 0) {
-            *ptr = NUL;                     /* end of line */
+            *ptr = '\0';                     /* end of line */
             len = (colnr_T) (ptr - line_start + 1);
             if (ml_append(lnum, line_start, len, newfile) == FAIL) {
               error = TRUE;
@@ -1643,17 +1643,17 @@ rewind_retry:
     } else {
       --ptr;
       while (++ptr, --size >= 0) {
-        if ((c = *ptr) != NUL && c != NL)          /* catch most common case */
+        if ((c = *ptr) != '\0' && c != NL)          /* catch most common case */
           continue;
-        if (c == NUL)
+        if (c == '\0')
           *ptr = NL;            /* NULs are replaced by newlines! */
         else {
           if (skip_count == 0) {
-            *ptr = NUL;                         /* end of line */
+            *ptr = '\0';                         /* end of line */
             len = (colnr_T)(ptr - line_start + 1);
             if (fileformat == EOL_DOS) {
               if (ptr[-1] == CAR) {             /* remove CR */
-                ptr[-1] = NUL;
+                ptr[-1] = '\0';
                 --len;
               }
               /*
@@ -1719,7 +1719,7 @@ failed:
     /* remember for when writing */
     if (set_options)
       curbuf->b_p_eol = FALSE;
-    *ptr = NUL;
+    *ptr = '\0';
     len = (colnr_T)(ptr - line_start + 1);
     if (ml_append(lnum, line_start, len, newfile) == FAIL)
       error = TRUE;
@@ -2030,8 +2030,8 @@ static int is_dev_fd_file(char_u *fname)
 {
   return STRNCMP(fname, "/dev/fd/", 8) == 0
          && VIM_ISDIGIT(fname[8])
-         && *skipdigits(fname + 9) == NUL
-         && (fname[9] != NUL
+         && *skipdigits(fname + 9) == '\0'
+         && (fname[9] != '\0'
              || (fname[8] != '0' && fname[8] != '1' && fname[8] != '2'));
 }
 #endif
@@ -2086,7 +2086,7 @@ void set_file_options(int set_options, exarg_T *eap)
   if (set_options) {
     if (eap != NULL && eap->force_ff != 0)
       set_fileformat(get_fileformat_force(curbuf, eap), OPT_LOCAL);
-    else if (*p_ffs != NUL)
+    else if (*p_ffs != '\0')
       set_fileformat(default_fileformat(), OPT_LOCAL);
   }
 
@@ -2126,7 +2126,7 @@ static char_u *next_fenc(char_u **pp)
   char_u      *p;
   char_u      *r;
 
-  if (**pp == NUL) {
+  if (**pp == '\0') {
     *pp = NULL;
     return (char_u *)"";
   }
@@ -2281,7 +2281,7 @@ check_for_cryptkey (
         *did_ask = TRUE;
 
         /* check if empty key entered */
-        if (cryptkey != NULL && *cryptkey == NUL) {
+        if (cryptkey != NULL && *cryptkey == '\0') {
           if (cryptkey != curbuf->b_p_key)
             vim_free(cryptkey);
           cryptkey = NULL;
@@ -2313,7 +2313,7 @@ check_for_cryptkey (
   }
   /* When starting to edit a new file which does not have encryption, clear
    * the 'key' option, except when starting up (called with -x argument) */
-  else if (newfile && *curbuf->b_p_key != NUL && !starting)
+  else if (newfile && *curbuf->b_p_key != '\0' && !starting)
     set_option_value((char_u *)"key", 0L, (char_u *)"", OPT_LOCAL);
 
   return cryptkey;
@@ -2503,7 +2503,7 @@ buf_write (
   context_sha256_T sha_ctx;
   int crypt_method_used;
 
-  if (fname == NULL || *fname == NUL)   /* safety check */
+  if (fname == NULL || *fname == '\0')   /* safety check */
     return FAIL;
   if (buf->b_ml.ml_mfp == NULL) {
     /* This can happen during startup when there is a stray "w" in the
@@ -2866,8 +2866,8 @@ buf_write (
   /*
    * If 'backupskip' is not empty, don't make a backup for some files.
    */
-  dobackup = (p_wb || p_bk || *p_pm != NUL);
-  if (dobackup && *p_bsk != NUL && match_file_list(p_bsk, sfname, ffname))
+  dobackup = (p_wb || p_bk || *p_pm != '\0');
+  if (dobackup && *p_bsk != '\0' && match_file_list(p_bsk, sfname, ffname))
     dobackup = FALSE;
 
   /*
@@ -2889,7 +2889,7 @@ buf_write (
    * Do not make any backup, if 'writebackup' and 'backup' are both switched
    * off.  This helps when editing large files on almost-full disks.
    */
-  if (!(append && *p_pm == NUL) && !filtering && perm >= 0 && dobackup) {
+  if (!(append && *p_pm == '\0') && !filtering && perm >= 0 && dobackup) {
 #if defined(UNIX) || defined(WIN32)
     struct stat st;
 #endif
@@ -2984,7 +2984,7 @@ buf_write (
 #endif
 
     /* make sure we have a valid backup extension to use */
-    if (*p_bex == NUL)
+    if (*p_bex == '\0')
       backup_ext = (char_u *)".bak";
     else
       backup_ext = p_bex;
@@ -3405,7 +3405,7 @@ nobackup:
      * writing, write to a temp file instead and let the conversion
      * overwrite the original file.
      */
-    if (*p_ccv != NUL) {
+    if (*p_ccv != '\0') {
       wfname = vim_tempname('w');
       if (wfname == NULL) {             /* Can't write without a tempfile! */
         errmsg = (char_u *)_("E214: Can't find temp file for writing");
@@ -3523,7 +3523,7 @@ restore_backup:
 
   write_info.bw_fd = fd;
 
-  if (*buf->b_p_key != NUL && !filtering) {
+  if (*buf->b_p_key != '\0' && !filtering) {
     char_u *header;
     int header_len;
 
@@ -3592,9 +3592,9 @@ restore_backup:
     ptr = ml_get_buf(buf, lnum, FALSE) - 1;
     if (write_undo_file)
       sha256_update(&sha_ctx, ptr + 1, (uint32_t)(STRLEN(ptr + 1) + 1));
-    while ((c = *++ptr) != NUL) {
+    while ((c = *++ptr) != '\0') {
       if (c == NL)
-        *s = NUL;                       /* replace newlines with NULs */
+        *s = '\0';                       /* replace newlines with NULs */
       else if (c == CAR && fileformat == EOL_MAC)
         *s = NL;                        /* Mac: replace CRs with NLs */
       else
@@ -3983,7 +3983,7 @@ nofail:
 #endif
         );                      /* put file name in IObuff with quotes */
     if (STRLEN(IObuff) + STRLEN(errmsg) + numlen >= IOSIZE)
-      IObuff[IOSIZE - STRLEN(errmsg) - numlen - 1] = NUL;
+      IObuff[IOSIZE - STRLEN(errmsg) - numlen - 1] = '\0';
     /* If the error message has the form "is ...", put the error number in
      * front of the file name. */
     if (errnum != NULL) {
@@ -4091,7 +4091,7 @@ static int set_rw_fname(char_u *fname, char_u *sfname)
     return FAIL;
 
   /* Do filetype detection now if 'filetype' is empty. */
-  if (*curbuf->b_p_ft == NUL) {
+  if (*curbuf->b_p_ft == '\0') {
     if (au_has_group((char_u *)"filetypedetect"))
       (void)do_doautocmd((char_u *)"filetypedetect BufRead", FALSE);
     do_modelines(0);
@@ -4500,7 +4500,7 @@ static int need_conversion(char_u *fenc)
   int enc_flags;
   int fenc_flags;
 
-  if (*fenc == NUL || STRCMP(p_enc, fenc) == 0) {
+  if (*fenc == '\0' || STRCMP(p_enc, fenc) == 0) {
     same_encoding = TRUE;
     fenc_flags = 0;
   } else {
@@ -4530,7 +4530,7 @@ static int get_fio_flags(char_u *ptr)
 {
   int prop;
 
-  if (*ptr == NUL)
+  if (*ptr == '\0')
     ptr = p_enc;
 
   prop = enc_canon_props(ptr);
@@ -4718,7 +4718,7 @@ buf_modname (
    * If there is no file name we must get the name of the current directory
    * (we need the full path in case :cd is used).
    */
-  if (fname == NULL || *fname == NUL) {
+  if (fname == NULL || *fname == '\0') {
     retval = alloc((unsigned)(MAXPATHL + extlen + 3));
     if (os_dirname(retval, MAXPATHL) == FAIL ||
         (fnamelen = (int)STRLEN(retval)) == 0) {
@@ -4727,7 +4727,7 @@ buf_modname (
     }
     if (!after_pathsep(retval, retval + fnamelen)) {
       retval[fnamelen++] = PATHSEP;
-      retval[fnamelen] = NUL;
+      retval[fnamelen] = '\0';
     }
 #ifndef SHORT_FNAME
     prepend_dot = FALSE;            /* nothing to prepend a dot to */
@@ -4785,7 +4785,7 @@ buf_modname (
      * extension starts with '.', put a '_' before the dot, because just
      * ".ext" is invalid.
      */
-    if (fname == NULL || *fname == NUL
+    if (fname == NULL || *fname == '\0'
         || vim_ispathsep(fname[STRLEN(fname) - 1])) {
       if (*ext == '.')
         *s++ = '_';
@@ -4819,7 +4819,7 @@ buf_modname (
    * '_' before the dot, because just ".ext" may be invalid if it's on a
    * FAT partition, and on HPFS it doesn't matter.
    */
-  else if ((fname == NULL || *fname == NUL) && *ext == '.')
+  else if ((fname == NULL || *fname == '\0') && *ext == '.')
     *s++ = '_';
 #endif
 
@@ -4871,24 +4871,24 @@ int vim_fgets(char_u *buf, int size, FILE *fp)
 #define FGETS_SIZE 200
   char tbuf[FGETS_SIZE];
 
-  buf[size - 2] = NUL;
+  buf[size - 2] = '\0';
 #ifdef USE_CR
   eof = fgets_cr((char *)buf, size, fp);
 #else
   eof = fgets((char *)buf, size, fp);
 #endif
-  if (buf[size - 2] != NUL && buf[size - 2] != '\n') {
-    buf[size - 1] = NUL;            /* Truncate the line */
+  if (buf[size - 2] != '\0' && buf[size - 2] != '\n') {
+    buf[size - 1] = '\0';            /* Truncate the line */
 
     /* Now throw away the rest of the line: */
     do {
-      tbuf[FGETS_SIZE - 2] = NUL;
+      tbuf[FGETS_SIZE - 2] = '\0';
 #ifdef USE_CR
       ignoredp = fgets_cr((char *)tbuf, FGETS_SIZE, fp);
 #else
       ignoredp = fgets((char *)tbuf, FGETS_SIZE, fp);
 #endif
-    } while (tbuf[FGETS_SIZE - 2] != NUL && tbuf[FGETS_SIZE - 2] != '\n');
+    } while (tbuf[FGETS_SIZE - 2] != '\0' && tbuf[FGETS_SIZE - 2] != '\n');
   }
   return eof == NULL;
 }
@@ -4925,7 +4925,7 @@ int tag_fgets(char_u *buf, int size, FILE *fp)
     if (c == '\n')
       break;
   }
-  buf[i] = NUL;
+  buf[i] = '\0';
   return eof;
 }
 #endif
@@ -5236,7 +5236,7 @@ buf_check_timestamp (
    * this buffer. */
   if (buf->b_ffname == NULL
       || buf->b_ml.ml_mfp == NULL
-      || *buf->b_p_bt != NUL
+      || *buf->b_p_bt != '\0'
       || buf->b_saving
       || busy
       )
@@ -5364,7 +5364,7 @@ buf_check_timestamp (
        * mesg2 has been appended. */
       set_vim_var_string(VV_WARNINGMSG, tbuf, -1);
       if (can_reload) {
-        if (*mesg2 != NUL) {
+        if (*mesg2 != '\0') {
           STRCAT(tbuf, "\n");
           STRCAT(tbuf, mesg2);
         }
@@ -5373,7 +5373,7 @@ buf_check_timestamp (
           reload = TRUE;
       } else if (State > NORMAL_BUSY || (State & CMDLINE) ||
                  already_warned) {
-        if (*mesg2 != NUL) {
+        if (*mesg2 != '\0') {
           STRCAT(tbuf, "; ");
           STRCAT(tbuf, mesg2);
         }
@@ -5383,7 +5383,7 @@ buf_check_timestamp (
         if (!autocmd_busy) {
           msg_start();
           msg_puts_attr(tbuf, hl_attr(HLF_E) + MSG_HIST);
-          if (*mesg2 != NUL)
+          if (*mesg2 != '\0')
             msg_puts_attr((char_u *)mesg2,
                 hl_attr(HLF_W) + MSG_HIST);
           msg_clr_eos();
@@ -5603,7 +5603,7 @@ void vim_deltempdir(void)
         os_remove((char *)files[i]);
       FreeWild(file_count, files);
     }
-    path_tail(NameBuff)[-1] = NUL;
+    path_tail(NameBuff)[-1] = '\0';
     os_rmdir((char *)NameBuff);
 
     vim_free(vim_tempdir);
@@ -5755,7 +5755,7 @@ vim_tempname (
 
   /* tmpnam() will make its own name */
   p = tmpnam((char *)itmp);
-  if (p == NULL || *p == NUL)
+  if (p == NULL || *p == '\0')
     return NULL;
 #  else
   char_u      *p;
@@ -5779,7 +5779,7 @@ void forward_slash(char_u *fname)
 {
   char_u      *p;
 
-  for (p = fname; *p != NUL; ++p)
+  for (p = fname; *p != '\0'; ++p)
     /* The Big5 encoding can have '\' in the trail byte. */
     if (enc_dbcs != 0 && (*mb_ptr2len)(p) > 1)
       ++p;
@@ -6239,7 +6239,7 @@ void do_augroup(char_u *arg, int del_group)
   int i;
 
   if (del_group) {
-    if (*arg == NUL)
+    if (*arg == '\0')
       EMSG(_(e_argreq));
     else
       au_del_group(arg);
@@ -6352,8 +6352,8 @@ static int event_ignored(event_T event)
 {
   char_u      *p = p_ei;
 
-  while (*p != NUL) {
-    if (STRNICMP(p, "all", 3) == 0 && (p[3] == NUL || p[3] == ','))
+  while (*p != '\0') {
+    if (STRNICMP(p, "all", 3) == 0 && (p[3] == '\0' || p[3] == ','))
       return TRUE;
     if (event_name2nr(p, &p) == event)
       return TRUE;
@@ -6370,7 +6370,7 @@ int check_ei(void)
   char_u      *p = p_ei;
 
   while (*p) {
-    if (STRNICMP(p, "all", 3) == 0 && (p[3] == NUL || p[3] == ',')) {
+    if (STRNICMP(p, "all", 3) == 0 && (p[3] == '\0' || p[3] == ',')) {
       p += 3;
       if (*p == ',')
         ++p;
@@ -6395,7 +6395,7 @@ char_u *au_event_disable(char *what)
   if (save_ei != NULL) {
     new_ei = vim_strnsave(p_ei, (int)(STRLEN(p_ei) + STRLEN(what)));
     if (new_ei != NULL) {
-      if (*what == ',' && *p_ei == NUL)
+      if (*what == ',' && *p_ei == '\0')
         STRCPY(new_ei, what + 1);
       else
         STRCAT(new_ei, what);
@@ -6474,14 +6474,14 @@ void do_autocmd(char_u *arg, int forceit)
     return;
 
   /*
-   * Scan over the pattern.  Put a NUL at the end.
+   * Scan over the pattern.  Put a '\0' at the end.
    */
   pat = skipwhite(pat);
   cmd = pat;
   while (*cmd && (!vim_iswhite(*cmd) || cmd[-1] == '\\'))
     cmd++;
   if (*cmd)
-    *cmd++ = NUL;
+    *cmd++ = '\0';
 
   /* Expand environment variables in the pattern.  Set 'shellslash', we want
    * forward slashes here. */
@@ -6503,7 +6503,7 @@ void do_autocmd(char_u *arg, int forceit)
    * Check for "nested" flag.
    */
   cmd = skipwhite(cmd);
-  if (*cmd != NUL && STRNCMP(cmd, "nested", 6) == 0 && vim_iswhite(cmd[6])) {
+  if (*cmd != '\0' && STRNCMP(cmd, "nested", 6) == 0 && vim_iswhite(cmd[6])) {
     nested = TRUE;
     cmd = skipwhite(cmd + 6);
   }
@@ -6512,7 +6512,7 @@ void do_autocmd(char_u *arg, int forceit)
    * Find the start of the commands.
    * Expand <sfile> in it.
    */
-  if (*cmd != NUL) {
+  if (*cmd != '\0') {
     cmd = expand_sfile(cmd);
     if (cmd == NULL)                /* some error */
       return;
@@ -6522,7 +6522,7 @@ void do_autocmd(char_u *arg, int forceit)
   /*
    * Print header when showing autocommands.
    */
-  if (!forceit && *cmd == NUL) {
+  if (!forceit && *cmd == '\0') {
     /* Highlight title */
     MSG_PUTS_TITLE(_("\n--- Auto-Commands ---"));
   }
@@ -6532,7 +6532,7 @@ void do_autocmd(char_u *arg, int forceit)
    */
   last_event = (event_T)-1;             /* for listing the event name */
   last_group = AUGROUP_ERROR;           /* for listing the group name */
-  if (*arg == '*' || *arg == NUL) {
+  if (*arg == '*' || *arg == '\0') {
     for (event = (event_T)0; (int)event < (int)NUM_EVENTS;
          event = (event_T)((int)event + 1))
       if (do_autocmd_event(event, pat,
@@ -6604,12 +6604,12 @@ static int do_autocmd_event(event_T event, char_u *pat, int nested, char_u *cmd,
     findgroup = current_augroup;
   else
     findgroup = group;
-  allgroups = (group == AUGROUP_ALL && !forceit && *cmd == NUL);
+  allgroups = (group == AUGROUP_ALL && !forceit && *cmd == '\0');
 
   /*
    * Show or delete all patterns for an event.
    */
-  if (*pat == NUL) {
+  if (*pat == '\0') {
     for (ap = first_autopat[(int)event]; ap != NULL; ap = ap->next) {
       if (forceit) {      /* delete the AutoPat, if it's in the current group */
         if (ap->group == findgroup)
@@ -6694,7 +6694,7 @@ static int do_autocmd_event(event_T event, char_u *pat, int nested, char_u *cmd,
            * this list.
            */
           if (forceit) {
-            if (*cmd != NUL && ap->next == NULL) {
+            if (*cmd != '\0' && ap->next == NULL) {
               au_remove_cmds(ap);
               break;
             }
@@ -6703,7 +6703,7 @@ static int do_autocmd_event(event_T event, char_u *pat, int nested, char_u *cmd,
           /*
            * Show autocmd's for this autopat, or buflocals <buffer=X>
            */
-          else if (*cmd == NUL)
+          else if (*cmd == '\0')
             show_autocmd(ap, event);
 
           /*
@@ -6719,7 +6719,7 @@ static int do_autocmd_event(event_T event, char_u *pat, int nested, char_u *cmd,
     /*
      * Add a new command.
      */
-    if (*cmd != NUL) {
+    if (*cmd != '\0') {
       /*
        * If the pattern we want to add a command to does appear at the
        * end of the list (or not is not in the list at all), add the
@@ -7309,7 +7309,7 @@ apply_autocmds_group (
   if (fname_io == NULL) {
     if (event == EVENT_COLORSCHEME)
       autocmd_fname = NULL;
-    else if (fname != NULL && *fname != NUL)
+    else if (fname != NULL && *fname != '\0')
       autocmd_fname = fname;
     else if (buf != NULL)
       autocmd_fname = buf->b_ffname;
@@ -7334,7 +7334,7 @@ apply_autocmds_group (
    * Always use the full path of the file name to match with, in case
    * "allow_dirs" is set.
    */
-  if (fname == NULL || *fname == NUL) {
+  if (fname == NULL || *fname == '\0') {
     if (buf == NULL)
       fname = NULL;
     else {
@@ -7755,16 +7755,16 @@ set_context_in_autocmd (
   if (group == AUGROUP_ERROR)
     return NULL;
   /* If there only is a group name that's what we expand. */
-  if (*arg == NUL && group != AUGROUP_ALL && !vim_iswhite(arg[-1])) {
+  if (*arg == '\0' && group != AUGROUP_ALL && !vim_iswhite(arg[-1])) {
     arg = p;
     group = AUGROUP_ALL;
   }
 
   /* skip over event name */
-  for (p = arg; *p != NUL && !vim_iswhite(*p); ++p)
+  for (p = arg; *p != '\0' && !vim_iswhite(*p); ++p)
     if (*p == ',')
       arg = p + 1;
-  if (*p == NUL) {
+  if (*p == '\0') {
     if (group == AUGROUP_ALL)
       include_groups = TRUE;
     xp->xp_context = EXPAND_EVENTS;         /* expand event name */
@@ -7840,7 +7840,7 @@ int au_exists(char_u *arg)
     return FALSE;
   p = vim_strchr(arg_save, '#');
   if (p != NULL)
-    *p++ = NUL;
+    *p++ = '\0';
 
   /* First, look for an autocmd group name */
   group = au_find_group(arg_save);
@@ -7859,7 +7859,7 @@ int au_exists(char_u *arg)
     event_name = p;
     p = vim_strchr(event_name, '#');
     if (p != NULL)
-      *p++ = NUL;           /* "Group#Event#pat" */
+      *p++ = '\0';           /* "Group#Event#pat" */
   }
 
   pattern = p;              /* "pattern" is NULL when there is no pattern */
@@ -7941,7 +7941,7 @@ match_file_pat (
 
     for (type_start = pattern + 1; (c = *pattern); pattern++) {
       if ((c == ';' || c == '>') && match == FALSE) {
-        *pattern = NUL;             /* Terminate the string */
+        *pattern = '\0';             /* Terminate the string */
         /* TODO: match with 'filetype' of buffer that "fname" comes
          * from. */
         match = mch_check_filetype(fname, type_start);
@@ -7953,9 +7953,9 @@ match_file_pat (
     }
 
     /* (c should never be NUL, but check anyway) */
-    if (match == FALSE || c == NUL)
+    if (match == FALSE || c == '\0')
       regmatch.regprog = NULL;          /* Doesn't match - don't check pat. */
-    else if (*pattern == NUL) {
+    else if (*pattern == '\0') {
       regmatch.regprog = NULL;          /* Vim will try to free regprog later */
       no_pattern = TRUE;        /* Always matches - don't check pat. */
     } else
@@ -8151,7 +8151,7 @@ file_pat_to_reg_pat (
       reg_pat[i++] = '.';
       break;
     case '\\':
-      if (p[1] == NUL)
+      if (p[1] == '\0')
         break;
 #ifdef BACKSLASH_IN_FILENAME
       if (!no_bslash) {
@@ -8245,7 +8245,7 @@ file_pat_to_reg_pat (
   }
   if (add_dollar)
     reg_pat[i++] = '$';
-  reg_pat[i] = NUL;
+  reg_pat[i] = '\0';
   if (nested != 0) {
     if (nested < 0)
       EMSG(_("E219: Missing {."));
diff --git a/src/fold.c b/src/fold.c
index 38432a79aa..c8dc537eb9 100644
--- a/src/fold.c
+++ b/src/fold.c
@@ -499,7 +499,7 @@ static void newFoldLevelWin(win_T *wp)
  */
 void foldCheckClose(void)
 {
-  if (*p_fcl != NUL) {  /* can only be "all" right now */
+  if (*p_fcl != '\0') {  /* can only be "all" right now */
     checkupdate(curwin);
     if (checkCloseRec(&curwin->w_folds, curwin->w_cursor.lnum,
             (int)curwin->w_p_fdl))
@@ -1662,13 +1662,13 @@ static void foldDelMarker(linenr_T lnum, char_u *marker, int markerlen)
   char_u      *cms2;
 
   line = ml_get(lnum);
-  for (p = line; *p != NUL; ++p)
+  for (p = line; *p != '\0'; ++p)
     if (STRNCMP(p, marker, markerlen) == 0) {
       /* Found the marker, include a digit if it's there. */
       len = markerlen;
       if (VIM_ISDIGIT(p[len]))
         ++len;
-      if (*cms != NUL) {
+      if (*cms != '\0') {
         /* Also delete 'commentstring' if it matches. */
         cms2 = (char_u *)strstr((char *)cms, "%s");
         if (p - line >= cms2 - cms
@@ -1713,7 +1713,7 @@ char_u *get_foldtext(win_T *wp, linenr_T lnum, linenr_T lnume, foldinfo_T *foldi
     /* a previous error should not abort evaluating 'foldexpr' */
     did_emsg = FALSE;
 
-  if (*wp->w_p_fdt != NUL) {
+  if (*wp->w_p_fdt != '\0') {
     char_u dashes[MAX_LEVEL + 2];
     win_T   *save_curwin;
     int level;
@@ -1729,7 +1729,7 @@ char_u *get_foldtext(win_T *wp, linenr_T lnum, linenr_T lnume, foldinfo_T *foldi
     if (level > (int)sizeof(dashes) - 1)
       level = (int)sizeof(dashes) - 1;
     memset(dashes, '-', (size_t)level);
-    dashes[level] = NUL;
+    dashes[level] = '\0';
     set_vim_var_string(VV_FOLDDASHES, dashes, -1);
     set_vim_var_nr(VV_FOLDLEVEL, (long)level);
 
@@ -1760,7 +1760,7 @@ char_u *get_foldtext(win_T *wp, linenr_T lnum, linenr_T lnume, foldinfo_T *foldi
     if (text != NULL) {
       /* Replace unprintable characters, if there are any.  But
        * replace a TAB with a space. */
-      for (p = text; *p != NUL; ++p) {
+      for (p = text; *p != '\0'; ++p) {
         int len;
 
         if (has_mbyte && (len = (*mb_ptr2len)(p)) > 1) {
@@ -1772,7 +1772,7 @@ char_u *get_foldtext(win_T *wp, linenr_T lnum, linenr_T lnume, foldinfo_T *foldi
         else if (ptr2cells(p) > 1)
           break;
       }
-      if (*p != NUL) {
+      if (*p != '\0') {
         p = transstr(text);
         vim_free(text);
         text = p;
@@ -1826,7 +1826,7 @@ void foldtext_cleanup(char_u *str)
   }
   parseMarker(curwin);
 
-  for (s = str; *s != NUL; ) {
+  for (s = str; *s != '\0'; ) {
     len = 0;
     if (STRNCMP(s, curwin->w_p_fmr, foldstartmarkerlen) == 0)
       len = foldstartmarkerlen;
@@ -2687,7 +2687,7 @@ static void foldlevelIndent(fline_T *flp)
 
   /* empty line or lines starting with a character in 'foldignore': level
    * depends on surrounding lines */
-  if (*s == NUL || vim_strchr(flp->wp->w_p_fdi, *s) != NULL) {
+  if (*s == '\0' || vim_strchr(flp->wp->w_p_fdi, *s) != NULL) {
     /* first and last line can't be undefined, use level 0 */
     if (lnum == 1 || lnum == buf->b_ml.ml_line_count)
       flp->lvl = 0;
diff --git a/src/garray.c b/src/garray.c
index bb035935a8..032d61f150 100644
--- a/src/garray.c
+++ b/src/garray.c
@@ -189,7 +189,7 @@ void append_ga_line(garray_T *gap)
       && (((char_u *)gap->ga_data)[gap->ga_len - 1] == CAR)) {
     gap->ga_len--;
   }
-  ga_append(gap, NUL);
+  ga_append(gap, '\0');
   ml_append(curwin->w_cursor.lnum++, gap->ga_data, 0, FALSE);
   gap->ga_len = 0;
 }
diff --git a/src/getchar.c b/src/getchar.c
index 62e0c9bf61..3ad6ff3390 100644
--- a/src/getchar.c
+++ b/src/getchar.c
@@ -66,17 +66,17 @@
 
 #define MINIMAL_SIZE 20                 /* minimal size for b_str */
 
-static buffheader_T redobuff = {{NULL, {NUL}}, NULL, 0, 0};
-static buffheader_T old_redobuff = {{NULL, {NUL}}, NULL, 0, 0};
-static buffheader_T save_redobuff = {{NULL, {NUL}}, NULL, 0, 0};
-static buffheader_T save_old_redobuff = {{NULL, {NUL}}, NULL, 0, 0};
-static buffheader_T recordbuff = {{NULL, {NUL}}, NULL, 0, 0};
+static buffheader_T redobuff = {{NULL, {'\0'}}, NULL, 0, 0};
+static buffheader_T old_redobuff = {{NULL, {'\0'}}, NULL, 0, 0};
+static buffheader_T save_redobuff = {{NULL, {'\0'}}, NULL, 0, 0};
+static buffheader_T save_old_redobuff = {{NULL, {'\0'}}, NULL, 0, 0};
+static buffheader_T recordbuff = {{NULL, {'\0'}}, NULL, 0, 0};
 
 // First read ahead buffer. Used for translated commands.
-static buffheader_T readbuf1 = {{NULL, {NUL}}, NULL, 0, 0};
+static buffheader_T readbuf1 = {{NULL, {'\0'}}, NULL, 0, 0};
 
 // Second read ahead buffer. Used for redo.
-static buffheader_T readbuf2 = {{NULL, {NUL}}, NULL, 0, 0};
+static buffheader_T readbuf2 = {{NULL, {'\0'}}, NULL, 0, 0};
 
 static int typeahead_char = 0;          /* typeahead char that's not flushed */
 
@@ -203,7 +203,7 @@ static char_u *get_buffcont(buffheader_T *buffer,
     for (bp = buffer->bh_first.b_next; bp != NULL; bp = bp->b_next)
       for (str = bp->b_str; *str; )
         *p2++ = *str++;
-    *p2 = NUL;
+    *p2 = '\0';
   }
   return p;
 }
@@ -228,7 +228,7 @@ char_u *get_recorded(void)
   len = STRLEN(p);
   if ((int)len >= last_recorded_len) {
     len -= last_recorded_len;
-    p[len] = NUL;
+    p[len] = '\0';
   }
 
   /*
@@ -236,7 +236,7 @@ char_u *get_recorded(void)
    * CTRL-O.
    */
   if (len > 0 && restart_edit != 0 && p[len - 1] == Ctrl_O)
-    p[len - 1] = NUL;
+    p[len - 1] = '\0';
 
   return p;
 }
@@ -331,15 +331,15 @@ static void add_char_buff(buffheader_T *buf, int c)
     if (!IS_SPECIAL(c))
       c = bytes[i];
 
-    if (IS_SPECIAL(c) || c == K_SPECIAL || c == NUL) {
+    if (IS_SPECIAL(c) || c == K_SPECIAL || c == '\0') {
       /* translate special key code into three byte sequence */
       temp[0] = K_SPECIAL;
       temp[1] = K_SECOND(c);
       temp[2] = K_THIRD(c);
-      temp[3] = NUL;
+      temp[3] = '\0';
     } else {
       temp[0] = c;
-      temp[1] = NUL;
+      temp[1] = '\0';
     }
     add_buff(buf, temp, -1L);
   }
@@ -356,7 +356,7 @@ static int read_readbuffers(int advance)
   int c;
 
   c = read_readbuf(&readbuf1, advance);
-  if (c == NUL)
+  if (c == '\0')
     c = read_readbuf(&readbuf2, advance);
   return c;
 }
@@ -367,13 +367,13 @@ static int read_readbuf(buffheader_T *buf, int advance)
   buffblock_T *curr;
 
   if (buf->bh_first.b_next == NULL) /* buffer is empty */
-    return NUL;
+    return '\0';
 
   curr = buf->bh_first.b_next;
   c = curr->b_str[buf->bh_index];
 
   if (advance) {
-    if (curr->b_str[++buf->bh_index] == NUL) {
+    if (curr->b_str[++buf->bh_index] == '\0') {
       buf->bh_first.b_next = curr->b_next;
       vim_free(curr);
       buf->bh_index = 0;
@@ -432,7 +432,7 @@ void flush_buffers(int flush_typeahead)
   init_typebuf();
 
   start_stuff();
-  while (read_readbuffers(TRUE) != NUL) {
+  while (read_readbuffers(TRUE) != '\0') {
   }
 
   if (flush_typeahead) {            /* remove all typeahead */
@@ -480,7 +480,7 @@ void CancelRedo(void)
     redobuff = old_redobuff;
     old_redobuff.bh_first.b_next = NULL;
     start_stuff();
-    while (read_readbuffers(TRUE) != NUL) {
+    while (read_readbuffers(TRUE) != '\0') {
     }
   }
 }
@@ -551,7 +551,7 @@ AppendToRedobuffLit (
   if (block_redo)
     return;
 
-  while (len < 0 ? *s != NUL : s - str < len) {
+  while (len < 0 ? *s != '\0' : s - str < len) {
     /* Put a string of normal characters in the redo buffer (that's
      * faster). */
     start = s;
@@ -562,12 +562,12 @@ AppendToRedobuffLit (
 
     /* Don't put '0' or '^' as last character, just in case a CTRL-D is
      * typed next. */
-    if (*s == NUL && (s[-1] == '0' || s[-1] == '^'))
+    if (*s == '\0' && (s[-1] == '0' || s[-1] == '^'))
       --s;
     if (s > start)
       add_buff(&redobuff, start, (long)(s - start));
 
-    if (*s == NUL || (len >= 0 && s - str >= len))
+    if (*s == '\0' || (len >= 0 && s - str >= len))
       break;
 
     /* Handle a special or multibyte character. */
@@ -576,11 +576,11 @@ AppendToRedobuffLit (
       c = mb_cptr2char_adv(&s);
     else
       c = *s++;
-    if (c < ' ' || c == DEL || (*s == NUL && (c == '0' || c == '^')))
+    if (c < ' ' || c == DEL || (*s == '\0' && (c == '0' || c == '^')))
       add_char_buff(&redobuff, Ctrl_V);
 
     /* CTRL-V '0' must be inserted as CTRL-V 048 (EBCDIC: xf0) */
-    if (*s == NUL && c == '0')
+    if (*s == '\0' && c == '0')
       add_buff(&redobuff, (char_u *)"048", 3L);
     else
       add_char_buff(&redobuff, c);
@@ -629,8 +629,8 @@ void stuffReadbuffSpec(char_u *s)
 {
   int c;
 
-  while (*s != NUL) {
-    if (*s == K_SPECIAL && s[1] != NUL && s[2] != NUL) {
+  while (*s != '\0') {
+    if (*s == K_SPECIAL && s[1] != '\0' && s[2] != '\0') {
       /* Insert special key literally. */
       stuffReadbuffLen(s, 3L);
       s += 3;
@@ -687,7 +687,7 @@ static int read_redo(int init, int old_redo)
     p = bp->b_str;
     return OK;
   }
-  if ((c = *p) != NUL) {
+  if ((c = *p) != '\0') {
     /* Reverse the conversion done by add_char_buff() */
     /* For a multi-byte character get all the bytes and return the
      * converted character. */
@@ -700,7 +700,7 @@ static int read_redo(int init, int old_redo)
         c = TO_SPECIAL(p[1], p[2]);
         p += 2;
       }
-      if (*++p == NUL && bp->b_next != NULL) {
+      if (*++p == '\0' && bp->b_next != NULL) {
         bp = bp->b_next;
         p = bp->b_str;
       }
@@ -711,7 +711,7 @@ static int read_redo(int init, int old_redo)
         break;
       }
       c = *p;
-      if (c == NUL)             /* cannot happen? */
+      if (c == '\0')             /* cannot happen? */
         break;
     }
   }
@@ -728,7 +728,7 @@ static void copy_redo(int old_redo)
 {
   int c;
 
-  while ((c = read_redo(FALSE, old_redo)) != NUL) {
+  while ((c = read_redo(FALSE, old_redo)) != '\0') {
     add_char_buff(&readbuf2, c);
   }
 }
@@ -800,7 +800,7 @@ int start_redo_ins(void)
   start_stuff();
 
   /* skip the count and the command character */
-  while ((c = read_redo(FALSE, FALSE)) != NUL) {
+  while ((c = read_redo(FALSE, FALSE)) != '\0') {
     if (vim_strchr((char_u *)"AaIiRrOo", c) != NULL) {
       if (c == 'O' || c == 'o') {
         add_buff(&readbuf2, NL_STR, -1L);
@@ -984,9 +984,9 @@ void ins_char_typebuf(int c)
     buf[0] = K_SPECIAL;
     buf[1] = K_SECOND(c);
     buf[2] = K_THIRD(c);
-    buf[3] = NUL;
+    buf[3] = '\0';
   } else {
-    buf[(*mb_char2bytes)(c, buf)] = NUL;
+    buf[(*mb_char2bytes)(c, buf)] = '\0';
   }
   (void)ins_typebuf(buf, KeyNoremap, 0, !KeyTyped, cmd_silent);
 }
@@ -1111,7 +1111,7 @@ static void gotchars(char_u *chars, int len)
   if (Recording)
     last_recorded_len += len;
 
-  buf[1] = NUL;
+  buf[1] = '\0';
   while (todo--) {
     /* Handle one byte at a time; no translation to be done. */
     c = *s++;
@@ -1543,7 +1543,7 @@ int safe_vgetc(void)
   int c;
 
   c = vgetc();
-  if (c == NUL)
+  if (c == '\0')
     c = get_keystroke();
   return c;
 }
@@ -1600,7 +1600,7 @@ int vpeekc_any(void)
   int c;
 
   c = vpeekc();
-  if (c == NUL && typebuf.tb_len > 0)
+  if (c == '\0' && typebuf.tb_len > 0)
     c = ESC;
   return c;
 }
@@ -1616,7 +1616,7 @@ int char_avail(void)
   ++no_mapping;
   retval = vpeekc();
   --no_mapping;
-  return retval != NUL;
+  return retval != '\0';
 }
 
 void 
@@ -1690,7 +1690,7 @@ static int vgetorpeek(int advance)
   if (vgetc_busy > 0
       && ex_normal_busy == 0
       )
-    return NUL;
+    return '\0';
 
   local_State = get_real_state();
 
@@ -1714,7 +1714,7 @@ static int vgetorpeek(int advance)
     } else {
       c = read_readbuffers(advance);
     }
-    if (c != NUL && !got_int) {
+    if (c != '\0' && !got_int) {
       if (advance) {
         /* KeyTyped = FALSE;  When the command that stuffed something
          * was typed, behave like the stuffed command was typed.
@@ -1920,13 +1920,13 @@ static int vgetorpeek(int advance)
           }
 
           /* Check for match with 'pastetoggle' */
-          if (*p_pt != NUL && mp == NULL && (State & (INSERT|NORMAL))) {
+          if (*p_pt != '\0' && mp == NULL && (State & (INSERT|NORMAL))) {
             for (mlen = 0; mlen < typebuf.tb_len && p_pt[mlen];
                  ++mlen)
               if (p_pt[mlen] != typebuf.tb_buf[typebuf.tb_off
                                                + mlen])
                 break;
-            if (p_pt[mlen] == NUL) {            /* match */
+            if (p_pt[mlen] == '\0') {            /* match */
               /* write chars to script file(s) */
               if (mlen > typebuf.tb_maplen)
                 gotchars(typebuf.tb_buf + typebuf.tb_off
@@ -2103,7 +2103,7 @@ static int vgetorpeek(int advance)
               vgetc_busy = 0;
               save_m_keys = vim_strsave(mp->m_keys);
               save_m_str = vim_strsave(mp->m_str);
-              s = eval_map_expr(save_m_str, NUL);
+              s = eval_map_expr(save_m_str, '\0');
               vgetc_busy = save_vgetc_busy;
             } else
               s = mp->m_str;
@@ -2362,7 +2362,7 @@ static int vgetorpeek(int advance)
 
         if (c < 0)
           continue;                     /* end of input script reached */
-        if (c == NUL) {                 /* no character available */
+        if (c == '\0') {                 /* no character available */
           if (!advance)
             break;
           if (wait_tb_len > 0) {                /* timed out */
@@ -2371,7 +2371,7 @@ static int vgetorpeek(int advance)
           }
         } else {          /* allow mapping for just typed characters */
           while (typebuf.tb_buf[typebuf.tb_off
-                                + typebuf.tb_len] != NUL)
+                                + typebuf.tb_len] != '\0')
             typebuf.tb_noremap[typebuf.tb_off
                                + typebuf.tb_len++] = RM_YES;
 #ifdef USE_IM_CONTROL
@@ -2384,7 +2384,7 @@ static int vgetorpeek(int advance)
     }           /* if (!character from stuffbuf) */
 
     /* if advance is FALSE don't loop on NULs */
-  } while (c < 0 || (advance && c == NUL));
+  } while (c < 0 || (advance && c == '\0'));
 
   /*
    * The "INSERT" message is taken care of here:
@@ -2555,7 +2555,7 @@ fix_input_buffer (
    * Don't replace K_SPECIAL when reading a script file.
    */
   for (i = len; --i >= 0; ++p) {
-    if (p[0] == NUL
+    if (p[0] == '\0'
         || (p[0] == K_SPECIAL
           && !script
           && (i < 2 || p[1] != KS_EXTRA || is_user_input(p[2])))) {
@@ -2567,7 +2567,7 @@ fix_input_buffer (
       len += 2;
     }
   }
-  *p = NUL;             /* add trailing NUL */
+  *p = '\0';             /* add trailing NUL */
   return len;
 }
 
@@ -2750,17 +2750,17 @@ do_map (
   do_backslash = (vim_strchr(p_cpo, CPO_BSLASH) == NULL);
   while (*p && (maptype == 1 || !vim_iswhite(*p))) {
     if ((p[0] == Ctrl_V || (do_backslash && p[0] == '\\')) &&
-        p[1] != NUL)
+        p[1] != '\0')
       ++p;                      /* skip CTRL-V or backslash */
     ++p;
   }
-  if (*p != NUL)
-    *p++ = NUL;
+  if (*p != '\0')
+    *p++ = '\0';
 
   p = skipwhite(p);
   rhs = p;
-  hasarg = (*rhs != NUL);
-  haskey = (*keys != NUL);
+  hasarg = (*rhs != '\0');
+  haskey = (*keys != '\0');
 
   /* check for :unmap without argument */
   if (maptype == 1 && !haskey) {
@@ -2955,7 +2955,7 @@ do_map (
                * "lhs", since an abbreviation can't have
                * trailing space. */
               if (n != len && (!abbrev || round || n > len
-                               || *skipwhite(keys + n) != NUL)) {
+                               || *skipwhite(keys + n) != '\0')) {
                 mpp = &(mp->m_next);
                 continue;
               }
@@ -3168,7 +3168,7 @@ void map_clear(char_u *cmdp, char_u *arg, int forceit, int abbr)
   int local;
 
   local = (STRCMP(arg, "<buffer>") == 0);
-  if (!local && *arg != NUL) {
+  if (!local && *arg != '\0') {
     EMSG(_(e_invarg));
     return;
   }
@@ -3275,7 +3275,7 @@ char_u *map_mode_to_chars(int mode)
     }
   }
 
-  ga_append(&mapmode, NUL);
+  ga_append(&mapmode, '\0');
   return (char_u *)mapmode.ga_data;
 }
 
@@ -3325,7 +3325,7 @@ showmap (
 
   /* Use FALSE below if we only want things like <Up> to show up as such on
    * the rhs, and not M-x etc, TRUE gets both -- webb */
-  if (*mp->m_str == NUL)
+  if (*mp->m_str == '\0')
     msg_puts_attr((char_u *)"<Nop>", hl_attr(HLF_8));
   else {
     /* Remove escaping of CSI, because "m_str" is in a format to be used
@@ -3731,7 +3731,7 @@ int check_abbr(int c, char_u *ptr, int col, int mincol)
           } else
             tb[j++] = c;
         }
-        tb[j] = NUL;
+        tb[j] = '\0';
         /* insert the last typed char */
         (void)ins_typebuf(tb, 1, 0, TRUE, mp->m_silent);
       }
@@ -3749,7 +3749,7 @@ int check_abbr(int c, char_u *ptr, int col, int mincol)
       }
 
       tb[0] = Ctrl_H;
-      tb[1] = NUL;
+      tb[1] = '\0';
       if (has_mbyte)
         len = clen;             /* Delete characters instead of bytes */
       while (len-- > 0)                 /* delete the from string */
@@ -3836,8 +3836,8 @@ char_u *vim_strsave_escape_csi(char_u *p)
   /* Need a buffer to hold up to three times as much. */
   res = alloc((unsigned)(STRLEN(p) * 3) + 1);
   d = res;
-  for (s = p; *s != NUL; ) {
-    if (s[0] == K_SPECIAL && s[1] != NUL && s[2] != NUL) {
+  for (s = p; *s != '\0'; ) {
+    if (s[0] == K_SPECIAL && s[1] != '\0' && s[2] != '\0') {
       /* Copy special key unmodified. */
       *d++ = *s++;
       *d++ = *s++;
@@ -3856,7 +3856,7 @@ char_u *vim_strsave_escape_csi(char_u *p)
       mb_ptr_adv(s);
     }
   }
-  *d = NUL;
+  *d = '\0';
 
   return res;
 }
@@ -3869,7 +3869,7 @@ void vim_unescape_csi(char_u *p)
 {
   char_u      *s = p, *d = p;
 
-  while (*s != NUL) {
+  while (*s != '\0') {
     if (s[0] == K_SPECIAL && s[1] == KS_SPECIAL && s[2] == KE_FILLER) {
       *d++ = K_SPECIAL;
       s += 3;
@@ -3880,7 +3880,7 @@ void vim_unescape_csi(char_u *p)
     } else
       *d++ = *s++;
   }
-  *d = NUL;
+  *d = '\0';
 }
 
 /*
@@ -3931,19 +3931,19 @@ makemap (
 
         /* skip mappings that contain a <SNR> (script-local thing),
          * they probably don't work when loaded again */
-        for (p = mp->m_str; *p != NUL; ++p)
+        for (p = mp->m_str; *p != '\0'; ++p)
           if (p[0] == K_SPECIAL && p[1] == KS_EXTRA
               && p[2] == (int)KE_SNR)
             break;
-        if (*p != NUL)
+        if (*p != '\0')
           continue;
 
         /* It's possible to create a mapping and then ":unmap" certain
          * modes.  We recreate this here by mapping the individual
          * modes, which requires up to three of them. */
-        c1 = NUL;
-        c2 = NUL;
-        c3 = NUL;
+        c1 = '\0';
+        c2 = '\0';
+        c3 = '\0';
         if (abbr)
           cmd = "abbr";
         else
@@ -4025,7 +4025,7 @@ makemap (
           /* When outputting <> form, need to make sure that 'cpo'
            * is set to the Vim default. */
           if (!did_cpo) {
-            if (*mp->m_str == NUL)                      /* will use <Nop> */
+            if (*mp->m_str == '\0')                      /* will use <Nop> */
               did_cpo = TRUE;
             else
               for (i = 0; i < 2; ++i)
@@ -4066,8 +4066,8 @@ makemap (
             return FAIL;
           c1 = c2;
           c2 = c3;
-          c3 = NUL;
-        } while (c1 != NUL);
+          c3 = '\0';
+        } while (c1 != '\0');
       }
     }
 
@@ -4093,20 +4093,20 @@ int put_escstr(FILE *fd, char_u *strstart, int what)
   int modifiers;
 
   /* :map xx <Nop> */
-  if (*str == NUL && what == 1) {
+  if (*str == '\0' && what == 1) {
     if (fprintf(fd, "<Nop>") < 0)
       return FAIL;
     return OK;
   }
 
-  for (; *str != NUL; ++str) {
+  for (; *str != '\0'; ++str) {
     char_u  *p;
 
     /* Check for a multi-byte character, which may contain escaped
      * K_SPECIAL and CSI bytes */
     p = mb_unescape(&str);
     if (p != NULL) {
-      while (*p != NUL)
+      while (*p != '\0')
         if (fputc(*p++, fd) < 0)
           return FAIL;
       --str;
@@ -4231,7 +4231,7 @@ void check_map_keycodes(void)
                 if (*p < 128) {                 /* for "normal" tcap entries */
                   buf[0] = p[0];
                   buf[1] = p[1];
-                  buf[2] = NUL;
+                  buf[2] = '\0';
                   (void)add_termcap_entry(buf, FALSE);
                 }
                 ++p;
@@ -4296,7 +4296,7 @@ check_map (
             minlen = len;
           s = mp->m_keys;
           if (ign_mod && s[0] == K_SPECIAL && s[1] == KS_MODIFIER
-              && s[2] != NUL) {
+              && s[2] != '\0') {
             s += 3;
             if (len > mp->m_keylen - 3)
               minlen = mp->m_keylen - 3;
diff --git a/src/globals.h b/src/globals.h
index d856a4c155..7d0681dc45 100644
--- a/src/globals.h
+++ b/src/globals.h
@@ -550,7 +550,7 @@ EXTERN colnr_T ai_col INIT(= 0);
  * the "end" comment leader when the COM_AUTO_END flag is given for that
  * comment end in 'comments'.  It is only valid when did_ai is TRUE.
  */
-EXTERN int end_comment_pending INIT(= NUL);
+EXTERN int end_comment_pending INIT(= '\0');
 
 /*
  * This flag is set after a ":syncbind" to let the check_scrollbind() function
@@ -900,12 +900,12 @@ EXTERN char_u   *globaldir INIT(= NULL);
 
 /* Characters from 'listchars' option */
 EXTERN int lcs_eol INIT(= '$');
-EXTERN int lcs_ext INIT(= NUL);
-EXTERN int lcs_prec INIT(= NUL);
-EXTERN int lcs_nbsp INIT(= NUL);
-EXTERN int lcs_tab1 INIT(= NUL);
-EXTERN int lcs_tab2 INIT(= NUL);
-EXTERN int lcs_trail INIT(= NUL);
+EXTERN int lcs_ext INIT(= '\0');
+EXTERN int lcs_prec INIT(= '\0');
+EXTERN int lcs_nbsp INIT(= '\0');
+EXTERN int lcs_tab1 INIT(= '\0');
+EXTERN int lcs_tab2 INIT(= '\0');
+EXTERN int lcs_trail INIT(= '\0');
 EXTERN int lcs_conceal INIT(= '-');
 
 /* Characters from 'fillchars' option */
diff --git a/src/hardcopy.c b/src/hardcopy.c
index 2e8939eece..f6636aa561 100644
--- a/src/hardcopy.c
+++ b/src/hardcopy.c
@@ -441,7 +441,7 @@ static void prt_header(prt_settings_T *psettings, int pagenum, linenr_T lnum)
 
   tbuf = xmalloc(width + IOSIZE);
 
-  if (*p_header != NUL) {
+  if (*p_header != '\0') {
     linenr_T tmp_lnum, tmp_topline, tmp_botline;
     int use_sandbox = FALSE;
 
@@ -478,7 +478,7 @@ static void prt_header(prt_settings_T *psettings, int pagenum, linenr_T lnum)
   /* Use a negative line number to indicate printing in the top margin. */
   page_line = 0 - prt_header_height();
   mch_print_start_line(TRUE, page_line);
-  for (p = tbuf; *p != NUL; ) {
+  for (p = tbuf; *p != '\0'; ) {
     if (mch_print_text_out(p,
             (l = (*mb_ptr2len)(p))
             )) {
@@ -535,7 +535,7 @@ void ex_hardcopy(exarg_T *eap)
       return;
     }
     settings.outfile = skipwhite(eap->arg + 1);
-  } else if (*eap->arg != NUL)
+  } else if (*eap->arg != '\0')
     settings.arguments = eap->arg;
 
   /*
@@ -778,7 +778,7 @@ static colnr_T hardcopy_line(prt_settings_T *psettings, int page_line, prt_pos_T
   /*
    * Loop over the columns until the end of the file line or right margin.
    */
-  for (col = ppos->column; line[col] != NUL && !need_break; col += outputlen) {
+  for (col = ppos->column; line[col] != '\0' && !need_break; col += outputlen) {
     outputlen = 1;
     if (has_mbyte && (outputlen = (*mb_ptr2len)(line + col)) < 1)
       outputlen = 1;
@@ -843,7 +843,7 @@ static colnr_T hardcopy_line(prt_settings_T *psettings, int page_line, prt_pos_T
    * line, unless we are doing a formfeed.
    */
   if (!ppos->ff
-      && (line[col] == NUL
+      && (line[col] == '\0'
           || (printer_opts[OPT_PRINT_WRAP].present
               && TOLOWER_ASC(printer_opts[OPT_PRINT_WRAP].string[0])
               == 'n')))
@@ -1584,7 +1584,7 @@ static void prt_resource_name(char_u *filename, void *cookie)
   char_u *resource_filename = cookie;
 
   if (STRLEN(filename) >= MAXPATHL)
-    *resource_filename = NUL;
+    *resource_filename = '\0';
   else
     STRCPY(resource_filename, filename);
 }
@@ -1602,10 +1602,10 @@ static int prt_find_resource(char *name, struct prt_ps_resource_S *resource)
   add_pathsep(buffer);
   vim_strcat(buffer, (char_u *)name, MAXPATHL);
   vim_strcat(buffer, (char_u *)".ps", MAXPATHL);
-  resource->filename[0] = NUL;
+  resource->filename[0] = '\0';
   retval = (do_in_runtimepath(buffer, FALSE, prt_resource_name,
                 resource->filename)
-            && resource->filename[0] != NUL);
+            && resource->filename[0] != '\0');
   vim_free(buffer);
   return retval;
 }
@@ -1746,7 +1746,7 @@ static int prt_open_resource(struct prt_ps_resource_S *resource)
     EMSG2(_("E624: Can't open file \"%s\""), resource->filename);
     return FALSE;
   }
-  memset(prt_resfile.buffer, NUL, PRT_FILE_BUFFER_LEN);
+  memset(prt_resfile.buffer, '\0', PRT_FILE_BUFFER_LEN);
 
   /* Parse first line to ensure valid resource file */
   prt_resfile.len = (int)fread((char *)prt_resfile.buffer, sizeof(char_u),
@@ -2149,7 +2149,7 @@ static int prt_match_charset(char *p_charset, struct prt_ps_mbfont_S *p_cmap, st
   struct prt_ps_charset_S *p_mbchar;
 
   /* Look for recognised character set, using default if one is not given */
-  if (*p_charset == NUL)
+  if (*p_charset == '\0')
     p_charset = p_cmap->defcs;
   char_len = (int)STRLEN(p_charset);
   p_mbchar = p_cmap->charsets;
@@ -2186,7 +2186,7 @@ int mch_print_init(prt_settings_T *psettings, char_u *jobname, int forceit)
    * Set up font and encoding.
    */
   p_encoding = enc_skip(p_penc);
-  if (*p_encoding == NUL)
+  if (*p_encoding == '\0')
     p_encoding = enc_skip(p_enc);
 
   /* Look for a multi-byte font that matches the encoding and character set.
@@ -2195,7 +2195,7 @@ int mch_print_init(prt_settings_T *psettings, char_u *jobname, int forceit)
    * uniquely identify a CJK character set to use. */
   p_mbenc = NULL;
   props = enc_canon_props(p_encoding);
-  if (!(props & ENC_8BIT) && ((*p_pmcs != NUL) || !(props & ENC_UNICODE))) {
+  if (!(props & ENC_8BIT) && ((*p_pmcs != '\0') || !(props & ENC_UNICODE))) {
     p_mbenc_first = NULL;
     for (cmap = 0; cmap < (int)NUM_ELEMENTS(prt_ps_mbfonts); cmap++)
       if (prt_match_encoding((char *)p_encoding, &prt_ps_mbfonts[cmap],
@@ -2215,7 +2215,7 @@ int mch_print_init(prt_settings_T *psettings, char_u *jobname, int forceit)
   prt_out_mbyte = (p_mbenc != NULL);
   if (prt_out_mbyte) {
     /* Build CMap name - will be same for all multi-byte fonts used */
-    prt_cmap[0] = NUL;
+    prt_cmap[0] = '\0';
 
     prt_custom_cmap = (p_mbchar == NULL);
     if (!prt_custom_cmap) {
@@ -2233,7 +2233,7 @@ int mch_print_init(prt_settings_T *psettings, char_u *jobname, int forceit)
       }
     } else {
       /* Add custom CMap character set name */
-      if (*p_pmcs == NUL) {
+      if (*p_pmcs == '\0') {
         EMSG(_("E674: printmbcharset cannot be empty with multi-byte encoding."));
         return FALSE;
       }
@@ -2527,7 +2527,7 @@ int mch_print_begin(prt_settings_T *psettings)
   /* Note: ctime() adds a \n so we have to remove it :-( */
   p = vim_strchr((char_u *)p_time, '\n');
   if (p != NULL)
-    *p = NUL;
+    *p = '\0';
   prt_dsc_textline("CreationDate", p_time);
   prt_dsc_textline("DocumentData", "Clean8Bit");
   prt_dsc_textline("Orientation", "Portrait");
@@ -2605,7 +2605,7 @@ int mch_print_begin(prt_settings_T *psettings)
    */
   if (!prt_out_mbyte) {
     p_encoding = enc_skip(p_penc);
-    if (*p_encoding == NUL
+    if (*p_encoding == '\0'
         || !prt_find_resource((char *)p_encoding, res_encoding)) {
       /* 'printencoding' not set or not supported - find alternate */
       int props;
@@ -2630,7 +2630,7 @@ int mch_print_begin(prt_settings_T *psettings)
      * perform */
   } else {
     p_encoding = enc_skip(p_penc);
-    if (*p_encoding == NUL)
+    if (*p_encoding == '\0')
       p_encoding = enc_skip(p_enc);
     if (prt_use_courier) {
       /* Include ASCII range encoding vector */
@@ -3119,7 +3119,7 @@ int mch_print_text_out(char_u *p, int len)
   }
 
   /* Need to free any translated characters */
-  if (prt_do_conv && (*p != NUL))
+  if (prt_do_conv && (*p != '\0'))
     vim_free(p);
 
   prt_text_run += char_width;
diff --git a/src/hashtab.c b/src/hashtab.c
index 1d3587b3ea..72928f8f0c 100644
--- a/src/hashtab.c
+++ b/src/hashtab.c
@@ -423,7 +423,7 @@ hash_T hash_hash(char_u *key)
 
   // A simplistic algorithm that appears to do very well.
   // Suggested by George Reilly.
-  while (*p != NUL) {
+  while (*p != '\0') {
     hash = hash * 101 + *p++;
   }
 
diff --git a/src/if_cscope.c b/src/if_cscope.c
index 2f283774fc..dc0768f278 100644
--- a/src/if_cscope.c
+++ b/src/if_cscope.c
@@ -181,11 +181,11 @@ void set_context_in_cscope_cmd(expand_T *xp, char_u *arg, cmdidx_T cmdidx)
                 ? EXP_SCSCOPE_SUBCMD : EXP_CSCOPE_SUBCMD;
 
   /* (part of) subcommand already typed */
-  if (*arg != NUL) {
+  if (*arg != '\0') {
     p = skiptowhite(arg);
-    if (*p != NUL) {                /* past first word */
+    if (*p != '\0') {                /* past first word */
       xp->xp_pattern = skipwhite(p);
-      if (*skiptowhite(xp->xp_pattern) != NUL)
+      if (*skiptowhite(xp->xp_pattern) != '\0')
         xp->xp_context = EXPAND_NOTHING;
       else if (STRNICMP(arg, "add", p - arg) == 0)
         xp->xp_context = EXPAND_FILES;
@@ -262,7 +262,7 @@ void do_cstag(exarg_T *eap)
 {
   int ret = FALSE;
 
-  if (*eap->arg == NUL) {
+  if (*eap->arg == '\0') {
     (void)EMSG(_("E562: Usage: cstag <ident>"));
     return;
   }
@@ -598,7 +598,7 @@ static int cs_check_for_connections(void)
 
 static int cs_check_for_tags(void)
 {
-  return p_tags[0] != NUL && curbuf->b_p_tags != NULL;
+  return p_tags[0] != '\0' && curbuf->b_p_tags != NULL;
 } /* cs_check_for_tags */
 
 /*
@@ -960,7 +960,7 @@ static int cs_find(exarg_T *eap)
    * spaces to correctly display the quickfix/location list window's title.
    */
   for (i = 0; i < eap_arg_len; ++i)
-    if (NUL == eap->arg[i])
+    if ('\0' == eap->arg[i])
       eap->arg[i] = ' ';
 
   return cs_find_common(opt, pat, eap->forceit, TRUE,
@@ -1881,11 +1881,11 @@ static int cs_read_prompt(int i)
         if (buf != NULL) {
           /* append character to the message */
           buf[bufpos++] = ch;
-          buf[bufpos] = NUL;
+          buf[bufpos] = '\0';
           if (bufpos >= epromptlen
               && strcmp(&buf[bufpos - epromptlen], eprompt) == 0) {
             /* remove eprompt from buf */
-            buf[bufpos - epromptlen] = NUL;
+            buf[bufpos - epromptlen] = '\0';
 
             /* print message to user */
             (void)EMSG2(cs_emsg, buf);
@@ -1896,7 +1896,7 @@ static int cs_read_prompt(int i)
 
             /* clear buf */
             bufpos = 0;
-            buf[bufpos] = NUL;
+            buf[bufpos] = '\0';
           }
         }
       }
@@ -1906,7 +1906,7 @@ static int cs_read_prompt(int i)
         ch = getc(csinfo[i].fr_fp);
       if (ch == EOF) {
         PERROR("cs_read_prompt EOF");
-        if (buf != NULL && buf[0] != NUL)
+        if (buf != NULL && buf[0] != '\0')
           (void)EMSG2(cs_emsg, buf);
         else if (p_csverbose)
           cs_reading_emsg(i);           /* don't have additional information */
@@ -2159,7 +2159,7 @@ static char *cs_resolve_file(int i, char *name)
       ) {
     fullname = xmalloc(len);
     (void)sprintf(fullname, "%s/%s", csinfo[i].ppath, name);
-  } else if (csdir != NULL && csinfo[i].fname != NULL && *csdir != NUL) {
+  } else if (csdir != NULL && csinfo[i].fname != NULL && *csdir != '\0') {
     /* Check for csdir to be non empty to avoid empty path concatenated to
      * cscope output. */
     fullname = (char *)concat_fnames(csdir, (char_u *)name, TRUE);
diff --git a/src/indent.c b/src/indent.c
index 10530b71f8..05db6838b7 100644
--- a/src/indent.c
+++ b/src/indent.c
@@ -418,7 +418,7 @@ int get_number_indent(linenr_T lnum)
     vim_regfree(regmatch.regprog);
   }
 
-  if ((pos.lnum == 0) || (*ml_get_pos(&pos) == NUL)) {
+  if ((pos.lnum == 0) || (*ml_get_pos(&pos) == '\0')) {
     return -1;
   }
   getvcol(curwin, &pos, &col, NULL, NULL);
@@ -548,29 +548,29 @@ int get_lisp_indent(void)
         continue;
       }
 
-      for (that = ml_get_curline(); *that != NUL; ++that) {
+      for (that = ml_get_curline(); *that != '\0'; ++that) {
         if (*that == ';') {
-          while (*(that + 1) != NUL) {
+          while (*(that + 1) != '\0') {
             that++;
           }
           continue;
         }
 
         if (*that == '\\') {
-          if (*(that + 1) != NUL) {
+          if (*(that + 1) != '\0') {
             that++;
           }
           continue;
         }
 
-        if ((*that == '"') && (*(that + 1) != NUL)) {
+        if ((*that == '"') && (*(that + 1) != '\0')) {
           while (*++that && *that != '"') {
             // Skipping escaped characters in the string
             if (*that == '\\') {
-              if (*++that == NUL) {
+              if (*++that == '\0') {
                 break;
               }
-              if (that[1] == NUL) {
+              if (that[1] == '\0') {
                 that++;
                 break;
               }
@@ -649,7 +649,7 @@ int get_lisp_indent(void)
                 if (((*that == ')') || (*that == ']')) && !quotecount) {
                   parencount--;
                 }
-                if ((*that == '\\') && (*(that + 1) != NUL)) {
+                if ((*that == '\\') && (*(that + 1) != '\0')) {
                   amount += lbr_chartabsize_adv(&that, (colnr_T)amount);
                 }
 
@@ -684,7 +684,7 @@ static int lisp_match(char_u *p)
   int len;
   char_u *word = p_lispwords;
 
-  while (*word != NUL) {
+  while (*word != '\0') {
     (void)copy_option_part(&word, buf, LSIZE, ",");
     len = (int)STRLEN(buf);
 
diff --git a/src/indent_c.c b/src/indent_c.c
index 08b28c73f2..8b90b0f855 100644
--- a/src/indent_c.c
+++ b/src/indent_c.c
@@ -83,7 +83,7 @@ static char_u *skip_string(char_u *p)
       }
     } else if (p[0] == '"') {             /* start of string */
       for (++p; p[0]; ++p) {
-        if (p[0] == '\\' && p[1] != NUL)
+        if (p[0] == '\\' && p[1] != '\0')
           ++p;
         else if (p[0] == '"')               /* end of string */
           break;
@@ -214,7 +214,7 @@ static char_u *cin_skipcomment(char_u *s)
  */
 static int cin_nocode(char_u *s)
 {
-  return *cin_skipcomment(s) == NUL;
+  return *cin_skipcomment(s) == '\0';
 }
 
 /*
@@ -234,7 +234,7 @@ static pos_T *find_line_comment(void)   /* XXX */
       pos.col = (int)(p - line);
       return &pos;
     }
-    if (*p != NUL)
+    if (*p != '\0')
       break;
   }
   return NULL;
@@ -299,7 +299,7 @@ int cin_islabel(void)
       line = ml_get_curline();
       if (cin_ispreproc(line))          /* ignore #defines, #if, etc. */
         continue;
-      if (*(line = cin_skipcomment(line)) == NUL)
+      if (*(line = cin_skipcomment(line)) == '\0')
         continue;
 
       curwin->w_cursor = cursor_save;
@@ -435,9 +435,9 @@ static int cin_is_cpp_namespace(char_u *s)
   int has_name = FALSE;
 
   s = cin_skipcomment(s);
-  if (STRNCMP(s, "namespace", 9) == 0 && (s[9] == NUL || !vim_iswordc(s[9]))) {
+  if (STRNCMP(s, "namespace", 9) == 0 && (s[9] == '\0' || !vim_iswordc(s[9]))) {
     p = cin_skipcomment(skipwhite(s + 9));
-    while (*p != NUL) {
+    while (*p != '\0') {
       if (vim_iswhite(*p)) {
         has_name = TRUE;         /* found end of a name */
         p = cin_skipcomment(skipwhite(p));
@@ -473,10 +473,10 @@ static char_u *after_label(char_u *l)
     } else if (*l == '\'' && l[1] && l[2] == '\'')
       l += 2;                       /* skip over 'x' */
   }
-  if (*l == NUL)
+  if (*l == '\0')
     return NULL;
   l = cin_skipcomment(l + 1);
-  if (*l == NUL)
+  if (*l == '\0')
     return NULL;
   return l;
 }
@@ -601,12 +601,12 @@ static int cin_get_equal_amount(linenr_T lnum)
 
   if (lnum > 1) {
     line = ml_get(lnum - 1);
-    if (*line != NUL && line[STRLEN(line) - 1] == '\\')
+    if (*line != '\0' && line[STRLEN(line) - 1] == '\\')
       return -1;
   }
 
   line = s = ml_get(lnum);
-  while (*s != NUL && vim_strchr((char_u *)"=;{}\"'", *s) == NULL) {
+  while (*s != '\0' && vim_strchr((char_u *)"=;{}\"'", *s) == NULL) {
     if (cin_iscomment(s))       /* ignore comments */
       s = cin_skipcomment(s);
     else
@@ -658,7 +658,7 @@ static int cin_ispreproc_cont(char_u **pp, linenr_T *lnump)
     if (lnum == 1)
       break;
     line = ml_get(--lnum);
-    if (*line == NUL || line[STRLEN(line) - 1] != '\\')
+    if (*line == '\0' || line[STRLEN(line) - 1] != '\\')
       break;
   }
 
@@ -787,11 +787,11 @@ static int cin_isfuncdecl(char_u **sp, linenr_T first_lnum, linenr_T min_lnum)
        */
       lnum = first_lnum - 1;
       s = ml_get(lnum);
-      if (*s == NUL || s[STRLEN(s) - 1] != '\\')
+      if (*s == '\0' || s[STRLEN(s) - 1] != '\\')
         retval = TRUE;
       goto done;
     }
-    if ((*s == ',' && cin_nocode(s + 1)) || s[1] == NUL || cin_nocode(s)) {
+    if ((*s == ',' && cin_nocode(s + 1)) || s[1] == '\0' || cin_nocode(s)) {
       int comma = (*s == ',');
 
       /* ',' at the end: continue looking in the next line.
@@ -941,7 +941,7 @@ static int cin_iswhileofdo_end(int terminated)
     return FALSE;
 
   p = line = ml_get_curline();
-  while (*p != NUL) {
+  while (*p != '\0') {
     p = cin_skipcomment(p);
     if (*p == ')') {
       s = skipwhite(p + 1);
@@ -966,7 +966,7 @@ static int cin_iswhileofdo_end(int terminated)
         p = line + i;
       }
     }
-    if (*p != NUL)
+    if (*p != '\0')
       ++p;
   }
   return FALSE;
@@ -1006,7 +1006,7 @@ cin_is_cpp_baseclass (
   if (*s == '#')                /* skip #define FOO x ? (x) : x */
     return FALSE;
   s = cin_skipcomment(s);
-  if (*s == NUL)
+  if (*s == '\0')
     return FALSE;
 
   cpp_base_class = lookfor_ctor_init = class_or_struct = FALSE;
@@ -1027,17 +1027,17 @@ cin_is_cpp_baseclass (
   while (lnum > 1) {
     line = ml_get(lnum - 1);
     s = skipwhite(line);
-    if (*s == '#' || *s == NUL)
+    if (*s == '#' || *s == '\0')
       break;
-    while (*s != NUL) {
+    while (*s != '\0') {
       s = cin_skipcomment(s);
       if (*s == '{' || *s == '}'
           || (*s == ';' && cin_nocode(s + 1)))
         break;
-      if (*s != NUL)
+      if (*s != '\0')
         ++s;
     }
-    if (*s != NUL)
+    if (*s != '\0')
       break;
     --lnum;
   }
@@ -1045,13 +1045,13 @@ cin_is_cpp_baseclass (
   line = ml_get(lnum);
   s = cin_skipcomment(line);
   for (;; ) {
-    if (*s == NUL) {
+    if (*s == '\0') {
       if (lnum == curwin->w_cursor.lnum)
         break;
       /* Continue in the cursor line. */
       line = ml_get(++lnum);
       s = cin_skipcomment(line);
-      if (*s == NUL)
+      if (*s == '\0')
         continue;
     }
 
@@ -1150,7 +1150,7 @@ static int cin_ends_in(char_u *s, char_u *find, char_u *ignore)
   char_u      *r;
   int len = (int)STRLEN(find);
 
-  while (*p != NUL) {
+  while (*p != '\0') {
     p = cin_skipcomment(p);
     if (STRNCMP(p, find, len) == 0) {
       r = skipwhite(p + len);
@@ -1159,7 +1159,7 @@ static int cin_ends_in(char_u *s, char_u *find, char_u *ignore)
       if (cin_nocode(r))
         return TRUE;
     }
-    if (*p != NUL)
+    if (*p != '\0')
       ++p;
   }
   return FALSE;
@@ -1286,7 +1286,7 @@ static int find_last_paren(char_u *l, int start, int end)
 
   curwin->w_cursor.col = 0;                 /* default is start of line */
 
-  for (i = 0; l[i] != NUL; i++) {
+  for (i = 0; l[i] != '\0'; i++) {
     i = (int)(cin_skipcomment(l + i) - l);     /* ignore parens in comments */
     i = (int)(skip_string(l + i) - l);        /* ignore parens in quotes */
     if (l[i] == start)
@@ -1586,7 +1586,7 @@ int get_c_indent(void)
   if ((State & INSERT)
       && curwin->w_cursor.col < (colnr_T)STRLEN(linecopy)
       && linecopy[curwin->w_cursor.col] == ')')
-    linecopy[curwin->w_cursor.col] = NUL;
+    linecopy[curwin->w_cursor.col] = '\0';
 
   theline = skipwhite(linecopy);
 
@@ -1641,16 +1641,16 @@ int get_c_indent(void)
     /* find how indented the line beginning the comment is */
     getvcol(curwin, trypos, &col, NULL, NULL);
     amount = col;
-    *lead_start = NUL;
-    *lead_middle = NUL;
+    *lead_start = '\0';
+    *lead_middle = '\0';
 
     p = curbuf->b_p_com;
-    while (*p != NUL) {
+    while (*p != '\0') {
       int align = 0;
       int off = 0;
       int what = 0;
 
-      while (*p != NUL && *p != ':') {
+      while (*p != '\0' && *p != ':') {
         if (*p == COM_START || *p == COM_END || *p == COM_MIDDLE)
           what = *p++;
         else if (*p == COM_LEFT || *p == COM_RIGHT)
@@ -1749,12 +1749,12 @@ int get_c_indent(void)
         if (!curbuf->b_ind_in_comment2) {
           start = ml_get(trypos->lnum);
           look = start + trypos->col + 2;           /* skip / and * */
-          if (*look != NUL)                         /* if something after it */
+          if (*look != '\0')                         /* if something after it */
             trypos->col = (colnr_T)(skipwhite(look) - start);
         }
         getvcol(curwin, trypos, &col, NULL, NULL);
         amount = col;
-        if (curbuf->b_ind_in_comment2 || *look == NUL)
+        if (curbuf->b_ind_in_comment2 || *look == '\0')
           amount += curbuf->b_ind_in_comment;
       }
     }
@@ -1919,7 +1919,7 @@ int get_c_indent(void)
               col = our_paren_pos.col + 1;
               while (vim_iswhite(l[col]))
                 col++;
-              if (l[col] != NUL)                /* In case of trailing space */
+              if (l[col] != '\0')                /* In case of trailing space */
                 our_paren_pos.col = col;
               else
                 our_paren_pos.col++;
@@ -2561,7 +2561,7 @@ int get_c_indent(void)
             if (terminated == ',') {
               while (curwin->w_cursor.lnum > 1) {
                 l = ml_get(curwin->w_cursor.lnum - 1);
-                if (*l == NUL || l[STRLEN(l) - 1] != '\\')
+                if (*l == '\0' || l[STRLEN(l) - 1] != '\\')
                   break;
                 --curwin->w_cursor.lnum;
                 curwin->w_cursor.col = 0;
@@ -2746,7 +2746,7 @@ int get_c_indent(void)
                   cont_amount = cin_first_id_amount();
                 } else {
                   if (lookfor == LOOKFOR_INITIAL
-                      && *l != NUL
+                      && *l != '\0'
                       && l[STRLEN(l) - 1] == '\\')
                     /* XXX */
                     cont_amount = cin_get_equal_amount(
@@ -3053,7 +3053,7 @@ term_again:
          */
         n = 0;
         if (cin_ends_in(l, (char_u *)",", NULL)
-            || (*l != NUL && (n = l[STRLEN(l) - 1]) == '\\')) {
+            || (*l != '\0' && (n = l[STRLEN(l) - 1]) == '\\')) {
           /* take us back to opening paren */
           if (find_last_paren(l, '(', ')')
               && (trypos = find_match_paren(
@@ -3068,7 +3068,7 @@ term_again:
            */
           while (n == 0 && curwin->w_cursor.lnum > 1) {
             l = ml_get(curwin->w_cursor.lnum - 1);
-            if (*l == NUL || l[STRLEN(l) - 1] != '\\')
+            if (*l == '\0' || l[STRLEN(l) - 1] != '\\')
               break;
             --curwin->w_cursor.lnum;
             curwin->w_cursor.col = 0;
@@ -3148,7 +3148,7 @@ term_again:
         if (cin_ends_in(l, (char_u *)";", NULL)) {
           l = ml_get(curwin->w_cursor.lnum - 1);
           if (cin_ends_in(l, (char_u *)",", NULL)
-              || (*l != NUL && l[STRLEN(l) - 1] == '\\'))
+              || (*l != '\0' && l[STRLEN(l) - 1] == '\\'))
             break;
           l = ml_get_curline();
         }
@@ -3180,7 +3180,7 @@ term_again:
        */
       if (cur_curpos.lnum > 1) {
         l = ml_get(cur_curpos.lnum - 1);
-        if (*l != NUL && l[STRLEN(l) - 1] == '\\') {
+        if (*l != '\0' && l[STRLEN(l) - 1] == '\\') {
           cur_amount = cin_get_equal_amount(cur_curpos.lnum - 1);
           if (cur_amount > 0)
             amount = cur_amount;
@@ -3309,7 +3309,7 @@ static int find_match(int lookfor, linenr_T ourscope)
  */
 void do_c_expr_indent(void)
 {
-  if (*curbuf->b_p_inde != NUL)
+  if (*curbuf->b_p_inde != '\0')
     fixthisline(get_expr_indent);
   else
     fixthisline(get_c_indent);
diff --git a/src/keymap.c b/src/keymap.c
index bec179c913..02d5903272 100644
--- a/src/keymap.c
+++ b/src/keymap.c
@@ -29,7 +29,7 @@ static struct modmasktable {
   {MOD_MASK_MULTI_CLICK,      MOD_MASK_4CLICK,        (char_u)'4'},
   /* 'A' must be the last one */
   {MOD_MASK_ALT,              MOD_MASK_ALT,           (char_u)'A'},
-  {0, 0, NUL}
+  {0, 0, '\0'}
 };
 
 /*
@@ -125,7 +125,7 @@ static char_u modifier_keys_table[] =
   /* TAB pseudo code*/
   MOD_MASK_SHIFT, 'k', 'B',                   KS_EXTRA, (int)KE_TAB,
 
-  NUL
+  '\0'
 };
 
 static struct key_name_entry {
@@ -349,7 +349,7 @@ int simplify_key(int key, int *modifiers)
     }
     key0 = KEY2TERMCAP0(key);
     key1 = KEY2TERMCAP1(key);
-    for (i = 0; modifier_keys_table[i] != NUL; i += MOD_KEYS_ENTRY_SIZE)
+    for (i = 0; modifier_keys_table[i] != '\0'; i += MOD_KEYS_ENTRY_SIZE)
       if (key0 == modifier_keys_table[i + 3]
           && key1 == modifier_keys_table[i + 4]
           && (*modifiers & modifier_keys_table[i])) {
@@ -477,7 +477,7 @@ char_u *get_special_key_name(int c, int modifiers)
     idx = (int)STRLEN(string);
   }
   string[idx++] = '>';
-  string[idx] = NUL;
+  string[idx] = '\0';
   return string;
 }
 
@@ -555,7 +555,7 @@ find_special_key (
   for (bp = src + 1; *bp == '-' || vim_isIDc(*bp); bp++) {
     if (*bp == '-') {
       last_dash = bp;
-      if (bp[1] != NUL) {
+      if (bp[1] != '\0') {
         if (has_mbyte)
           l = mb_ptr2len(bp + 1);
         else
@@ -617,7 +617,7 @@ find_special_key (
        * get_special_key_code() may return NUL for invalid
        * special key name.
        */
-      if (key != NUL) {
+      if (key != '\0') {
         /*
          * Only use a modifier when there is no special key code that
          * includes the modifier.
@@ -711,19 +711,19 @@ int get_special_key_code(char_u *name)
   /*
    * If it's <t_xx> we get the code for xx from the termcap
    */
-  if (name[0] == 't' && name[1] == '_' && name[2] != NUL && name[3] != NUL) {
+  if (name[0] == 't' && name[1] == '_' && name[2] != '\0' && name[3] != '\0') {
     string[0] = name[2];
     string[1] = name[3];
-    string[2] = NUL;
+    string[2] = '\0';
     if (add_termcap_entry(string, FALSE) == OK)
       return TERMCAP2KEY(name[2], name[3]);
   } else
     for (i = 0; key_names_table[i].name != NULL; i++) {
       table_name = key_names_table[i].name;
-      for (j = 0; vim_isIDc(name[j]) && table_name[j] != NUL; j++)
+      for (j = 0; vim_isIDc(name[j]) && table_name[j] != '\0'; j++)
         if (TOLOWER_ASC(table_name[j]) != TOLOWER_ASC(name[j]))
           break;
-      if (!vim_isIDc(name[j]) && table_name[j] == NUL)
+      if (!vim_isIDc(name[j]) && table_name[j] == '\0')
         return key_names_table[i].key;
     }
   return 0;
diff --git a/src/keymap.h b/src/keymap.h
index 244e9b3ddf..da968dba00 100644
--- a/src/keymap.h
+++ b/src/keymap.h
@@ -131,10 +131,10 @@
  * get second or third byte when translating special key code into three bytes
  */
 #define K_SECOND(c)     ((c) == K_SPECIAL ? KS_SPECIAL : (c) == \
-                         NUL ? KS_ZERO : KEY2TERMCAP0(c))
+                         '\0' ? KS_ZERO : KEY2TERMCAP0(c))
 
 #define K_THIRD(c)      (((c) == K_SPECIAL || (c) == \
-                          NUL) ? KE_FILLER : KEY2TERMCAP1(c))
+                          '\0') ? KE_FILLER : KEY2TERMCAP1(c))
 
 /*
  * get single int code from second byte after K_SPECIAL
diff --git a/src/macros.h b/src/macros.h
index 8dd412f838..7793ab326c 100644
--- a/src/macros.h
+++ b/src/macros.h
@@ -31,13 +31,13 @@
 /*
  * lineempty() - return TRUE if the line is empty
  */
-#define lineempty(p) (*ml_get(p) == NUL)
+#define lineempty(p) (*ml_get(p) == '\0')
 
 /*
  * bufempty() - return TRUE if the current buffer is empty
  */
 #define bufempty() (curbuf->b_ml.ml_line_count == 1 && *ml_get((linenr_T)1) == \
-                    NUL)
+                    '\0')
 
 /*
  * toupper() and tolower() that use the current locale.
diff --git a/src/main.c b/src/main.c
index 105f676bbd..83886b94b0 100644
--- a/src/main.c
+++ b/src/main.c
@@ -375,7 +375,7 @@ static char *(main_errors[]) =
    * Read in registers, history etc, but not marks, from the viminfo file.
    * This is where v:oldfiles gets filled.
    */
-  if (*p_viminfo != NUL) {
+  if (*p_viminfo != '\0') {
     read_viminfo(NULL, VIF_WANT_INFO | VIF_GET_OLDFILES);
     TIME_MSG("reading viminfo");
   }
@@ -413,7 +413,7 @@ static char *(main_errors[]) =
 #if defined(UNIX)
   /* When switching screens and something caused a message from a vimrc
    * script, need to output an extra newline on exit. */
-  if ((did_emsg || msg_didout) && *T_TI != NUL)
+  if ((did_emsg || msg_didout) && *T_TI != '\0')
     newline_on_exit = TRUE;
 #endif
 
@@ -830,7 +830,7 @@ void getout(int exitval)
     apply_autocmds(EVENT_VIMLEAVEPRE, NULL, NULL, FALSE, curbuf);
   }
 
-  if (p_viminfo && *p_viminfo != NUL)
+  if (p_viminfo && *p_viminfo != '\0')
     /* Write out the registers, history, marks etc, to the viminfo file */
     write_viminfo(NULL, FALSE);
 
@@ -899,7 +899,7 @@ static void init_locale(void)
     /* expand_env() doesn't work yet, because chartab[] is not initialized
      * yet, call vim_getenv() directly */
     p = vim_getenv((char_u *)"VIMRUNTIME", &mustfree);
-    if (p != NULL && *p != NUL) {
+    if (p != NULL && *p != '\0') {
       vim_snprintf((char *)NameBuff, MAXPATHL, "%s/lang", p);
       bindtextdomain(VIMPACKAGE, (char *)NameBuff);
     }
@@ -1012,7 +1012,7 @@ static void command_line_scan(mparm_T *parmp)
       if (parmp->n_commands >= MAX_ARG_CMDS)
         mainerr(ME_EXTRA_CMD, NULL);
       argv_idx = -1;                /* skip to next argument */
-      if (argv[0][1] == NUL)
+      if (argv[0][1] == '\0')
         parmp->commands[parmp->n_commands++] = (char_u *)"$";
       else
         parmp->commands[parmp->n_commands++] = (char_u *)&(argv[0][1]);
@@ -1024,7 +1024,7 @@ static void command_line_scan(mparm_T *parmp)
       want_argument = FALSE;
       c = argv[0][argv_idx++];
       switch (c) {
-        case NUL:                 /* "vim -"  read from stdin */
+        case '\0':                 /* "vim -"  read from stdin */
           /* "ex -" silent mode */
           if (exmode_active)
             silent_mode = TRUE;
@@ -1226,7 +1226,7 @@ static void command_line_scan(mparm_T *parmp)
         case 'V':                 /* "-V{N}"	Verbose level */
           /* default is 10: a little bit verbose */
           p_verbose = get_number_arg((char_u *)argv[0], &argv_idx, 10);
-          if (argv[0][argv_idx] != NUL) {
+          if (argv[0][argv_idx] != '\0') {
             set_option_value((char_u *)"verbosefile", 0L,
                 (char_u *)argv[0] + argv_idx, 0);
             argv_idx = (int)STRLEN(argv[0]);
@@ -1260,7 +1260,7 @@ static void command_line_scan(mparm_T *parmp)
 
         case 'c':                 /* "-c{command}" or "-c {command}" execute
                                      command */
-          if (argv[0][argv_idx] != NUL) {
+          if (argv[0][argv_idx] != '\0') {
             if (parmp->n_commands >= MAX_ARG_CMDS)
               mainerr(ME_EXTRA_CMD, NULL);
             parmp->commands[parmp->n_commands++] = (char_u *)argv[0]
@@ -1289,7 +1289,7 @@ static void command_line_scan(mparm_T *parmp)
         /*
          * Check for garbage immediately after the option letter.
          */
-        if (argv[0][argv_idx] != NUL)
+        if (argv[0][argv_idx] != '\0')
           mainerr(ME_GARBAGE, (char_u *)argv[0]);
 
         --argc;
@@ -1456,7 +1456,7 @@ scripterror:
      * argument.  argv_idx is set to -1 when the current argument is to be
      * skipped.
      */
-    if (argv_idx <= 0 || argv[0][argv_idx] == NUL) {
+    if (argv_idx <= 0 || argv[0][argv_idx] == '\0') {
       --argc;
       ++argv;
       argv_idx = 1;
@@ -2102,7 +2102,7 @@ process_env (
   scid_T save_sid;
 
   initstr = (char_u *)os_getenv((char *)env);
-  if (initstr != NULL && *initstr != NUL) {
+  if (initstr != NULL && *initstr != '\0') {
     if (is_viminit)
       vimrc_found(NULL, NULL);
     save_sourcing_name = sourcing_name;
diff --git a/src/mark.c b/src/mark.c
index bd37f93eb1..09eb8a845a 100644
--- a/src/mark.c
+++ b/src/mark.c
@@ -110,7 +110,7 @@ int setmark_pos(int c, pos_T *pos, int fnum)
       curbuf->b_visual.vi_start = *pos;
     else
       curbuf->b_visual.vi_end = *pos;
-    if (curbuf->b_visual.vi_mode == NUL)
+    if (curbuf->b_visual.vi_mode == '\0')
       /* Visual_mode has not yet been set, use a sane default. */
       curbuf->b_visual.vi_mode = 'v';
     return OK;
@@ -336,7 +336,7 @@ pos_T *getmark_buf_fnum(buf_T *buf, int c, int changefile, int *fnum)
     pos = curwin->w_cursor;
     listcmd_busy = TRUE;            /* avoid that '' is changed */
     if (findpar(&oa.inclusive,
-            c == '}' ? FORWARD : BACKWARD, 1L, NUL, FALSE)) {
+            c == '}' ? FORWARD : BACKWARD, 1L, '\0', FALSE)) {
       pos_copy = curwin->w_cursor;
       posp = &pos_copy;
     }
@@ -609,12 +609,12 @@ static char_u *mark_line(pos_T *mp, int lead_len)
     return NULL;
   /* Truncate the line to fit it in the window */
   len = 0;
-  for (p = s; *p != NUL; mb_ptr_adv(p)) {
+  for (p = s; *p != '\0'; mb_ptr_adv(p)) {
     len += ptr2cells(p);
     if (len >= Columns - lead_len)
       break;
   }
-  *p = NUL;
+  *p = '\0';
   return s;
 }
 
@@ -627,7 +627,7 @@ void do_marks(exarg_T *eap)
   int i;
   char_u      *name;
 
-  if (arg != NULL && *arg == NUL)
+  if (arg != NULL && *arg == '\0')
     arg = NULL;
 
   show_one_mark('\'', arg, &curwin->w_pcmark, NULL, TRUE);
@@ -717,16 +717,16 @@ void ex_delmarks(exarg_T *eap)
   int digit;
   int n;
 
-  if (*eap->arg == NUL && eap->forceit)
+  if (*eap->arg == '\0' && eap->forceit)
     /* clear all marks */
     clrallmarks(curbuf);
   else if (eap->forceit)
     EMSG(_(e_invarg));
-  else if (*eap->arg == NUL)
+  else if (*eap->arg == '\0')
     EMSG(_(e_argreq));
   else {
     /* clear specified marks only */
-    for (p = eap->arg; *p != NUL; ++p) {
+    for (p = eap->arg; *p != '\0'; ++p) {
       lower = ASCII_ISLOWER(*p);
       digit = VIM_ISDIGIT(*p);
       if (lower || digit || ASCII_ISUPPER(*p)) {
@@ -1293,7 +1293,7 @@ static void write_one_filemark(FILE *fp, xfmark_T *fm, int c1, int c2)
     name = buflist_nr2name(fm->fmark.fnum, TRUE, FALSE);
   else
     name = fm->fname;                   /* use name from .viminfo */
-  if (name != NULL && *name != NUL) {
+  if (name != NULL && *name != '\0') {
     fprintf(fp, "%c%c  %" PRId64 "  %" PRId64 "  ",
             c1, c2, (int64_t)fm->fmark.mark.lnum, (int64_t)fm->fmark.mark.col);
     viminfo_writestring(fp, name);
@@ -1370,7 +1370,7 @@ int write_viminfo_marks(FILE *fp_out)
           }
       }
       if (is_mark_set && buf->b_ffname != NULL
-          && buf->b_ffname[0] != NUL && !removable(buf->b_ffname)) {
+          && buf->b_ffname[0] != '\0' && !removable(buf->b_ffname)) {
         home_replace(NULL, buf->b_ffname, IObuff, IOSIZE, TRUE);
         fprintf(fp_out, "\n> ");
         viminfo_writestring(fp_out, IObuff);
@@ -1418,7 +1418,7 @@ void copy_viminfo_marks(vir_T *virp, FILE *fp_out, int count, int eof, int flags
   list_T      *list = NULL;
 
   name_buf = alloc(LSIZE);
-  *name_buf = NUL;
+  *name_buf = '\0';
 
   if (fp_out == NULL && (flags & (VIF_GET_OLDFILES | VIF_FORCEIT))) {
     list = list_alloc();
@@ -1447,11 +1447,11 @@ void copy_viminfo_marks(vir_T *virp, FILE *fp_out, int count, int eof, int flags
     if (str == NULL)
       continue;
     p = str + STRLEN(str);
-    while (p != str && (*p == NUL || vim_isspace(*p)))
+    while (p != str && (*p == '\0' || vim_isspace(*p)))
       p--;
     if (*p)
       p++;
-    *p = NUL;
+    *p = '\0';
 
     if (list != NULL)
       list_append_string(list, str, -1);
@@ -1463,7 +1463,7 @@ void copy_viminfo_marks(vir_T *virp, FILE *fp_out, int count, int eof, int flags
     load_marks = copy_marks_out = FALSE;
     if (fp_out == NULL) {
       if ((flags & VIF_WANT_MARKS) && curbuf->b_ffname != NULL) {
-        if (*name_buf == NUL)               /* only need to do this once */
+        if (*name_buf == '\0')               /* only need to do this once */
           home_replace(NULL, curbuf->b_ffname, name_buf, LSIZE, TRUE);
         if (fnamecmp(str, name_buf) == 0)
           load_marks = TRUE;
@@ -1492,7 +1492,7 @@ void copy_viminfo_marks(vir_T *virp, FILE *fp_out, int count, int eof, int flags
     pos.coladd = 0;
     while (!(eof = viminfo_readline(virp)) && line[0] == TAB) {
       if (load_marks) {
-        if (line[1] != NUL) {
+        if (line[1] != '\0') {
           int64_t lnum_64;
           unsigned u;
           sscanf((char *)line + 2, "%" SCNd64 "%u", &lnum_64, &u);
diff --git a/src/mbyte.c b/src/mbyte.c
index f150b9b9ca..46471d1181 100644
--- a/src/mbyte.c
+++ b/src/mbyte.c
@@ -526,7 +526,7 @@ char_u * mb_init()
       n = 1;
     else {
       char buf[MB_MAXBYTES + 1];
-      if (i == NUL)             /* just in case mblen() can't handle "" */
+      if (i == '\0')             /* just in case mblen() can't handle "" */
         n = 1;
       else {
         buf[0] = i;
@@ -608,7 +608,7 @@ int bomb_size()
   int n = 0;
 
   if (curbuf->b_p_bomb && !curbuf->b_p_bin) {
-    if (*curbuf->b_p_fenc == NUL) {
+    if (*curbuf->b_p_fenc == '\0') {
       if (enc_utf8) {
         if (enc_unicode != 0)
           n = enc_unicode;
@@ -658,13 +658,13 @@ int mb_get_class(char_u *p)
 int mb_get_class_buf(char_u *p, buf_T *buf)
 {
   if (MB_BYTE2LEN(p[0]) == 1) {
-    if (p[0] == NUL || vim_iswhite(p[0]))
+    if (p[0] == '\0' || vim_iswhite(p[0]))
       return 0;
     if (vim_iswordc_buf(p[0], buf))
       return 2;
     return 1;
   }
-  if (enc_dbcs != 0 && p[0] != NUL && p[1] != NUL)
+  if (enc_dbcs != 0 && p[0] != '\0' && p[1] != '\0')
     return dbcs_class(p[0], p[1]);
   if (enc_utf8)
     return utf_class(utf_ptr2char(p));
@@ -849,7 +849,7 @@ static int dbcs_char2bytes(int c, char_u *buf)
     buf[1] = c;
     /* Never use a NUL byte, it causes lots of trouble.  It's an invalid
      * character anyway. */
-    if (buf[1] == NUL)
+    if (buf[1] == '\0')
       buf[1] = '\n';
     return 2;
   }
@@ -874,7 +874,7 @@ static int dbcs_ptr2len(char_u *p)
 
   /* Check if second byte is not missing. */
   len = MB_BYTE2LEN(*p);
-  if (len == 2 && p[1] == NUL)
+  if (len == 2 && p[1] == '\0')
     len = 1;
   return len;
 }
@@ -887,7 +887,7 @@ static int dbcs_ptr2len(char_u *p)
  */
 int latin_ptr2len_len(char_u *p, int size)
 {
-  if (size < 1 || *p == NUL)
+  if (size < 1 || *p == '\0')
     return 0;
   return 1;
 }
@@ -896,13 +896,13 @@ static int dbcs_ptr2len_len(char_u *p, int size)
 {
   int len;
 
-  if (size < 1 || *p == NUL)
+  if (size < 1 || *p == '\0')
     return 0;
   if (size == 1)
     return 1;
   /* Check that second byte is not missing. */
   len = MB_BYTE2LEN(*p);
-  if (len == 2 && p[1] == NUL)
+  if (len == 2 && p[1] == '\0')
     len = 1;
   return len;
 }
@@ -1225,7 +1225,7 @@ int utf_ptr2cells(char_u *p)
   if (*p >= 0x80) {
     c = utf_ptr2char(p);
     /* An illegal byte is displayed as <xx>. */
-    if (utf_ptr2len(p) == 1 || c == NUL)
+    if (utf_ptr2len(p) == 1 || c == '\0')
       return 4;
     /* If the char is ASCII it must be an overlong sequence. */
     if (c < 0x80)
@@ -1264,7 +1264,7 @@ static int utf_ptr2cells_len(char_u *p, int size)
       return 1;        /* truncated */
     c = utf_ptr2char(p);
     /* An illegal byte is displayed as <xx>. */
-    if (utf_ptr2len(p) == 1 || c == NUL)
+    if (utf_ptr2len(p) == 1 || c == '\0')
       return 4;
     /* If the char is ASCII it must be an overlong sequence. */
     if (c < 0x80)
@@ -1312,7 +1312,7 @@ int mb_string2cells(char_u *p, int len)
   int i;
   int clen = 0;
 
-  for (i = 0; (len < 0 || i < len) && p[i] != NUL; i += (*mb_ptr2len)(p + i))
+  for (i = 0; (len < 0 || i < len) && p[i] != '\0'; i += (*mb_ptr2len)(p + i))
     clen += (*mb_ptr2cells)(p + i);
   return clen;
 }
@@ -1356,7 +1356,7 @@ int latin_ptr2char(char_u *p)
 
 static int dbcs_ptr2char(char_u *p)
 {
-  if (MB_BYTE2LEN(*p) > 1 && p[1] != NUL)
+  if (MB_BYTE2LEN(*p) > 1 && p[1] != '\0')
     return (p[0] << 8) + p[1];
   return *p;
 }
@@ -1614,7 +1614,7 @@ int utf_ptr2len(char_u *p)
   int len;
   int i;
 
-  if (*p == NUL)
+  if (*p == '\0')
     return 0;
   len = utf8len_tab[*p];
   for (i = 1; i < len; ++i)
@@ -1670,7 +1670,7 @@ int utfc_ptr2len(char_u *p)
   int b0 = *p;
   int prevlen;
 
-  if (b0 == NUL)
+  if (b0 == '\0')
     return 0;
   if (b0 < 0x80 && p[1] < 0x80)         /* be quick for ASCII */
     return 1;
@@ -1708,7 +1708,7 @@ int utfc_ptr2len_len(char_u *p, int size)
   int len;
   int prevlen;
 
-  if (size < 1 || *p == NUL)
+  if (size < 1 || *p == '\0')
     return 0;
   if (p[0] < 0x80 && (size == 1 || p[1] < 0x80))   /* be quick for ASCII */
     return 1;
@@ -2132,7 +2132,7 @@ int utf_class(int c)
 
   /* First quick check for Latin1 characters, use 'iskeyword'. */
   if (c < 0x100) {
-    if (c == ' ' || c == '\t' || c == NUL || c == 0xa0)
+    if (c == ' ' || c == '\t' || c == '\0' || c == 0xa0)
       return 0;             /* blank */
     if (vim_iswordc(c))
       return 2;             /* word character */
@@ -2781,7 +2781,7 @@ static int utf_strnicmp(char_u *s1, char_u *s2, size_t n1, size_t n2)
     s2 = buffer;
   }
 
-  while (n1 > 0 && n2 > 0 && *s1 != NUL && *s2 != NUL) {
+  while (n1 > 0 && n2 > 0 && *s1 != '\0' && *s2 != '\0') {
     cdiff = (int)(*s1) - (int)(*s2);
     if (cdiff != 0)
       return cdiff;
@@ -2792,9 +2792,9 @@ static int utf_strnicmp(char_u *s1, char_u *s2, size_t n1, size_t n2)
     n2--;
   }
 
-  if (n1 > 0 && *s1 == NUL)
+  if (n1 > 0 && *s1 == '\0')
     n1 = 0;
-  if (n2 > 0 && *s2 == NUL)
+  if (n2 > 0 && *s2 == '\0')
     n2 = 0;
 
   if (n1 == 0 && n2 == 0)
@@ -2820,7 +2820,7 @@ int mb_strnicmp(char_u *s1, char_u *s2, size_t nn)
     return utf_strnicmp(s1, s2, nn, nn);
   } else {
     for (i = 0; i < n; i += l) {
-      if (s1[i] == NUL && s2[i] == NUL)         /* both strings end */
+      if (s1[i] == '\0' && s2[i] == '\0')         /* both strings end */
         return 0;
 
       l = (*mb_ptr2len)(s1 + i);
@@ -2861,7 +2861,7 @@ void show_utf8()
   line = ml_get_cursor();
   len = utfc_ptr2len(line);
   if (len == 0) {
-    MSG("NUL");
+    MSG("'\0'");
     return;
   }
 
@@ -2876,7 +2876,7 @@ void show_utf8()
       clen = utf_ptr2len(line + i);
     }
     sprintf((char *)IObuff + rlen, "%02x ",
-        (line[i] == NL) ? NUL : line[i]);          /* NUL is stored as NL */
+        (line[i] == NL) ? '\0' : line[i]);          /* NUL is stored as NL */
     --clen;
     rlen += (int)STRLEN(IObuff + rlen);
     if (rlen > IOSIZE - 20)
@@ -2903,7 +2903,7 @@ int dbcs_head_off(char_u *base, char_u *p)
 
   /* It can't be a trailing byte when not using DBCS, at the start of the
    * string or the previous byte can't start a double-byte. */
-  if (p <= base || MB_BYTE2LEN(p[-1]) == 1 || *p == NUL)
+  if (p <= base || MB_BYTE2LEN(p[-1]) == 1 || *p == '\0')
     return 0;
 
   /* This is slow: need to start at the base and go forward until the
@@ -2929,7 +2929,7 @@ int dbcs_screen_head_off(char_u *base, char_u *p)
   if (p <= base
       || (enc_dbcs == DBCS_JPNU && p[-1] == 0x8e)
       || MB_BYTE2LEN(p[-1]) == 1
-      || *p == NUL)
+      || *p == '\0')
     return 0;
 
   /* This is slow: need to start at the base and go forward until the
@@ -3049,7 +3049,7 @@ int mb_tail_off(char_u *base, char_u *p)
   int i;
   int j;
 
-  if (*p == NUL)
+  if (*p == '\0')
     return 0;
 
   if (enc_utf8) {
@@ -3067,7 +3067,7 @@ int mb_tail_off(char_u *base, char_u *p)
 
   /* It can't be the first byte if a double-byte when not using DBCS, at the
    * end of the string or the byte can't start a double-byte. */
-  if (enc_dbcs == 0 || p[1] == NUL || MB_BYTE2LEN(*p) == 1)
+  if (enc_dbcs == 0 || p[1] == '\0' || MB_BYTE2LEN(*p) == 1)
     return 0;
 
   /* Return 1 when on the lead byte, 0 when on the tail byte. */
@@ -3104,7 +3104,7 @@ void utf_find_illegal()
       p = tofree;
     }
 
-    while (*p != NUL) {
+    while (*p != '\0') {
       /* Illegal means that there are not enough trail bytes (checked by
        * utf_ptr2len()) or too many of them (overlong sequence). */
       len = utf_ptr2len(p);
@@ -3116,7 +3116,7 @@ void utf_find_illegal()
           int l;
 
           len = (int)(p - tofree);
-          for (p = ml_get_cursor(); *p != NUL && len-- > 0; p += l) {
+          for (p = ml_get_cursor(); *p != '\0' && len-- > 0; p += l) {
             l = utf_ptr2len(p);
             curwin->w_cursor.col += l;
           }
@@ -3198,7 +3198,7 @@ int mb_charlen(char_u *str)
   if (p == NULL)
     return 0;
 
-  for (count = 0; *p != NUL; count++)
+  for (count = 0; *p != '\0'; count++)
     p += (*mb_ptr2len)(p);
 
   return count;
@@ -3212,7 +3212,7 @@ int mb_charlen_len(char_u *str, int len)
   char_u      *p = str;
   int count;
 
-  for (count = 0; *p != NUL && p < str + len; count++)
+  for (count = 0; *p != '\0' && p < str + len; count++)
     p += (*mb_ptr2len)(p);
 
   return count;
@@ -3235,7 +3235,7 @@ char_u * mb_unescape(char_u **pp)
   /* Must translate K_SPECIAL KS_SPECIAL KE_FILLER to K_SPECIAL and CSI
    * KS_EXTRA KE_CSI to CSI.
    * Maximum length of a utf-8 character is 4 bytes. */
-  for (n = 0; str[n] != NUL && m < 4; ++n) {
+  for (n = 0; str[n] != '\0' && m < 4; ++n) {
     if (str[n] == K_SPECIAL
         && str[n + 1] == KS_SPECIAL
         && str[n + 2] == KE_FILLER) {
@@ -3252,7 +3252,7 @@ char_u * mb_unescape(char_u **pp)
       break;                    /* a special key can't be a multibyte char */
     else
       buf[m++] = str[n];
-    buf[m] = NUL;
+    buf[m] = '\0';
 
     /* Return a multi-byte character if it's found.  An illegal sequence
      * will result in a 1 here. */
@@ -3289,7 +3289,7 @@ int mb_fix_col(int col, int row)
   row = check_row(row);
   if (has_mbyte && ScreenLines != NULL && col > 0
       && ((enc_dbcs
-          && ScreenLines[LineOffset[row] + col] != NUL
+          && ScreenLines[LineOffset[row] + col] != '\0'
           && dbcs_screen_head_off(ScreenLines + LineOffset[row],
             ScreenLines + LineOffset[row] + col))
         || (enc_utf8 && ScreenLines[LineOffset[row] + col] == 0)))
@@ -3335,13 +3335,13 @@ char_u * enc_canonize(char_u *enc)
   r = alloc((unsigned)(STRLEN(enc) + 3));
   /* Make it all lower case and replace '_' with '-'. */
   p = r;
-  for (s = enc; *s != NUL; ++s) {
+  for (s = enc; *s != '\0'; ++s) {
     if (*s == '_')
       *p++ = '-';
     else
       *p++ = TOLOWER_ASC(*s);
   }
-  *p = NUL;
+  *p = '\0';
 
   /* Skip "2byte-" and "8bit-". */
   p = enc_skip(r);
@@ -3408,16 +3408,16 @@ char_u * enc_locale()
   int i;
   char buf[50];
 # ifdef HAVE_NL_LANGINFO_CODESET
-  if ((s = nl_langinfo(CODESET)) == NULL || *s == NUL)
+  if ((s = nl_langinfo(CODESET)) == NULL || *s == '\0')
 # endif
 #  if defined(HAVE_LOCALE_H) || defined(X_LOCALE)
-    if ((s = setlocale(LC_CTYPE, NULL)) == NULL || *s == NUL)
+    if ((s = setlocale(LC_CTYPE, NULL)) == NULL || *s == '\0')
 #  endif
-      if ((s = (char *)os_getenv("LC_ALL")) == NULL || *s == NUL)
-        if ((s = (char *)os_getenv("LC_CTYPE")) == NULL || *s == NUL)
+      if ((s = (char *)os_getenv("LC_ALL")) == NULL || *s == '\0')
+        if ((s = (char *)os_getenv("LC_CTYPE")) == NULL || *s == '\0')
           s = (char *)os_getenv("LANG");
 
-  if (s == NULL || *s == NUL)
+  if (s == NULL || *s == '\0')
     return FAIL;
 
   /* The most generic locale format is:
@@ -3440,7 +3440,7 @@ char_u * enc_locale()
     } else
       s = p + 1;
   }
-  for (i = 0; s[i] != NUL && i < (int)sizeof(buf) - 1; ++i) {
+  for (i = 0; s[i] != '\0' && i < (int)sizeof(buf) - 1; ++i) {
     if (s[i] == '_' || s[i] == '-')
       buf[i] = '-';
     else if (isalnum((int)s[i]))
@@ -3448,7 +3448,7 @@ char_u * enc_locale()
     else
       break;
   }
-  buf[i] = NUL;
+  buf[i] = '\0';
 
   return enc_canonize((char_u *)buf);
 }
@@ -3549,7 +3549,7 @@ static char_u * iconv_string(vimconv_T *vcp, char_u *str, int slen, int *unconvl
     if (iconv(vcp->vc_fd, (void *)&from, &fromlen, &to, &tolen)
         != (size_t)-1) {
       /* Finished, append a NUL. */
-      *to = NUL;
+      *to = '\0';
       break;
     }
 
@@ -3558,7 +3558,7 @@ static char_u * iconv_string(vimconv_T *vcp, char_u *str, int slen, int *unconvl
     if (!vcp->vc_fail && unconvlenp != NULL
         && (ICONV_ERRNO == ICONV_EINVAL || ICONV_ERRNO == EINVAL)) {
       /* Handle an incomplete sequence at the end. */
-      *to = NUL;
+      *to = '\0';
       *unconvlenp = (int)fromlen;
       break;
     }
@@ -3766,7 +3766,7 @@ int convert_setup_ext(vcp, from, from_unicode_is_utf8, to, to_unicode_is_utf8)
   vcp->vc_fail = FALSE;
 
   /* No conversion when one of the names is empty or they are equal. */
-  if (from == NULL || *from == NUL || to == NULL || *to == NUL
+  if (from == NULL || *from == '\0' || to == NULL || *to == '\0'
       || STRCMP(from, to) == 0)
     return OK;
 
@@ -3918,7 +3918,7 @@ char_u * string_convert_ext(vcp, ptr, lenp, unconvlenp)
           *d++ = 0x80 + (c & 0x3f);
         }
       }
-      *d = NUL;
+      *d = '\0';
       if (lenp != NULL)
         *lenp = (int)(d - retval);
       break;
@@ -3940,7 +3940,7 @@ char_u * string_convert_ext(vcp, ptr, lenp, unconvlenp)
         }
         d += utf_char2bytes(c, d);
       }
-      *d = NUL;
+      *d = '\0';
       if (lenp != NULL)
         *lenp = (int)(d - retval);
       break;
@@ -3952,7 +3952,7 @@ char_u * string_convert_ext(vcp, ptr, lenp, unconvlenp)
       for (i = 0; i < len; ++i) {
         l = utf_ptr2len_len(ptr + i, len - i);
         if (l == 0)
-          *d++ = NUL;
+          *d++ = '\0';
         else if (l == 1) {
           int l_w = utf8len_tab_zero[ptr[i]];
 
@@ -4003,7 +4003,7 @@ char_u * string_convert_ext(vcp, ptr, lenp, unconvlenp)
           i += l - 1;
         }
       }
-      *d = NUL;
+      *d = '\0';
       if (lenp != NULL)
         *lenp = (int)(d - retval);
       break;
diff --git a/src/memfile.c b/src/memfile.c
index 9f8d482768..fa5b604304 100644
--- a/src/memfile.c
+++ b/src/memfile.c
@@ -540,7 +540,7 @@ int mf_sync(memfile_T *mfp, int flags)
   if (hp == NULL || status == FAIL)
     mfp->mf_dirty = FALSE;
 
-  if ((flags & MFS_FLUSH) && *p_sws != NUL) {
+  if ((flags & MFS_FLUSH) && *p_sws != '\0') {
 #if defined(UNIX)
 # ifdef HAVE_FSYNC
     if (STRCMP(p_sws, "fsync") == 0) {
@@ -838,7 +838,7 @@ static int mf_read(memfile_T *mfp, bhdr_T *hp)
   }
 
   /* Decrypt if 'key' is set and this is a data block. */
-  if (*mfp->mf_buffer->b_p_key != NUL)
+  if (*mfp->mf_buffer->b_p_key != '\0')
     ml_decrypt_data(mfp, hp->bh_data, offset, size);
 
   return OK;
@@ -926,7 +926,7 @@ static int mf_write_block(memfile_T *mfp, bhdr_T *hp, off_t offset, unsigned siz
   int result = OK;
 
   /* Encrypt if 'key' is set and this is a data block. */
-  if (*mfp->mf_buffer->b_p_key != NUL) {
+  if (*mfp->mf_buffer->b_p_key != '\0') {
     data = ml_encrypt_data(mfp, data, offset, size);
   }
 
diff --git a/src/memline.c b/src/memline.c
index 5df8034e69..e0f9b0b232 100644
--- a/src/memline.c
+++ b/src/memline.c
@@ -347,11 +347,11 @@ int ml_open(buf_T *buf)
     b0p->b0_flags = get_fileformat(buf) + 1;
     set_b0_fname(b0p, buf);
     (void)os_get_user_name((char *)b0p->b0_uname, B0_UNAME_SIZE);
-    b0p->b0_uname[B0_UNAME_SIZE - 1] = NUL;
+    b0p->b0_uname[B0_UNAME_SIZE - 1] = '\0';
     os_get_hostname((char *)b0p->b0_hname, B0_HNAME_SIZE);
-    b0p->b0_hname[B0_HNAME_SIZE - 1] = NUL;
+    b0p->b0_hname[B0_HNAME_SIZE - 1] = '\0';
     long_to_char(os_get_pid(), b0p->b0_pid);
-    if (*buf->b_p_key != NUL)
+    if (*buf->b_p_key != '\0')
       ml_set_b0_crypt(buf, b0p);
   }
 
@@ -397,7 +397,7 @@ int ml_open(buf_T *buf)
   dp->db_index[0] = --dp->db_txt_start;         /* at end of block */
   dp->db_free -= 1 + INDEX_SIZE;
   dp->db_line_count = 1;
-  *((char_u *)dp + dp->db_txt_start) = NUL;     /* empty line */
+  *((char_u *)dp + dp->db_txt_start) = '\0';     /* empty line */
 
   return OK;
 
@@ -416,7 +416,7 @@ error:
  */
 static void ml_set_b0_crypt(buf_T *buf, ZERO_BL *b0p)
 {
-  if (*buf->b_p_key == NUL)
+  if (*buf->b_p_key == '\0')
     b0p->b0_id[1] = BLOCK0_ID1;
   else {
     if (get_crypt_method(buf) == 0)
@@ -574,7 +574,7 @@ void ml_setname(buf_T *buf)
    */
   dirp = p_dir;
   for (;; ) {
-    if (*dirp == NUL)               /* tried all directories, fail */
+    if (*dirp == '\0')               /* tried all directories, fail */
       break;
     fname = findswapname(buf, &dirp, mfp->mf_fname);
     /* alloc's fname */
@@ -671,7 +671,7 @@ void ml_open_file(buf_T *buf)
    */
   dirp = p_dir;
   for (;; ) {
-    if (*dirp == NUL)
+    if (*dirp == '\0')
       break;
     /* There is a small chance that between choosing the swap file name
      * and creating it, another Vim creates the file.  In that case the
@@ -835,7 +835,7 @@ static void set_b0_fname(ZERO_BL *b0p, buf_T *buf)
   struct stat st;
 
   if (buf->b_ffname == NULL)
-    b0p->b0_fname[0] = NUL;
+    b0p->b0_fname[0] = '\0';
   else {
     char uname[B0_UNAME_SIZE];
 
@@ -909,7 +909,7 @@ static void add_b0_fenc(ZERO_BL *b0p, buf_T *buf)
   /* Without encryption use the same offset as in Vim 7.2 to be compatible.
    * With encryption it's OK to move elsewhere, the swap file is not
    * compatible anyway. */
-  if (*buf->b_p_key != NUL)
+  if (*buf->b_p_key != '\0')
     size = B0_FNAME_SIZE_CRYPT;
 
   n = (int)STRLEN(buf->b_p_fenc);
@@ -918,7 +918,7 @@ static void add_b0_fenc(ZERO_BL *b0p, buf_T *buf)
   else {
     memmove((char *)b0p->b0_fname + size - n,
         (char *)buf->b_p_fenc, (size_t)n);
-    *(b0p->b0_fname + size - n - 1) = NUL;
+    *(b0p->b0_fname + size - n - 1) = '\0';
     b0p->b0_flags |= B0_HAS_FENC;
   }
 }
@@ -1087,7 +1087,7 @@ void ml_recover(void)
         attr | MSG_HIST);
     MSG_PUTS_ATTR(_("The file was created on "), attr | MSG_HIST);
     /* avoid going past the end of a corrupted hostname */
-    b0p->b0_fname[0] = NUL;
+    b0p->b0_fname[0] = '\0';
     MSG_PUTS_ATTR(b0p->b0_hname, attr | MSG_HIST);
     MSG_PUTS_ATTR(_(",\nor the file has been damaged."), attr | MSG_HIST);
     msg_end();
@@ -1173,7 +1173,7 @@ void ml_recover(void)
     /* Use the same size as in add_b0_fenc(). */
     if (b0p->b0_id[1] != BLOCK0_ID1)
       fnsize = B0_FNAME_SIZE_CRYPT;
-    for (p = b0p->b0_fname + fnsize; p > b0p->b0_fname && p[-1] != NUL; --p)
+    for (p = b0p->b0_fname + fnsize; p > b0p->b0_fname && p[-1] != '\0'; --p)
       ;
     b0_fenc = vim_strnsave(p, (int)(b0p->b0_fname + fnsize - p));
   }
@@ -1200,7 +1200,7 @@ void ml_recover(void)
   if (b0_cm >= 0) {
     /* Need to ask the user for the crypt key.  If this fails we continue
      * without a key, will probably get garbage text. */
-    if (*curbuf->b_p_key != NUL) {
+    if (*curbuf->b_p_key != '\0') {
       smsg((char_u *)_("Swap file is encrypted: \"%s\""), fname_used);
       MSG_PUTS(_(
               "\nIf you entered a new crypt key but did not write the text file,"));
@@ -1213,7 +1213,7 @@ void ml_recover(void)
     buf->b_p_key = get_crypt_key(FALSE, FALSE);
     if (buf->b_p_key == NULL)
       buf->b_p_key = curbuf->b_p_key;
-    else if (*buf->b_p_key == NUL) {
+    else if (*buf->b_p_key == '\0') {
       vim_free(buf->b_p_key);
       buf->b_p_key = curbuf->b_p_key;
     }
@@ -1349,7 +1349,7 @@ void ml_recover(void)
           }
 
           /* make sure there is a NUL at the end of the block */
-          *((char_u *)dp + dp->db_txt_end - 1) = NUL;
+          *((char_u *)dp + dp->db_txt_end - 1) = '\0';
 
           /*
            * check number of lines in block
@@ -1403,7 +1403,7 @@ void ml_recover(void)
   if (orig_file_status != OK || curbuf->b_ml.ml_line_count != lnum * 2 + 1) {
     /* Recovering an empty file results in two lines and the first line is
      * empty.  Don't set the modified flag then. */
-    if (!(curbuf->b_ml.ml_line_count == 2 && *ml_get(1) == NUL)) {
+    if (!(curbuf->b_ml.ml_line_count == 2 && *ml_get(1) == '\0')) {
       changed_int();
       ++curbuf->b_changedtick;
     }
@@ -1452,7 +1452,7 @@ void ml_recover(void)
     MSG_PUTS(_("\nYou may want to delete the .swp file now.\n\n"));
     cmdline_row = msg_row;
   }
-  if (*buf->b_p_key != NUL && STRCMP(curbuf->b_p_key, buf->b_p_key) != 0) {
+  if (*buf->b_p_key != '\0' && STRCMP(curbuf->b_p_key, buf->b_p_key) != 0) {
     MSG_PUTS(_("Using crypt key from swap file for the text file.\n"));
     set_option_value((char_u *)"key", 0L, buf->b_p_key, OPT_LOCAL);
   }
@@ -1546,7 +1546,7 @@ recover_names (
      */
     (void)copy_option_part(&dirp, dir_name, 31000, ",");
 
-    if (dir_name[0] == '.' && dir_name[1] == NUL) {     /* check current dir */
+    if (dir_name[0] == '.' && dir_name[1] == '\0') {     /* check current dir */
       if (fname == NULL) {
         names[0] = vim_strsave((char_u *)"*.sw?");
 #if defined(UNIX) || defined(WIN3264)
@@ -1606,7 +1606,7 @@ recover_names (
      * not able to execute the shell).
      * Try finding a swap file by simply adding ".swp" to the file name.
      */
-    if (*dirp == NUL && file_count + num_files == 0 && fname != NULL) {
+    if (*dirp == '\0' && file_count + num_files == 0 && fname != NULL) {
       struct stat st;
       char_u          *swapname;
 
@@ -1650,7 +1650,7 @@ recover_names (
         dirp = (char_u *)"";                        /* stop searching */
       }
     } else if (list) {
-      if (dir_name[0] == '.' && dir_name[1] == NUL) {
+      if (dir_name[0] == '.' && dir_name[1] == '\0') {
         if (fname == NULL)
           MSG_PUTS(_("   In current directory:\n"));
         else
@@ -1698,7 +1698,7 @@ static char_u *make_percent_swname(char_u *dir, char_u *name)
   d = NULL;
   if (f != NULL) {
     s = (char_u *)xstrdup((char *)f);
-    for (d = s; *d != NUL; mb_ptr_adv(d))
+    for (d = s; *d != '\0'; mb_ptr_adv(d))
       if (vim_ispathsep(*d))
         *d = '%';
     d = concat_fnames(dir, s, TRUE);
@@ -1759,7 +1759,7 @@ static time_t swapfile_info(char_u *fname)
         MSG_PUTS(_("         [does not look like a Vim swap file]"));
       } else {
         MSG_PUTS(_("         file name: "));
-        if (b0.b0_fname[0] == NUL)
+        if (b0.b0_fname[0] == '\0')
           MSG_PUTS(_("[No Name]"));
         else
           msg_outtrans(b0.b0_fname);
@@ -1767,13 +1767,13 @@ static time_t swapfile_info(char_u *fname)
         MSG_PUTS(_("\n          modified: "));
         MSG_PUTS(b0.b0_dirty ? _("YES") : _("no"));
 
-        if (*(b0.b0_uname) != NUL) {
+        if (*(b0.b0_uname) != '\0') {
           MSG_PUTS(_("\n         user name: "));
           msg_outtrans(b0.b0_uname);
         }
 
-        if (*(b0.b0_hname) != NUL) {
-          if (*(b0.b0_uname) != NUL)
+        if (*(b0.b0_hname) != '\0') {
+          if (*(b0.b0_uname) != '\0')
             MSG_PUTS(_("   host name: "));
           else
             MSG_PUTS(_("\n         host name: "));
@@ -3349,7 +3349,7 @@ int resolve_symlink(char_u *fname, char_u *buf)
       /* There must be some error reading links, use original name. */
       return FAIL;
     }
-    buf[ret] = NUL;
+    buf[ret] = '\0';
 
     /*
      * Check whether the symlink is relative or absolute.
@@ -3422,7 +3422,7 @@ char_u *makeswapname(char_u *fname, char_u *ffname, buf_T *buf, char_u *dir_name
       FALSE
 #else
       /* Prepend a '.' to the swap file name for the current directory. */
-      dir_name[0] == '.' && dir_name[1] == NUL
+      dir_name[0] == '.' && dir_name[1] == '\0'
 #endif
       );
   if (r == NULL)            /* out of memory */
@@ -3458,14 +3458,14 @@ get_file_in_dir (
 
   tail = path_tail(fname);
 
-  if (dname[0] == '.' && dname[1] == NUL)
+  if (dname[0] == '.' && dname[1] == '\0')
     retval = vim_strsave(fname);
   else if (dname[0] == '.' && vim_ispathsep(dname[1])) {
     if (tail == fname)              /* no path before file name */
       retval = concat_fnames(dname + 2, tail, TRUE);
     else {
       save_char = *tail;
-      *tail = NUL;
+      *tail = '\0';
       t = concat_fnames(fname, dname + 2, TRUE);
       *tail = save_char;
       retval = concat_fnames(t, tail, TRUE);
@@ -3667,7 +3667,7 @@ findswapname (
           fname2[n - 1] = 'x';
         else if (*path_tail(fname) == '.') {
           fname2[n] = 'x';
-          fname2[n + 1] = NUL;
+          fname2[n + 1] = '\0';
         } else
           fname2[n - 5] += 1;
         /*
diff --git a/src/menu.c b/src/menu.c
index 01f7f4f9d3..641fae3f75 100644
--- a/src/menu.c
+++ b/src/menu.c
@@ -113,13 +113,13 @@ ex_menu (
   /* Locate an optional "icon=filename" argument. */
   if (STRNCMP(arg, "icon=", 5) == 0) {
     arg += 5;
-    while (*arg != NUL && *arg != ' ') {
+    while (*arg != '\0' && *arg != ' ') {
       if (*arg == '\\')
         STRMOVE(arg, arg + 1);
       mb_ptr_adv(arg);
     }
-    if (*arg != NUL) {
-      *arg++ = NUL;
+    if (*arg != '\0') {
+      *arg++ = '\0';
       arg = skipwhite(arg);
     }
   }
@@ -162,7 +162,7 @@ ex_menu (
   /*
    * If there is no argument, display all menus.
    */
-  if (*arg == NUL) {
+  if (*arg == '\0') {
     show_menus(arg, modes);
     return;
   }
@@ -179,10 +179,10 @@ ex_menu (
   /*
    * If there is only a menu name, display menus with that name.
    */
-  if (*map_to == NUL && !unmenu && enable == MAYBE) {
+  if (*map_to == '\0' && !unmenu && enable == MAYBE) {
     show_menus(menu_path, modes);
     goto theend;
-  } else if (*map_to != NUL && (unmenu || enable != MAYBE)) {
+  } else if (*map_to != '\0' && (unmenu || enable != MAYBE)) {
     EMSG(_(e_trailing));
     goto theend;
   }
@@ -325,7 +325,7 @@ add_menu_path (
     dname = menu_text(name, NULL, NULL);
     if (dname == NULL)
       goto erret;
-    if (*dname == NUL) {
+    if (*dname == '\0') {
       /* Only a mnemonic or accelerator is not valid. */
       EMSG(_("E792: Empty menu name"));
       goto erret;
@@ -336,12 +336,12 @@ add_menu_path (
     menu = *menup;
     while (menu != NULL) {
       if (menu_name_equal(name, menu) || menu_name_equal(dname, menu)) {
-        if (*next_name == NUL && menu->children != NULL) {
+        if (*next_name == '\0' && menu->children != NULL) {
           if (!sys_menu)
             EMSG(_("E330: Menu path must not lead to a sub-menu"));
           goto erret;
         }
-        if (*next_name != NUL && menu->children == NULL
+        if (*next_name != '\0' && menu->children == NULL
             ) {
           if (!sys_menu)
             EMSG(_(e_notsubmenu));
@@ -362,12 +362,12 @@ add_menu_path (
     }
 
     if (menu == NULL) {
-      if (*next_name == NUL && parent == NULL) {
+      if (*next_name == '\0' && parent == NULL) {
         EMSG(_("E331: Must not add menu items directly to menu bar"));
         goto erret;
       }
 
-      if (menu_is_separator(dname) && *next_name != NUL) {
+      if (menu_is_separator(dname) && *next_name != '\0') {
         EMSG(_("E332: Separator cannot be part of a menu path"));
         goto erret;
       }
@@ -446,7 +446,7 @@ add_menu_path (
          * Don't do this for "<Nop>". */
         c = 0;
         d = 0;
-        if (amenu && call_data != NULL && *call_data != NUL
+        if (amenu && call_data != NULL && *call_data != '\0'
             ) {
           switch (1 << i) {
           case MENU_VISUAL_MODE:
@@ -477,7 +477,7 @@ add_menu_path (
             /* Append CTRL-\ CTRL-G to obey 'insertmode'. */
             menu->strings[i][len] = Ctrl_BSL;
             menu->strings[i][len + 1] = Ctrl_G;
-            menu->strings[i][len + 2] = NUL;
+            menu->strings[i][len + 2] = '\0';
           }
         } else
           menu->strings[i] = p;
@@ -531,8 +531,8 @@ static int menu_nable_recurse(vimmenu_T *menu, char_u *name, int modes, int enab
 
   /* Find the menu */
   while (menu != NULL) {
-    if (*name == NUL || *name == '*' || menu_name_equal(name, menu)) {
-      if (*p != NUL) {
+    if (*name == '\0' || *name == '*' || menu_name_equal(name, menu)) {
+      if (*p != '\0') {
         if (menu->children == NULL) {
           EMSG(_(e_notsubmenu));
           return FAIL;
@@ -550,12 +550,12 @@ static int menu_nable_recurse(vimmenu_T *menu, char_u *name, int modes, int enab
        * modes, so keep looping, otherwise we are just doing the named
        * menu item (which has been found) so break here.
        */
-      if (*name != NUL && *name != '*')
+      if (*name != '\0' && *name != '*')
         break;
     }
     menu = menu->next;
   }
-  if (*name != NUL && *name != '*' && menu == NULL) {
+  if (*name != '\0' && *name != '*' && menu == NULL) {
     EMSG2(_(e_nomenu), name);
     return FAIL;
   }
@@ -588,8 +588,8 @@ remove_menu (
 
   /* Find the menu */
   while ((menu = *menup) != NULL) {
-    if (*name == NUL || menu_name_equal(name, menu)) {
-      if (*p != NUL && menu->children == NULL) {
+    if (*name == '\0' || menu_name_equal(name, menu)) {
+      if (*p != '\0' && menu->children == NULL) {
         if (!silent)
           EMSG(_(e_notsubmenu));
         return FAIL;
@@ -600,14 +600,14 @@ remove_menu (
          * If we are removing all entries for this menu,MENU_ALL_MODES,
          * Then kill any tearoff before we start
          */
-        if (*p == NUL && modes == MENU_ALL_MODES) {
+        if (*p == '\0' && modes == MENU_ALL_MODES) {
           if (IsWindow(menu->tearoff_handle))
             DestroyWindow(menu->tearoff_handle);
         }
 #endif
         if (remove_menu(&menu->children, p, modes, silent) == FAIL)
           return FAIL;
-      } else if (*name != NUL) {
+      } else if (*name != '\0') {
         if (!silent)
           EMSG(_(e_othermode));
         return FAIL;
@@ -618,7 +618,7 @@ remove_menu (
        * modes, so keep looping, otherwise we are just removing the named
        * menu item (which has been found) so break here.
        */
-      if (*name != NUL)
+      if (*name != '\0')
         break;
 
       /* Remove the menu item for the given mode[s].  If the menu item
@@ -633,7 +633,7 @@ remove_menu (
     } else
       menup = &menu->next;
   }
-  if (*name != NUL) {
+  if (*name != '\0') {
     if (menu == NULL) {
       if (!silent)
         EMSG2(_(e_nomenu), name);
@@ -736,7 +736,7 @@ static int show_menus(char_u *path_name, int modes)
     while (menu != NULL) {
       if (menu_name_equal(name, menu)) {
         /* Found menu */
-        if (*p != NUL && menu->children == NULL) {
+        if (*p != '\0' && menu->children == NULL) {
           EMSG(_(e_notsubmenu));
           vim_free(path_name);
           return FAIL;
@@ -817,7 +817,7 @@ static void show_menus_recursive(vimmenu_T *menu, int modes, int depth)
         else
           msg_putchar(' ');
         MSG_PUTS(" ");
-        if (*menu->strings[bit] == NUL)
+        if (*menu->strings[bit] == '\0')
           msg_puts_attr((char_u *)"<Nop>", hl_attr(HLF_8));
         else
           msg_outtrans_special(menu->strings[bit], FALSE);
@@ -867,22 +867,22 @@ char_u *set_context_in_menu_cmd(expand_T *xp, char_u *cmd, char_u *arg, int forc
 
   if (!vim_iswhite(*p)) {
     if (STRNCMP(arg, "enable", 6) == 0
-        && (arg[6] == NUL ||  vim_iswhite(arg[6])))
+        && (arg[6] == '\0' ||  vim_iswhite(arg[6])))
       p = arg + 6;
     else if (STRNCMP(arg, "disable", 7) == 0
-             && (arg[7] == NUL || vim_iswhite(arg[7])))
+             && (arg[7] == '\0' || vim_iswhite(arg[7])))
       p = arg + 7;
     else
       p = arg;
   }
 
-  while (*p != NUL && vim_iswhite(*p))
+  while (*p != '\0' && vim_iswhite(*p))
     ++p;
 
   arg = after_dot = p;
 
   for (; *p && !vim_iswhite(*p); ++p) {
-    if ((*p == '\\' || *p == Ctrl_V) && p[1] != NUL)
+    if ((*p == '\\' || *p == Ctrl_V) && p[1] != '\0')
       p++;
     else if (*p == '.')
       after_dot = p + 1;
@@ -893,7 +893,7 @@ char_u *set_context_in_menu_cmd(expand_T *xp, char_u *cmd, char_u *arg, int forc
   expand_emenu = (*cmd == 'e');
   if (expand_menus && vim_iswhite(*p))
     return NULL;        /* TODO: check for next command? */
-  if (*p == NUL) {              /* Complete the menu name */
+  if (*p == '\0') {              /* Complete the menu name */
     /*
      * With :unmenu, you only want to match menus for the appropriate mode.
      * With :menu though you might want to add a menu with the same name as
@@ -914,7 +914,7 @@ char_u *set_context_in_menu_cmd(expand_T *xp, char_u *cmd, char_u *arg, int forc
       while (menu != NULL) {
         if (menu_name_equal(name, menu)) {
           /* Found menu */
-          if ((*p != NUL && menu->children == NULL)
+          if ((*p != '\0' && menu->children == NULL)
               || ((menu->modes & expand_modes) == 0x0)) {
             /*
              * Menu path continues, but we have reached a leaf.
@@ -1065,12 +1065,12 @@ char_u *menu_name_skip(char_u *name)
   for (p = name; *p && *p != '.'; mb_ptr_adv(p)) {
     if (*p == '\\' || *p == Ctrl_V) {
       STRMOVE(p, p + 1);
-      if (*p == NUL)
+      if (*p == '\0')
         break;
     }
   }
   if (*p)
-    *p++ = NUL;
+    *p++ = '\0';
   return p;
 }
 
@@ -1091,11 +1091,11 @@ static int menu_namecmp(char_u *name, char_u *mname)
 {
   int i;
 
-  for (i = 0; name[i] != NUL && name[i] != TAB; ++i)
+  for (i = 0; name[i] != '\0' && name[i] != TAB; ++i)
     if (name[i] != mname[i])
       break;
-  return (name[i] == NUL || name[i] == TAB)
-         && (mname[i] == NUL || mname[i] == TAB);
+  return (name[i] == '\0' || name[i] == TAB)
+         && (mname[i] == '\0' || mname[i] == TAB);
 }
 
 /*
@@ -1208,7 +1208,7 @@ static char_u *menu_text(char_u *str, int *mnemonic, char_u **actext)
   for (p = text; p != NULL; ) {
     p = vim_strchr(p, '&');
     if (p != NULL) {
-      if (p[1] == NUL)              /* trailing "&" */
+      if (p[1] == '\0')              /* trailing "&" */
         break;
       if (mnemonic != NULL && p[1] != '&')
 #if !defined(__MVS__) || defined(MOTIF390_MNEMONIC_FIXED)
@@ -1274,7 +1274,7 @@ int menu_is_separator(char_u *name)
  */
 static int menu_is_hidden(char_u *name)
 {
-  return (name[0] == ']') || (menu_is_popup(name) && name[5] != NUL);
+  return (name[0] == ']') || (menu_is_popup(name) && name[5] != '\0');
 }
 
 #if defined(FEAT_CMDL_COMPL) \
@@ -1315,10 +1315,10 @@ void ex_emenu(exarg_T *eap)
 
     while (menu != NULL) {
       if (menu_name_equal(name, menu)) {
-        if (*p == NUL && menu->children != NULL) {
+        if (*p == '\0' && menu->children != NULL) {
           EMSG(_("E333: Menu path must lead to a menu item"));
           menu = NULL;
-        } else if (*p != NUL && menu->children == NULL) {
+        } else if (*p != '\0' && menu->children == NULL) {
           EMSG(_(e_notsubmenu));
           menu = NULL;
         }
@@ -1326,7 +1326,7 @@ void ex_emenu(exarg_T *eap)
       }
       menu = menu->next;
     }
-    if (menu == NULL || *p == NUL)
+    if (menu == NULL || *p == '\0')
       break;
     menu = menu->children;
     name = p;
@@ -1382,7 +1382,7 @@ void ex_emenu(exarg_T *eap)
 
     /* Adjust the cursor to make sure it is in the correct pos
      * for exclusive mode */
-    if (*p_sel == 'e' && gchar_cursor() != NUL)
+    if (*p_sel == 'e' && gchar_cursor() != '\0')
       ++curwin->w_cursor.col;
   } else {
     mode = (char_u *)"Normal";
@@ -1430,14 +1430,14 @@ vimmenu_T *gui_find_menu(char_u *path_name)
       if (menu_name_equal(name, menu)) {
         if (menu->children == NULL) {
           /* found a menu item instead of a sub-menu */
-          if (*p == NUL)
+          if (*p == '\0')
             EMSG(_("E336: Menu path must lead to a sub-menu"));
           else
             EMSG(_(e_notsubmenu));
           menu = NULL;
           goto theend;
         }
-        if (*p == NUL)              /* found a full match */
+        if (*p == '\0')              /* found a full match */
           goto theend;
         break;
       }
@@ -1504,7 +1504,7 @@ void ex_menutranslate(exarg_T *eap)
     from = arg;
     arg = menu_skip_part(arg);
     to = skipwhite(arg);
-    *arg = NUL;
+    *arg = '\0';
     arg = menu_skip_part(to);
     if (arg == to)
       EMSG(_(e_invarg));
@@ -1539,8 +1539,8 @@ void ex_menutranslate(exarg_T *eap)
  */
 static char_u *menu_skip_part(char_u *p)
 {
-  while (*p != NUL && *p != '.' && !vim_iswhite(*p)) {
-    if ((*p == '\\' || *p == Ctrl_V) && p[1] != NUL)
+  while (*p != '\0' && *p != '.' && !vim_iswhite(*p)) {
+    if ((*p == '\\' || *p == Ctrl_V) && p[1] != '\0')
       ++p;
     ++p;
   }
@@ -1558,12 +1558,12 @@ static char_u *menutrans_lookup(char_u *name, int len)
   char_u              *dname;
 
   for (i = 0; i < menutrans_ga.ga_len; ++i)
-    if (STRNCMP(name, tp[i].from, len) == 0 && tp[i].from[len] == NUL)
+    if (STRNCMP(name, tp[i].from, len) == 0 && tp[i].from[len] == '\0')
       return tp[i].to;
 
   /* Now try again while ignoring '&' characters. */
   i = name[len];
-  name[len] = NUL;
+  name[len] = '\0';
   dname = menu_text(name, NULL, NULL);
   name[len] = i;
   if (dname != NULL) {
@@ -1599,7 +1599,7 @@ static char_u *menu_translate_tab_and_shift(char_u *arg_start)
   char_u      *arg = arg_start;
 
   while (*arg && !vim_iswhite(*arg)) {
-    if ((*arg == '\\' || *arg == Ctrl_V) && arg[1] != NUL)
+    if ((*arg == '\\' || *arg == Ctrl_V) && arg[1] != '\0')
       arg++;
     else if (STRNICMP(arg, "<TAB>", 5) == 0) {
       *arg = TAB;
@@ -1607,8 +1607,8 @@ static char_u *menu_translate_tab_and_shift(char_u *arg_start)
     }
     arg++;
   }
-  if (*arg != NUL)
-    *arg++ = NUL;
+  if (*arg != '\0')
+    *arg++ = '\0';
   arg = skipwhite(arg);
 
   return arg;
diff --git a/src/message.c b/src/message.c
index 89bd787d5c..37f3392eb8 100644
--- a/src/message.c
+++ b/src/message.c
@@ -262,9 +262,9 @@ void trunc_string(char_u *s, char_u *buf, int room, int buflen)
 
   /* First part: Start of the string. */
   for (e = 0; len < half && e < buflen; ++e) {
-    if (s[e] == NUL) {
+    if (s[e] == '\0') {
       /* text fits without truncating! */
-      buf[e] = NUL;
+      buf[e] = '\0';
       return;
     }
     n = ptr2cells(s + e);
@@ -316,9 +316,9 @@ void trunc_string(char_u *s, char_u *buf, int room, int buflen)
     if (len >= buflen - e - 3)
       len = buflen - e - 3 - 1;
     memmove(buf + e + 3, s + i, len);
-    buf[e + 3 + len - 1] = NUL;
+    buf[e + 3 + len - 1] = '\0';
   } else {
-    buf[e - 1] = NUL;      /* make sure it is truncated */
+    buf[e - 1] = '\0';      /* make sure it is truncated */
   }
 }
 
@@ -712,7 +712,7 @@ void ex_messages(exarg_T *eap)
   msg_hist_off = TRUE;
 
   s = os_getenv("LANG");
-  if (s != NULL && *s != NUL)
+  if (s != NULL && *s != '\0')
     msg_attr((char_u *)
         _("Messages maintainer: Bram Moolenaar <Bram@vim.org>"),
         hl_attr(HLF_T));
@@ -1052,9 +1052,9 @@ void msg_putchar_attr(int c, int attr)
     buf[0] = K_SPECIAL;
     buf[1] = K_SECOND(c);
     buf[2] = K_THIRD(c);
-    buf[3] = NUL;
+    buf[3] = '\0';
   } else {
-    buf[(*mb_char2bytes)(c, buf)] = NUL;
+    buf[(*mb_char2bytes)(c, buf)] = '\0';
   }
   msg_puts_attr(buf, attr);
 }
@@ -1175,7 +1175,7 @@ int msg_outtrans_len_attr(char_u *msgstr, int len, int attr)
       str += mb_l;
     } else {
       s = transchar_byte(*str);
-      if (s[1] != NUL) {
+      if (s[1] != '\0') {
         /* unprintable char: print the printable chars so far and the
          * translation of the unprintable char. */
         if (str > plain_start)
@@ -1240,9 +1240,9 @@ msg_outtrans_special (
   int len;
 
   attr = hl_attr(HLF_8);
-  while (*str != NUL) {
+  while (*str != '\0') {
     /* Leading and trailing spaces need to be displayed in <> form. */
-    if ((str == strstart || str[1] == NUL) && *str == ' ') {
+    if ((str == strstart || str[1] == '\0') && *str == ' ') {
       string = (char_u *)"<Space>";
       ++str;
     } else
@@ -1271,9 +1271,9 @@ str2special_save (
   char_u      *p = str;
 
   ga_init(&ga, 1, 40);
-  while (*p != NUL)
+  while (*p != '\0')
     ga_concat(&ga, str2special(&p, is_lhs));
-  ga_append(&ga, NUL);
+  ga_append(&ga, '\0');
   return (char_u *)ga.ga_data;
 }
 
@@ -1305,17 +1305,17 @@ str2special (
   }
 
   c = *str;
-  if (c == K_SPECIAL && str[1] != NUL && str[2] != NUL) {
+  if (c == K_SPECIAL && str[1] != '\0' && str[2] != '\0') {
     if (str[1] == KS_MODIFIER) {
       modifiers = str[2];
       str += 3;
       c = *str;
     }
-    if (c == K_SPECIAL && str[1] != NUL && str[2] != NUL) {
+    if (c == K_SPECIAL && str[1] != '\0' && str[2] != '\0') {
       c = TO_SPECIAL(str[1], str[2]);
       str += 2;
       if (c == KS_ZERO)         /* display <Nul> as ^@ or <Nul> */
-        c = NUL;
+        c = '\0';
     }
     if (IS_SPECIAL(c) || modifiers)     /* special key */
       special = TRUE;
@@ -1342,7 +1342,7 @@ str2special (
   if (special || char2cells(c) > 1 || (from && c == ' '))
     return get_special_key_name(c, modifiers);
   buf[0] = c;
-  buf[1] = NUL;
+  buf[1] = '\0';
   return buf;
 }
 
@@ -1353,7 +1353,7 @@ void str2specialbuf(char_u *sp, char_u *buf, int len)
 {
   char_u      *s;
 
-  *buf = NUL;
+  *buf = '\0';
   while (*sp) {
     s = str2special(&sp, FALSE);
     if ((int)(STRLEN(s) + STRLEN(buf)) < len)
@@ -1389,7 +1389,7 @@ void msg_prt_line(char_u *s, int list)
 
   /* output a space for an empty line, otherwise the line will be
    * overwritten */
-  if (*s == NUL && !(list && lcs_eol != NUL))
+  if (*s == '\0' && !(list && lcs_eol != '\0'))
     msg_putchar(' ');
 
   while (!got_int) {
@@ -1401,12 +1401,12 @@ void msg_prt_line(char_u *s, int list)
         c = *p_extra++;
     } else if (has_mbyte && (l = (*mb_ptr2len)(s)) > 1) {
       col += (*mb_ptr2cells)(s);
-      if (lcs_nbsp != NUL && list && mb_ptr2char(s) == 160) {
+      if (lcs_nbsp != '\0' && list && mb_ptr2char(s) == 160) {
         mb_char2bytes(lcs_nbsp, buf);
-        buf[(*mb_ptr2len)(buf)] = NUL;
+        buf[(*mb_ptr2len)(buf)] = '\0';
       } else {
         memmove(buf, s, (size_t)l);
-        buf[l] = NUL;
+        buf[l] = '\0';
       }
       msg_puts(buf);
       s += l;
@@ -1425,20 +1425,20 @@ void msg_prt_line(char_u *s, int list)
           c_extra = lcs_tab2;
           attr = hl_attr(HLF_8);
         }
-      } else if (c == 160 && list && lcs_nbsp != NUL) {
+      } else if (c == 160 && list && lcs_nbsp != '\0') {
         c = lcs_nbsp;
         attr = hl_attr(HLF_8);
-      } else if (c == NUL && list && lcs_eol != NUL) {
+      } else if (c == '\0' && list && lcs_eol != '\0') {
         p_extra = (char_u *)"";
-        c_extra = NUL;
+        c_extra = '\0';
         n_extra = 1;
         c = lcs_eol;
         attr = hl_attr(HLF_AT);
         --s;
-      } else if (c != NUL && (n = byte2cells(c)) > 1) {
+      } else if (c != '\0' && (n = byte2cells(c)) > 1) {
         n_extra = n - 1;
         p_extra = transchar_byte(c);
-        c_extra = NUL;
+        c_extra = '\0';
         c = *p_extra++;
         /* Use special coloring to be able to distinguish <hex> from
          * the same in plain text. */
@@ -1449,7 +1449,7 @@ void msg_prt_line(char_u *s, int list)
       }
     }
 
-    if (c == NUL)
+    if (c == '\0')
       break;
 
     msg_putchar_attr(c, attr);
@@ -1603,7 +1603,7 @@ static void msg_puts_display(char_u *str, int maxlen, int attr, int recurse)
   int did_last_char;
 
   did_wait_return = FALSE;
-  while ((maxlen < 0 || (int)(s - str) < maxlen) && *s != NUL) {
+  while ((maxlen < 0 || (int)(s - str) < maxlen) && *s != '\0') {
     /*
      * We are at the end of the screen line when:
      * - When outputting a newline.
@@ -1684,7 +1684,7 @@ static void msg_puts_display(char_u *str, int maxlen, int attr, int recurse)
         --lines_left;
       if (p_more && lines_left == 0 && State != HITRETURN
           && !msg_no_more && !exmode_active) {
-        if (do_more_prompt(NUL))
+        if (do_more_prompt('\0'))
           s = confirm_msg_tail;
         if (quit_more)
           return;
@@ -1800,7 +1800,7 @@ static void msg_scroll_up(void)
  */
 static void inc_msg_scrolled(void)
 {
-  if (*get_vim_var_str(VV_SCROLLSTART) == NUL) {
+  if (*get_vim_var_str(VV_SCROLLSTART) == '\0') {
     char_u      *p = sourcing_name;
     char_u      *tofree = NULL;
     int len;
@@ -2014,7 +2014,7 @@ static void msg_puts_printf(char_u *str, int maxlen)
   char_u buf[4];
   char_u      *p;
 
-  while (*s != NUL && (maxlen < 0 || (int)(s - str) < maxlen)) {
+  while (*s != '\0' && (maxlen < 0 || (int)(s - str) < maxlen)) {
     if (!(silent_mode && p_verbose == 0)) {
       /* NL --> CR NL translation (for Unix, not for "--version") */
       /* NL --> CR translation (for Mac) */
@@ -2078,15 +2078,15 @@ static int do_more_prompt(int typed_char)
 
   State = ASKMORE;
   setmouse();
-  if (typed_char == NUL)
+  if (typed_char == '\0')
     msg_moremsg(FALSE);
   for (;; ) {
     /*
      * Get a typed character directly from the user.
      */
-    if (used_typed_char != NUL) {
+    if (used_typed_char != '\0') {
       c = used_typed_char;              /* was typed at hit-enter prompt */
-      used_typed_char = NUL;
+      used_typed_char = '\0';
     } else
       c = get_keystroke();
 
@@ -2510,7 +2510,7 @@ static void redir_write(char_u *str, int maxlen)
     return;
 
   /* If 'verbosefile' is set prepare for writing in that file. */
-  if (*p_vfile != NUL && verbose_fd == NULL)
+  if (*p_vfile != '\0' && verbose_fd == NULL)
     verbose_open();
 
   if (redirecting()) {
@@ -2535,7 +2535,7 @@ static void redir_write(char_u *str, int maxlen)
       var_redir_str(s, maxlen);
 
     /* Write and adjust the current column. */
-    while (*s != NUL && (maxlen < 0 || (int)(s - str) < maxlen)) {
+    while (*s != '\0' && (maxlen < 0 || (int)(s - str) < maxlen)) {
       if (!redir_reg && !redir_vname)
         if (redir_fd != NULL)
           putc(*s, redir_fd);
@@ -2557,7 +2557,7 @@ static void redir_write(char_u *str, int maxlen)
 
 int redirecting(void)
 {
-  return redir_fd != NULL || *p_vfile != NUL
+  return redir_fd != NULL || *p_vfile != '\0'
          || redir_reg || redir_vname
   ;
 }
@@ -2568,7 +2568,7 @@ int redirecting(void)
  */
 void verbose_enter(void)
 {
-  if (*p_vfile != NUL)
+  if (*p_vfile != '\0')
     ++msg_silent;
 }
 
@@ -2578,7 +2578,7 @@ void verbose_enter(void)
  */
 void verbose_leave(void)
 {
-  if (*p_vfile != NUL)
+  if (*p_vfile != '\0')
     if (--msg_silent < 0)
       msg_silent = 0;
 }
@@ -2588,7 +2588,7 @@ void verbose_leave(void)
  */
 void verbose_enter_scroll(void)
 {
-  if (*p_vfile != NUL)
+  if (*p_vfile != '\0')
     ++msg_silent;
   else
     /* always scroll up, don't overwrite */
@@ -2600,7 +2600,7 @@ void verbose_enter_scroll(void)
  */
 void verbose_leave_scroll(void)
 {
-  if (*p_vfile != NUL) {
+  if (*p_vfile != '\0') {
     if (--msg_silent < 0)
       msg_silent = 0;
   } else
@@ -2872,7 +2872,7 @@ static char_u *msg_show_console_dialog(char_u *message, char_u *buttons, int dfl
             hotkp += STRLEN(hotkp);
           else
             ++hotkp;
-          hotkp[copy_char(r + 1, hotkp, TRUE)] = NUL;
+          hotkp[copy_char(r + 1, hotkp, TRUE)] = '\0';
           if (dfltbutton)
             --dfltbutton;
 
@@ -2899,7 +2899,7 @@ static char_u *msg_show_console_dialog(char_u *message, char_u *buttons, int dfl
             *msgp++ = (dfltbutton == 1) ? ']' : ')';
 
             /* redefine hotkey */
-            hotkp[copy_char(r, hotkp, TRUE)] = NUL;
+            hotkp[copy_char(r, hotkp, TRUE)] = '\0';
           }
         } else {
           ++len;                    /* '&a' -> '[a]' */
@@ -2919,7 +2919,7 @@ static char_u *msg_show_console_dialog(char_u *message, char_u *buttons, int dfl
     if (copy) {
       *msgp++ = ':';
       *msgp++ = ' ';
-      *msgp = NUL;
+      *msgp = '\0';
     } else {
       len += (int)(STRLEN(message)
                    + 2                          /* for the NL's */
@@ -2938,7 +2938,7 @@ static char_u *msg_show_console_dialog(char_u *message, char_u *buttons, int dfl
        */
       vim_free(confirm_msg);
       confirm_msg = alloc(len);
-      *confirm_msg = NUL;
+      *confirm_msg = '\0';
       hotk = alloc(lenhotkey);
 
       *confirm_msg = '\n';
@@ -2949,7 +2949,7 @@ static char_u *msg_show_console_dialog(char_u *message, char_u *buttons, int dfl
 
       /* Define first default hotkey.  Keep the hotkey string NUL
        * terminated to avoid reading past the end. */
-      hotkp[copy_char(buttons, hotkp, TRUE)] = NUL;
+      hotkp[copy_char(buttons, hotkp, TRUE)] = '\0';
 
       /* Remember where the choices start, displaying starts here when
        * "hotkp" typed at the more prompt. */
@@ -3163,7 +3163,7 @@ int vim_vsnprintf(char *str, size_t str_m, char *fmt, va_list ap, typval_T *tvs)
 
   if (p == NULL)
     p = "";
-  while (*p != NUL) {
+  while (*p != '\0') {
     if (*p != '%') {
       char    *q = strchr(p + 1, '%');
       size_t n = (q == NULL) ? STRLEN(p) : (size_t)(q - p);
@@ -3646,7 +3646,7 @@ int vim_vsnprintf(char *str, size_t str_m, char *fmt, va_list ap, typval_T *tvs)
             l += sprintf(format + 1, ".%d", (int)precision);
           }
           format[l] = fmt_spec;
-          format[l + 1] = NUL;
+          format[l + 1] = '\0';
           str_arg_l = sprintf(tmp, format, f);
 
           if (remove_trailing_zeroes) {
@@ -3719,13 +3719,13 @@ int vim_vsnprintf(char *str, size_t str_m, char *fmt, va_list ap, typval_T *tvs)
          * the unrecognized conversion character	  */
         str_arg = p;
         str_arg_l = 0;
-        if (*p != NUL)
+        if (*p != '\0')
           str_arg_l++;            /* include invalid conversion specifier
                                      unchanged if not at end-of-string */
         break;
       }
 
-      if (*p != NUL)
+      if (*p != '\0')
         p++;             /* step over the just processed conversion specifier */
 
       /* insert padding to the left as requested by min_field_width;
diff --git a/src/misc1.c b/src/misc1.c
index 21fb8face1..63e5718e24 100644
--- a/src/misc1.c
+++ b/src/misc1.c
@@ -104,13 +104,13 @@ open_line (
   || defined(FEAT_CINDENT) || defined(FEAT_COMMENTS)
   char_u      *p;
 #endif
-  int saved_char = NUL;                 /* init for GCC */
+  int saved_char = '\0';                 /* init for GCC */
   pos_T       *pos;
   int do_si = (!p_paste && curbuf->b_p_si
                && !curbuf->b_p_cin
                );
   int no_si = FALSE;                    /* reset did_si afterwards */
-  int first_char = NUL;                 /* init for GCC */
+  int first_char = '\0';                 /* init for GCC */
   int vreplace_mode;
   int did_append;                       /* appended a new line */
   int saved_pi = curbuf->b_p_pi;           /* copy of preserveindent setting */
@@ -146,16 +146,16 @@ open_line (
      * might be replaced at the start of the next line (due to autoindent
      * etc) a bit later.
      */
-    replace_push(NUL);      /* Call twice because BS over NL expects it */
-    replace_push(NUL);
+    replace_push('\0');      /* Call twice because BS over NL expects it */
+    replace_push('\0');
     p = saved_line + curwin->w_cursor.col;
-    while (*p != NUL) {
+    while (*p != '\0') {
       if (has_mbyte)
         p += replace_push_mb(p);
       else
         replace_push(*p++);
     }
-    saved_line[curwin->w_cursor.col] = NUL;
+    saved_line[curwin->w_cursor.col] = '\0';
   }
 
   if ((State & INSERT)
@@ -168,7 +168,7 @@ open_line (
     }
     extra_len = (int)STRLEN(p_extra);
     saved_char = *p_extra;
-    *p_extra = NUL;
+    *p_extra = '\0';
   }
 
   u_clearline();                /* cannot do "U" command when adding lines */
@@ -204,7 +204,7 @@ open_line (
      * don't add an indent. Fixes inserting a NL before '{' in line
      *	"if (condition) {"
      */
-    if (!trunc_line && do_si && *saved_line != NUL
+    if (!trunc_line && do_si && *saved_line != '\0'
         && (p_extra == NULL || first_char != '{')) {
       char_u  *ptr;
       char_u last_char;
@@ -252,7 +252,7 @@ open_line (
                  * the comment
                  */
                 curwin->w_cursor.col = (colnr_T)(p - ptr);
-                if ((pos = findmatch(NULL, NUL)) != NULL) {
+                if ((pos = findmatch(NULL, '\0')) != NULL) {
                   curwin->w_cursor.lnum = pos->lnum;
                   newindent = get_indent();
                 }
@@ -349,7 +349,7 @@ open_line (
    * Find out if the current line starts with a comment leader.
    * This may then be inserted in front of the new line.
    */
-  end_comment_pending = NUL;
+  end_comment_pending = '\0';
   if (flags & OPENLINE_DO_COM)
     lead_len = get_leader_len(saved_line, &lead_flags, dir == BACKWARD, TRUE);
   else
@@ -442,7 +442,7 @@ open_line (
               && ((p_extra != NULL
                    && (int)curwin->w_cursor.col == lead_len)
                   || (p_extra == NULL
-                      && saved_line[lead_len] == NUL)
+                      && saved_line[lead_len] == '\0')
                   || require_blank))
             extra_space = TRUE;
         }
@@ -517,7 +517,7 @@ open_line (
         int c = 0;
         int off = 0;
 
-        for (p = lead_flags; *p != NUL && *p != ':'; ) {
+        for (p = lead_flags; *p != '\0' && *p != ':'; ) {
           if (*p == COM_RIGHT || *p == COM_LEFT)
             c = *p++;
           else if (VIM_ISDIGIT(*p) || *p == '-')
@@ -553,7 +553,7 @@ open_line (
           }
           memmove(p, lead_repl, (size_t)lead_repl_len);
           if (p + lead_repl_len > leader + lead_len)
-            p[lead_repl_len] = NUL;
+            p[lead_repl_len] = '\0';
 
           /* blank-out any other chars from the old leader. */
           while (--p >= leader) {
@@ -583,7 +583,7 @@ open_line (
             int i;
             int l;
 
-            for (i = 0; p[i] != NUL && i < lead_len; i += l) {
+            for (i = 0; p[i] != '\0' && i < lead_len; i += l) {
               l = (*mb_ptr2len)(p + i);
               if (vim_strnsize(p, i + l) > repl_size)
                 break;
@@ -623,7 +623,7 @@ open_line (
                 *p = ' ';
               }
             }
-          *p = NUL;
+          *p = '\0';
         }
 
         /* Recompute the indent, it may have changed. */
@@ -654,12 +654,12 @@ open_line (
          * extra space */
         if (lead_len > 0 && vim_iswhite(leader[lead_len - 1]))
           extra_space = FALSE;
-        leader[lead_len] = NUL;
+        leader[lead_len] = '\0';
       }
 
       if (extra_space) {
         leader[lead_len++] = ' ';
-        leader[lead_len] = NUL;
+        leader[lead_len] = '\0';
       }
 
       newcol = lead_len;
@@ -692,7 +692,7 @@ open_line (
           )) {
         old_cursor = curwin->w_cursor;
         curwin->w_cursor.col = (colnr_T)(comment_end - saved_line);
-        if ((pos = findmatch(NULL, NUL)) != NULL) {
+        if ((pos = findmatch(NULL, '\0')) != NULL) {
           curwin->w_cursor.lnum = pos->lnum;
           newindent = get_indent();
         }
@@ -713,7 +713,7 @@ open_line (
      * preceded by a NUL, so they can be put back when a BS is entered.
      */
     if (REPLACE_NORMAL(State))
-      replace_push(NUL);            /* end of extra blanks */
+      replace_push('\0');            /* end of extra blanks */
     if (curbuf->b_p_ai || (flags & OPENLINE_DELSPACES)) {
       while ((*p_extra == ' ' || *p_extra == '\t')
              && (!enc_utf8
@@ -725,7 +725,7 @@ open_line (
         ++less_cols_off;
       }
     }
-    if (*p_extra != NUL)
+    if (*p_extra != '\0')
       did_ai = FALSE;               /* append some text, don't truncate now */
 
     /* columns for marks adjusted for removed columns */
@@ -756,7 +756,7 @@ open_line (
     did_ai = TRUE;          /* So truncating blanks works with comments */
     less_cols -= lead_len;
   } else
-    end_comment_pending = NUL;      /* turns out there was no leader */
+    end_comment_pending = '\0';      /* turns out there was no leader */
 
   old_cursor = curwin->w_cursor;
   if (dir == BACKWARD)
@@ -820,7 +820,7 @@ open_line (
      */
     if (REPLACE_NORMAL(State))
       for (n = 0; n < (int)curwin->w_cursor.col; ++n)
-        replace_push(NUL);
+        replace_push('\0');
     newcol += curwin->w_cursor.col;
     if (no_si)
       did_si = FALSE;
@@ -832,14 +832,14 @@ open_line (
    */
   if (REPLACE_NORMAL(State))
     while (lead_len-- > 0)
-      replace_push(NUL);
+      replace_push('\0');
 
   curwin->w_cursor = old_cursor;
 
   if (dir == FORWARD) {
     if (trunc_line || (State & INSERT)) {
       /* truncate current line at cursor */
-      saved_line[curwin->w_cursor.col] = NUL;
+      saved_line[curwin->w_cursor.col] = '\0';
       /* Remove trailing white space, unless OPENLINE_KEEPTRAIL used. */
       if (trunc_line && !(flags & OPENLINE_KEEPTRAIL))
         truncate_spaces(saved_line);
@@ -897,7 +897,7 @@ open_line (
    */
   if (!p_paste
       && (curbuf->b_p_cin
-          || *curbuf->b_p_inde != NUL
+          || *curbuf->b_p_inde != '\0'
           )
       && in_cinkeys(dir == FORWARD
           ? KEY_OPEN_FORW
@@ -970,7 +970,7 @@ int get_leader_len(char_u *line, char_u **flags, int backward, int include_space
   /*
    * Repeat to match several nested comment strings.
    */
-  while (line[i] != NUL) {
+  while (line[i] != '\0') {
     /*
      * scan through the 'comments' option for a match
      */
@@ -985,7 +985,7 @@ int get_leader_len(char_u *line, char_u **flags, int backward, int include_space
       string = vim_strchr(part_buf, ':');
       if (string == NULL)           /* missing ':', ignore this part */
         continue;
-      *string++ = NUL;              /* isolate flags from string */
+      *string++ = '\0';              /* isolate flags from string */
 
       /* If we found a middle match previously, use that match when this
        * is not a middle or end. */
@@ -1013,15 +1013,15 @@ int get_leader_len(char_u *line, char_u **flags, int backward, int include_space
         while (vim_iswhite(string[0]))
           ++string;
       }
-      for (j = 0; string[j] != NUL && string[j] == line[i + j]; ++j)
+      for (j = 0; string[j] != '\0' && string[j] == line[i + j]; ++j)
         ;
-      if (string[j] != NUL)
+      if (string[j] != '\0')
         continue;          /* string doesn't match */
 
       /* When 'b' flag used, there must be white space or an
        * end-of-line after the string in the line. */
       if (vim_strchr(part_buf, COM_BLANK) != NULL
-          && !vim_iswhite(line[i + j]) && line[i + j] != NUL)
+          && !vim_iswhite(line[i + j]) && line[i + j] != '\0')
         continue;
 
       /* We have found a match, stop searching unless this is a middle
@@ -1118,7 +1118,7 @@ int get_last_leader_offset(char_u *line, char_u **flags)
                                  * happen. */
         continue;
       }
-      *string++ = NUL;          /* Isolate flags from string. */
+      *string++ = '\0';          /* Isolate flags from string. */
       com_leader = string;
 
       /*
@@ -1133,9 +1133,9 @@ int get_last_leader_offset(char_u *line, char_u **flags)
         while (vim_iswhite(string[0]))
           ++string;
       }
-      for (j = 0; string[j] != NUL && string[j] == line[i + j]; ++j)
+      for (j = 0; string[j] != '\0' && string[j] == line[i + j]; ++j)
         /* do nothing */;
-      if (string[j] != NUL)
+      if (string[j] != '\0')
         continue;
 
       /*
@@ -1143,7 +1143,7 @@ int get_last_leader_offset(char_u *line, char_u **flags)
        * end-of-line after the string in the line.
        */
       if (vim_strchr(part_buf, COM_BLANK) != NULL
-          && !vim_iswhite(line[i + j]) && line[i + j] != NUL) {
+          && !vim_iswhite(line[i + j]) && line[i + j] != '\0') {
         continue;
       }
 
@@ -1273,7 +1273,7 @@ int plines_win_nofold(win_T *wp, linenr_T lnum)
   int width;
 
   s = ml_get_buf(wp->w_buffer, lnum, FALSE);
-  if (*s == NUL)                /* empty line */
+  if (*s == '\0')                /* empty line */
     return 1;
   col = win_linetabsize(wp, s, (colnr_T)MAXCOL);
 
@@ -1281,7 +1281,7 @@ int plines_win_nofold(win_T *wp, linenr_T lnum)
    * If list mode is on, then the '$' at the end of the line may take up one
    * extra column.
    */
-  if (wp->w_p_list && lcs_eol != NUL)
+  if (wp->w_p_list && lcs_eol != '\0')
     col += 1;
 
   /*
@@ -1321,7 +1321,7 @@ int plines_win_col(win_T *wp, linenr_T lnum, long column)
   s = ml_get_buf(wp->w_buffer, lnum, FALSE);
 
   col = 0;
-  while (*s != NUL && --column >= 0) {
+  while (*s != '\0' && --column >= 0) {
     col += win_lbr_chartabsize(wp, s, (colnr_T)col, NULL);
     mb_ptr_adv(s);
   }
@@ -1478,7 +1478,7 @@ void ins_char_bytes(char_u *buf, int charlen)
        */
       getvcol(curwin, &curwin->w_cursor, NULL, &vcol, NULL);
       new_vcol = vcol + chartabsize(buf, vcol);
-      while (oldp[col + oldlen] != NUL && vcol < new_vcol) {
+      while (oldp[col + oldlen] != '\0' && vcol < new_vcol) {
         vcol += chartabsize(oldp + col + oldlen, vcol);
         /* Don't need to remove a TAB that takes us to the right
          * position. */
@@ -1490,7 +1490,7 @@ void ins_char_bytes(char_u *buf, int charlen)
           newlen += vcol - new_vcol;
       }
       curwin->w_p_list = old_list;
-    } else if (oldp[col] != NUL)  {
+    } else if (oldp[col] != '\0')  {
       /* normal replace */
       oldlen = (*mb_ptr2len)(oldp + col);
     }
@@ -1500,7 +1500,7 @@ void ins_char_bytes(char_u *buf, int charlen)
      * put back when BS is used.  The bytes of a multi-byte character are
      * done the other way around, so that the first byte is popped off
      * first (it tells the byte length of the character). */
-    replace_push(NUL);
+    replace_push('\0');
     for (i = 0; i < oldlen; ++i) {
       if (has_mbyte)
         i += replace_push_mb(oldp + col + i) - 1;
@@ -1599,7 +1599,7 @@ int del_char(int fixpos)
   if (has_mbyte) {
     /* Make sure the cursor is at the start of a character. */
     mb_adjust_cursor();
-    if (*ml_get_cursor() == NUL)
+    if (*ml_get_cursor() == '\0')
       return FAIL;
     return del_chars(1L, fixpos);
   }
@@ -1617,7 +1617,7 @@ int del_chars(long count, int fixpos)
   int l;
 
   p = ml_get_cursor();
-  for (i = 0; i < count && *p != NUL; ++i) {
+  for (i = 0; i < count && *p != '\0'; ++i) {
     l = (*mb_ptr2len)(p);
     bytes += l;
     p += l;
@@ -1664,7 +1664,7 @@ del_bytes (
     int n;
 
     (void)utfc_ptr2char(oldp + col, cc);
-    if (cc[0] != NUL) {
+    if (cc[0] != '\0') {
       /* Find the last composing char, there can be several. */
       n = col;
       do {
@@ -2461,7 +2461,7 @@ int get_keystroke(void)
     if (has_mbyte) {
       if (MB_BYTE2LEN(n) > len)
         continue;               /* more bytes to get */
-      buf[len >= buflen ? buflen - 1 : len] = NUL;
+      buf[len >= buflen ? buflen - 1 : len] = '\0';
       n = (*mb_ptr2char)(buf);
     }
 #ifdef UNIX
@@ -2672,7 +2672,7 @@ void init_homedir(void)
 
   var = (char_u *)os_getenv("HOME");
 
-  if (var != NULL && *var == NUL)       /* empty is same as not set */
+  if (var != NULL && *var == '\0')       /* empty is same as not set */
     var = NULL;
 
 
@@ -2795,7 +2795,7 @@ expand_env_esc (
         } else
 #endif
         {
-          while (c-- > 0 && *tail != NUL && ((vim_isIDc(*tail))
+          while (c-- > 0 && *tail != '\0' && ((vim_isIDc(*tail))
                                              )) {
             *var++ = *tail++;
           }
@@ -2816,14 +2816,14 @@ expand_env_esc (
 #endif
             ++tail;
 #endif
-        *var = NUL;
+        *var = '\0';
         var = vim_getenv(dst, &mustfree);
 #if defined(MSWIN) || defined(UNIX)
       }
 #endif
       }
       /* home directory */
-      else if (  src[1] == NUL
+      else if (  src[1] == '\0'
                  || vim_ispathsep(src[1])
                  || vim_strchr((char_u *)" ,\t\n", src[1]) != NULL) {
         var = homedir;
@@ -2841,7 +2841,7 @@ expand_env_esc (
                    && vim_isfilec(*tail)
                    && !vim_ispathsep(*tail))
           *var++ = *tail++;
-        *var = NUL;
+        *var = '\0';
         /*
          * Use os_get_user_directory() to get the user directory.
          * If this function fails, the shell is used to
@@ -2896,14 +2896,14 @@ expand_env_esc (
         }
       }
 
-      if (var != NULL && *var != NUL
+      if (var != NULL && *var != '\0'
           && (STRLEN(var) + STRLEN(tail) + 1 < (unsigned)dstlen)) {
         STRCPY(dst, var);
         dstlen -= (int)STRLEN(var);
         c = (int)STRLEN(var);
         /* if var[] ends in a path separator and tail[] starts
          * with it, skip a character */
-        if (*var != NUL && after_pathsep(dst, dst + c)
+        if (*var != '\0' && after_pathsep(dst, dst + c)
 #if defined(BACKSLASH_IN_FILENAME)
             && dst[-1] != ':'
 #endif
@@ -2924,7 +2924,7 @@ expand_env_esc (
        * ":edit foo ~ foo".
        */
       at_start = FALSE;
-      if (src[0] == '\\' && src[1] != NUL) {
+      if (src[0] == '\\' && src[1] != '\0') {
         *dst++ = *src++;
         --dstlen;
       } else if ((src[0] == ' ' || src[0] == ',') && !one)
@@ -2937,7 +2937,7 @@ expand_env_esc (
         at_start = TRUE;
     }
   }
-  *dst = NUL;
+  *dst = '\0';
 }
 
 /*
@@ -2955,7 +2955,7 @@ char_u *vim_getenv(char_u *name, int *mustfree)
 
 
   p = (char_u *)os_getenv((char *)name);
-  if (p != NULL && *p == NUL)       /* empty is the same as not set */
+  if (p != NULL && *p == '\0')       /* empty is the same as not set */
     p = NULL;
 
   if (p != NULL) {
@@ -2972,11 +2972,11 @@ char_u *vim_getenv(char_u *name, int *mustfree)
    */
   if (vimruntime
 #ifdef HAVE_PATHDEF
-      && *default_vimruntime_dir == NUL
+      && *default_vimruntime_dir == '\0'
 #endif
       ) {
     p = (char_u *)os_getenv("VIM");
-    if (p != NULL && *p == NUL)             /* empty is the same as not set */
+    if (p != NULL && *p == '\0')             /* empty is the same as not set */
       p = NULL;
     if (p != NULL) {
       p = vim_version_dir(p);
@@ -3053,10 +3053,10 @@ char_u *vim_getenv(char_u *name, int *mustfree)
    * default_vimruntime_dir */
   if (p == NULL) {
     /* Only use default_vimruntime_dir when it is not empty */
-    if (vimruntime && *default_vimruntime_dir != NUL) {
+    if (vimruntime && *default_vimruntime_dir != '\0') {
       p = default_vimruntime_dir;
       *mustfree = FALSE;
-    } else if (*default_vim_dir != NUL) {
+    } else if (*default_vim_dir != '\0') {
       if (vimruntime && (p = vim_version_dir(default_vim_dir)) != NULL)
         *mustfree = TRUE;
       else {
@@ -3091,7 +3091,7 @@ static char_u *vim_version_dir(char_u *vimdir)
 {
   char_u      *p;
 
-  if (vimdir == NULL || *vimdir == NUL)
+  if (vimdir == NULL || *vimdir == '\0')
     return NULL;
   p = concat_fnames(vimdir, (char_u *)VIM_VERSION_NODOT, TRUE);
   if (os_isdir(p))
@@ -3130,7 +3130,7 @@ void vim_setenv(char_u *name, char_u *val)
    * When setting $VIMRUNTIME adjust the directory to find message
    * translations to $VIMRUNTIME/lang.
    */
-  if (*val != NUL && STRICMP(name, "VIMRUNTIME") == 0) {
+  if (*val != '\0' && STRICMP(name, "VIMRUNTIME") == 0) {
     char_u  *buf = concat_str(val, (char_u *)"/lang");
     bindtextdomain(VIMPACKAGE, (char *)buf);
     vim_free(buf);
@@ -3227,7 +3227,7 @@ home_replace (
   char_u      *p;
 
   if (src == NULL) {
-    *dst = NUL;
+    *dst = '\0';
     return;
   }
 
@@ -3248,7 +3248,7 @@ home_replace (
 
   homedir_env_orig = homedir_env = (char_u *)os_getenv("HOME");
   /* Empty is the same as not set. */
-  if (homedir_env != NULL && *homedir_env == NUL)
+  if (homedir_env != NULL && *homedir_env == '\0')
     homedir_env = NULL;
 
   if (homedir_env != NULL && vim_strchr(homedir_env, '~') != NULL) {
@@ -3262,7 +3262,7 @@ home_replace (
     flen = (int)STRLEN(homedir_env);
     if (flen > 0 && vim_ispathsep(homedir_env[flen - 1]))
       /* Remove the trailing / that is added to a directory. */
-      homedir_env[flen - 1] = NUL;
+      homedir_env[flen - 1] = '\0';
   }
 
   if (homedir_env != NULL)
@@ -3287,7 +3287,7 @@ home_replace (
              && fnamencmp(src, p, len) == 0
              && (vim_ispathsep(src[len])
                  || (!one && (src[len] == ',' || src[len] == ' '))
-                 || src[len] == NUL)) {
+                 || src[len] == '\0')) {
         src += len;
         if (--dstlen > 0)
           *dst++ = '~';
@@ -3314,7 +3314,7 @@ home_replace (
   }
   /* if (dstlen == 0) out of space, what to do??? */
 
-  *dst = NUL;
+  *dst = '\0';
 
   if (homedir_env != homedir_env_orig)
     vim_free(homedir_env);
@@ -3510,10 +3510,10 @@ get_cmd_output (
   } else {
     /* Change NUL into SOH, otherwise the string is truncated. */
     for (i = 0; i < len; ++i)
-      if (buffer[i] == NUL)
+      if (buffer[i] == '\0')
         buffer[i] = 1;
 
-    buffer[len] = NUL;          /* make sure the buffer is terminated */
+    buffer[len] = '\0';          /* make sure the buffer is terminated */
   }
 
 done:
diff --git a/src/misc2.c b/src/misc2.c
index d946bd66f5..8dae4a79c8 100644
--- a/src/misc2.c
+++ b/src/misc2.c
@@ -163,7 +163,7 @@ coladvance2 (
   int head = 0;
 
   one_more = (State & INSERT)
-             || restart_edit != NUL
+             || restart_edit != '\0'
              || (VIsual_active && *p_sel != 'o')
              || ((ve_flags & VE_ONEMORE) && wcol < MAXCOL)
   ;
@@ -200,7 +200,7 @@ coladvance2 (
     }
 
     ptr = line;
-    while (col <= wcol && *ptr != NUL) {
+    while (col <= wcol && *ptr != '\0') {
       /* Count a tab for what it's worth (if list mode not on) */
       csize = win_lbr_chartabsize(curwin, ptr, col, &head);
       mb_ptr_adv(ptr);
@@ -226,7 +226,7 @@ coladvance2 (
       /* 'virtualedit' is set: The difference between wcol and col is
        * filled with spaces. */
 
-      if (line[idx] == NUL) {
+      if (line[idx] == '\0') {
         /* Append spaces */
         int correct = wcol - col;
         char_u  *newline = alloc(idx + correct + 1);
@@ -238,7 +238,7 @@ coladvance2 (
         for (t = 0; t < correct; ++t)
           newline[t + idx] = ' ';
 
-        newline[idx + correct] = NUL;
+        newline[idx + correct] = '\0';
 
         ml_replace(pos->lnum, newline, FALSE);
         changed_bytes(pos->lnum, (colnr_T)idx);
@@ -265,7 +265,7 @@ coladvance2 (
               newline[s++] = ' ';
         }
 
-        newline[linelen + csize - 1] = NUL;
+        newline[linelen + csize - 1] = '\0';
 
         ml_replace(pos->lnum, newline, FALSE);
         changed_bytes(pos->lnum, idx);
@@ -330,16 +330,16 @@ int inc(pos_T *lp)
 {
   char_u  *p = ml_get_pos(lp);
 
-  if (*p != NUL) {      /* still within line, move to next char (may be NUL) */
+  if (*p != '\0') {      /* still within line, move to next char (may be NUL) */
     if (has_mbyte) {
       int l = (*mb_ptr2len)(p);
 
       lp->col += l;
-      return (p[l] != NUL) ? 0 : 2;
+      return (p[l] != '\0') ? 0 : 2;
     }
     lp->col++;
     lp->coladd = 0;
-    return (p[1] != NUL) ? 0 : 2;
+    return (p[1] != '\0') ? 0 : 2;
   }
   if (lp->lnum != curbuf->b_ml.ml_line_count) {     /* there is a next line */
     lp->col = 0;
@@ -538,7 +538,7 @@ void adjust_cursor_col(void)
 {
   if (curwin->w_cursor.col > 0
       && (!VIsual_active || *p_sel == 'o')
-      && gchar_cursor() == NUL)
+      && gchar_cursor() == '\0')
     --curwin->w_cursor.col;
 }
 
@@ -618,7 +618,7 @@ char_u *vim_strnsave(char_u *string, int len)
 
   p = alloc((unsigned)(len + 1));
   STRNCPY(p, string, len);
-  p[len] = NUL;
+  p[len] = '\0';
   return p;
 }
 
@@ -672,7 +672,7 @@ char_u *vim_strsave_escaped_ext(char_u *string, char_u *esc_chars, int cc, int b
       *p2++ = cc;
     *p2++ = *p;
   }
-  *p2 = NUL;
+  *p2 = '\0';
 
   return escaped_string;
 }
@@ -712,7 +712,7 @@ char_u *vim_strsave_shellescape(char_u *string, bool do_special, bool do_newline
 
   /* First count the number of extra bytes required. */
   length = (unsigned)STRLEN(string) + 3;    /* two quotes and a trailing NUL */
-  for (p = string; *p != NUL; mb_ptr_adv(p)) {
+  for (p = string; *p != '\0'; mb_ptr_adv(p)) {
     if (*p == '\'')
       length += 3;                      /* ' => '\'' */
     if ((*p == '\n' && (csh_like || do_newline))
@@ -734,7 +734,7 @@ char_u *vim_strsave_shellescape(char_u *string, bool do_special, bool do_newline
   /* add opening quote */
   *d++ = '\'';
 
-  for (p = string; *p != NUL; ) {
+  for (p = string; *p != '\0'; ) {
     if (*p == '\'') {
       *d++ = '\'';
       *d++ = '\\';
@@ -763,7 +763,7 @@ char_u *vim_strsave_shellescape(char_u *string, bool do_special, bool do_newline
 
   /* add terminating quote and finish with a NUL */
   *d++ = '\'';
-  *d = NUL;
+  *d = '\0';
 
   return escaped_string;
 }
@@ -804,7 +804,7 @@ void vim_strup(char_u *p)
 
   if (p != NULL) {
     p2 = p;
-    while ((c = *p2) != NUL)
+    while ((c = *p2) != '\0')
       *p2++ = (c < 'a' || c > 'z') ? c : (c - 0x20);
   }
 }
@@ -822,7 +822,7 @@ char_u *strup_save(char_u *orig)
   res = p = vim_strsave(orig);
 
   if (res != NULL)
-    while (*p != NUL) {
+    while (*p != '\0') {
       int l;
 
       if (enc_utf8) {
@@ -893,7 +893,7 @@ void del_trailing_spaces(char_u *ptr)
 
   q = ptr + STRLEN(ptr);
   while (--q > ptr && vim_iswhite(q[0]) && q[-1] != '\\' && q[-1] != Ctrl_V)
-    *q = NUL;
+    *q = '\0';
 }
 
 /*
@@ -903,7 +903,7 @@ void del_trailing_spaces(char_u *ptr)
 void vim_strncpy(char_u *to, char_u *from, size_t len)
 {
   STRNCPY(to, from, len);
-  to[len] = NUL;
+  to[len] = '\0';
 }
 
 /*
@@ -917,7 +917,7 @@ void vim_strcat(char_u *to, char_u *from, size_t tosize)
 
   if (tolen + fromlen + 1 > tosize) {
     memmove(to + tolen, from, tosize - tolen - 1);
-    to[tosize - 1] = NUL;
+    to[tosize - 1] = '\0';
   } else
     STRCPY(to + tolen, from);
 }
@@ -937,7 +937,7 @@ int copy_option_part(char_u **option, char_u *buf, int maxlen, char *sep_chars)
   /* skip '.' at start of option part, for 'suffixes' */
   if (*p == '.')
     buf[len++] = *p++;
-  while (*p != NUL && vim_strchr((char_u *)sep_chars, *p) == NULL) {
+  while (*p != '\0' && vim_strchr((char_u *)sep_chars, *p) == NULL) {
     /*
      * Skip backslash before a separator character and space.
      */
@@ -947,9 +947,9 @@ int copy_option_part(char_u **option, char_u *buf, int maxlen, char *sep_chars)
       buf[len++] = *p;
     ++p;
   }
-  buf[len] = NUL;
+  buf[len] = '\0';
 
-  if (*p != NUL && *p != ',')   /* skip non-standard separator */
+  if (*p != '\0' && *p != ',')   /* skip non-standard separator */
     ++p;
   p = skip_to_option_part(p);   /* p points to next file name */
 
@@ -983,7 +983,7 @@ int vim_stricmp(char *s1, char *s2)
     i = (int)TOLOWER_LOC(*s1) - (int)TOLOWER_LOC(*s2);
     if (i != 0)
       return i;                             /* this character different */
-    if (*s1 == NUL)
+    if (*s1 == '\0')
       break;                                /* strings match until NUL */
     ++s1;
     ++s2;
@@ -1006,7 +1006,7 @@ int vim_strnicmp(char *s1, char *s2, size_t len)
     i = (int)TOLOWER_LOC(*s1) - (int)TOLOWER_LOC(*s2);
     if (i != 0)
       return i;                             /* this character different */
-    if (*s1 == NUL)
+    if (*s1 == '\0')
       break;                                /* strings match until NUL */
     ++s1;
     ++s2;
@@ -1028,7 +1028,7 @@ char_u *vim_strchr(char_u *string, int c)
 
   p = string;
   if (enc_utf8 && c >= 0x80) {
-    while (*p != NUL) {
+    while (*p != '\0') {
       if (utf_ptr2char(p) == c)
         return p;
       p += (*mb_ptr2len)(p);
@@ -1039,7 +1039,7 @@ char_u *vim_strchr(char_u *string, int c)
     int n2 = c & 0xff;
 
     c = ((unsigned)c >> 8) & 0xff;
-    while ((b = *p) != NUL) {
+    while ((b = *p) != '\0') {
       if (b == c && p[1] == n2)
         return p;
       p += (*mb_ptr2len)(p);
@@ -1047,14 +1047,14 @@ char_u *vim_strchr(char_u *string, int c)
     return NULL;
   }
   if (has_mbyte) {
-    while ((b = *p) != NUL) {
+    while ((b = *p) != '\0') {
       if (b == c)
         return p;
       p += (*mb_ptr2len)(p);
     }
     return NULL;
   }
-  while ((b = *p) != NUL) {
+  while ((b = *p) != '\0') {
     if (b == c)
       return p;
     ++p;
@@ -1071,7 +1071,7 @@ char_u *vim_strbyte(char_u *string, int c)
 {
   char_u      *p = string;
 
-  while (*p != NUL) {
+  while (*p != '\0') {
     if (*p == c)
       return p;
     ++p;
@@ -1217,19 +1217,19 @@ int call_shell(char_u *cmd, ShellOpts opts, char_u *extra_shell_arg)
   if (do_profiling == PROF_YES)
     prof_child_enter(&wait_time);
 
-  if (*p_sh == NUL) {
+  if (*p_sh == '\0') {
     EMSG(_(e_shellempty));
     retval = -1;
   } else {
     /* The external command may update a tags file, clear cached tags. */
     tag_freematch();
 
-    if (cmd == NULL || *p_sxq == NUL)
+    if (cmd == NULL || *p_sxq == '\0')
       retval = os_call_shell(cmd, opts, extra_shell_arg);
     else {
       char_u *ecmd = cmd;
 
-      if (*p_sxe != NUL && STRCMP(p_sxq, "(") == 0) {
+      if (*p_sxe != '\0' && STRCMP(p_sxq, "(") == 0) {
         ecmd = vim_strsave_escaped_ext(cmd, p_sxe, '^', FALSE);
         if (ecmd == NULL)
           ecmd = cmd;
@@ -1293,7 +1293,7 @@ int vim_chdirfile(char_u *fname)
   char_u dir[MAXPATHL];
 
   vim_strncpy(dir, fname, MAXPATHL - 1);
-  *path_tail_with_sep(dir) = NUL;
+  *path_tail_with_sep(dir) = '\0';
   return os_chdir((char *)dir) == 0 ? OK : FAIL;
 }
 #endif
@@ -1306,7 +1306,7 @@ int vim_chdirfile(char_u *fname)
  */
 int illegal_slash(char *name)
 {
-  if (name[0] == NUL)
+  if (name[0] == '\0')
     return FALSE;           /* no file name is not illegal */
   if (name[strlen(name) - 1] != '/')
     return FALSE;           /* no trailing slash */
@@ -1447,7 +1447,7 @@ char_u *read_string(FILE *fd, int cnt)
     }
     str[i] = c;
   }
-  str[i] = NUL;
+  str[i] = '\0';
 
   return str;
 }
@@ -1510,7 +1510,7 @@ int has_non_ascii(char_u *s)
   char_u      *p;
 
   if (s != NULL)
-    for (p = s; *p != NUL; ++p)
+    for (p = s; *p != '\0'; ++p)
       if (*p >= 128)
         return TRUE;
   return FALSE;
diff --git a/src/move.c b/src/move.c
index f234d968a6..1c7fd8b32d 100644
--- a/src/move.c
+++ b/src/move.c
@@ -796,7 +796,7 @@ curs_columns (
       /* When cursor wraps to first char of next line in Insert
        * mode, the 'showbreak' string isn't shown, backup to first
        * column */
-      if (*p_sbr && *ml_get_cursor() == NUL
+      if (*p_sbr && *ml_get_cursor() == '\0'
           && curwin->w_wcol == (int)vim_strsize(p_sbr))
         curwin->w_wcol = 0;
     }
diff --git a/src/msgpack_rpc.c b/src/msgpack_rpc.c
index 5e36830132..aa4cfa57cb 100644
--- a/src/msgpack_rpc.c
+++ b/src/msgpack_rpc.c
@@ -93,7 +93,7 @@ char *msgpack_rpc_raw_argument(msgpack_object *obj)
 {
   char *rv = xmalloc(obj->via.raw.size + 1);
   memcpy(rv, obj->via.raw.ptr, obj->via.raw.size);
-  rv[obj->via.raw.size] = NUL;
+  rv[obj->via.raw.size] = '\0';
 
   return rv;
 }
diff --git a/src/normal.c b/src/normal.c
index 0d6410f3a9..fcf3fb36d6 100644
--- a/src/normal.c
+++ b/src/normal.c
@@ -58,10 +58,10 @@
 /*
  * The Visual area is remembered for reselection.
  */
-static int resel_VIsual_mode = NUL;             /* 'v', 'V', or Ctrl-V */
+static int resel_VIsual_mode = '\0';             /* 'v', 'V', or Ctrl-V */
 static linenr_T resel_VIsual_line_count;        /* number of lines */
 static colnr_T resel_VIsual_vcol;               /* nr of cols or end col */
-static int VIsual_mode_orig = NUL;              /* saved Visual mode */
+static int VIsual_mode_orig = '\0';              /* saved Visual mode */
 
 static int restart_VIsual_select = 0;
 
@@ -222,7 +222,7 @@ static const struct nv_cmd {
   short cmd_arg;                /* value for ca.arg */
 } nv_cmds[] =
 {
-  {NUL,       nv_error,       0,                      0},
+  {'\0',      nv_error,       0,                      0},
   {Ctrl_A,    nv_addsub,      0,                      0},
   {Ctrl_B,    nv_page,        NV_STS,                 BACKWARD},
   {Ctrl_C,    nv_esc,         0,                      TRUE},
@@ -595,7 +595,7 @@ normal_cmd (
            || (VIsual_active && mapped_len == 0 && typebuf_maplen() > 0))
     old_mapped_len = typebuf_maplen();
 
-  if (c == NUL)
+  if (c == '\0')
     c = K_ZERO;
 
   /*
@@ -919,7 +919,7 @@ getcount:
         ca.nchar = ca.extra_char;
         idx = find_command(ca.cmdchar);
       } else if ((ca.nchar == 'n' || ca.nchar == 'N') && ca.cmdchar == 'g')
-        ca.oap->op_type = get_op_type(*cp, NUL);
+        ca.oap->op_type = get_op_type(*cp, '\0');
       else if (*cp == Ctrl_BSL) {
         long towait = (p_ttm >= 0 ? p_ttm : p_tm);
 
@@ -1194,7 +1194,7 @@ void do_pending_operator(cmdarg_T *cap, int old_col, int gui_yank)
   int restart_edit_save;
 
   /* The visual area is remembered for redo */
-  static int redo_VIsual_mode = NUL;        /* 'v', 'V', or Ctrl-V */
+  static int redo_VIsual_mode = '\0';        /* 'v', 'V', or Ctrl-V */
   static linenr_T redo_VIsual_line_count;   /* number of lines */
   static colnr_T redo_VIsual_vcol;          /* number of cols or end column */
   static long redo_VIsual_count;            /* count for Visual operator */
@@ -1303,9 +1303,9 @@ void do_pending_operator(cmdarg_T *cap, int old_col, int gui_yank)
         curbuf->b_visual.vi_start = VIsual;
         curbuf->b_visual.vi_end = curwin->w_cursor;
         curbuf->b_visual.vi_mode = VIsual_mode;
-        if (VIsual_mode_orig != NUL) {
+        if (VIsual_mode_orig != '\0') {
           curbuf->b_visual.vi_mode = VIsual_mode_orig;
-          VIsual_mode_orig = NUL;
+          VIsual_mode_orig = '\0';
         }
         curbuf->b_visual.vi_curswant = curwin->w_curswant;
         curbuf->b_visual_mode_eval = VIsual_mode;
@@ -1458,7 +1458,7 @@ void do_pending_operator(cmdarg_T *cap, int old_col, int gui_yank)
           && oap->op_type != OP_FOLDCLOSEREC
           && oap->op_type != OP_FOLDDEL
           && oap->op_type != OP_FOLDDELREC
-          && oap->motion_force == NUL
+          && oap->motion_force == '\0'
           ) {
         /* Prepare for redoing.  Only use the nchar field for "r",
          * otherwise it might be the second char of the operator. */
@@ -1468,11 +1468,11 @@ void do_pending_operator(cmdarg_T *cap, int old_col, int gui_yank)
               get_op_char(oap->op_type), get_extra_op_char(oap->op_type),
               oap->motion_force, cap->cmdchar, cap->nchar);
         else if (cap->cmdchar != ':')
-          prep_redo(oap->regname, 0L, NUL, 'v',
+          prep_redo(oap->regname, 0L, '\0', 'v',
               get_op_char(oap->op_type),
               get_extra_op_char(oap->op_type),
               oap->op_type == OP_REPLACE
-              ? cap->nchar : NUL);
+              ? cap->nchar : '\0');
         if (!redo_VIsual_busy) {
           redo_VIsual_mode = resel_VIsual_mode;
           redo_VIsual_vcol = resel_VIsual_vcol;
@@ -1486,13 +1486,13 @@ void do_pending_operator(cmdarg_T *cap, int old_col, int gui_yank)
        * If oap->end is on a NUL (empty line) oap->inclusive becomes
        * FALSE.  This makes "d}P" and "v}dP" work the same.
        */
-      if (oap->motion_force == NUL || oap->motion_type == MLINE)
+      if (oap->motion_force == '\0' || oap->motion_type == MLINE)
         oap->inclusive = TRUE;
       if (VIsual_mode == 'V')
         oap->motion_type = MLINE;
       else {
         oap->motion_type = MCHAR;
-        if (VIsual_mode != Ctrl_V && *ml_get_pos(&(oap->end)) == NUL
+        if (VIsual_mode != Ctrl_V && *ml_get_pos(&(oap->end)) == '\0'
             && (include_line_break || !virtual_op)
             ) {
           oap->inclusive = FALSE;
@@ -1535,7 +1535,7 @@ void do_pending_operator(cmdarg_T *cap, int old_col, int gui_yank)
              || oap->op_type == OP_COLON
              || oap->op_type == OP_FUNCTION
              || oap->op_type == OP_FILTER)
-            && oap->motion_force == NUL)
+            && oap->motion_force == '\0')
           redraw_curbuf_later(INVERTED);
       }
     }
@@ -1557,7 +1557,7 @@ void do_pending_operator(cmdarg_T *cap, int old_col, int gui_yank)
     oap->empty = (oap->motion_type == MCHAR
                   && (!oap->inclusive
                       || (oap->op_type == OP_YANK
-                          && gchar_pos(&oap->end) == NUL))
+                          && gchar_pos(&oap->end) == '\0'))
                   && equalpos(oap->start, oap->end)
                   && !(virtual_op && oap->start.coladd != oap->end.coladd)
                   );
@@ -1686,13 +1686,13 @@ void do_pending_operator(cmdarg_T *cap, int old_col, int gui_yank)
       /*
        * If 'equalprg' is empty, do the indenting internally.
        */
-      if (oap->op_type == OP_INDENT && *get_equalprg() == NUL) {
+      if (oap->op_type == OP_INDENT && *get_equalprg() == '\0') {
         if (curbuf->b_p_lisp) {
           op_reindent(oap, get_lisp_indent);
           break;
         }
         op_reindent(oap,
-            *curbuf->b_p_inde != NUL ? get_expr_indent :
+            *curbuf->b_p_inde != '\0' ? get_expr_indent :
             get_c_indent);
         break;
       }
@@ -1713,9 +1713,9 @@ void do_pending_operator(cmdarg_T *cap, int old_col, int gui_yank)
       break;
 
     case OP_FORMAT:
-      if (*curbuf->b_p_fex != NUL)
+      if (*curbuf->b_p_fex != '\0')
         op_formatexpr(oap);             /* use expression */
-      else if (*p_fp != NUL)
+      else if (*p_fp != '\0')
         op_colon(oap);                  /* use external command */
       else
         op_format(oap, FALSE);          /* use internal function */
@@ -1841,7 +1841,7 @@ static void op_colon(oparg_T *oap)
     stuffReadbuff(get_equalprg());
     stuffReadbuff((char_u *)"\n");
   } else if (oap->op_type == OP_FORMAT) {
-    if (*p_fp == NUL)
+    if (*p_fp == '\0')
       stuffReadbuff((char_u *)"fmt");
     else
       stuffReadbuff(p_fp);
@@ -1861,7 +1861,7 @@ static void op_function(oparg_T *oap)
   char_u      *(argv[1]);
   int save_virtual_op = virtual_op;
 
-  if (*p_opfunc == NUL)
+  if (*p_opfunc == '\0')
     EMSG(_("E774: 'operatorfunc' is empty"));
   else {
     /* Set '[ and '] marks to text to be operated on. */
@@ -1980,7 +1980,7 @@ do_mouse (
     if (is_drag) {
       /* If the next character is the same mouse event then use that
        * one. Speeds up dragging the status line. */
-      if (vpeekc() != NUL) {
+      if (vpeekc() != '\0') {
         int nc;
         int save_mouse_row = mouse_row;
         int save_mouse_col = mouse_col;
@@ -2448,9 +2448,9 @@ do_mouse (
       c2 = 'p';
     } else {
       c1 = (dir == FORWARD) ? 'p' : 'P';
-      c2 = NUL;
+      c2 = '\0';
     }
-    prep_redo(regname, count, NUL, c1, NUL, c2, NUL);
+    prep_redo(regname, count, '\0', c1, '\0', c2, '\0');
 
     /*
      * Remember where the paste started, so in edit() Insstart can be set
@@ -2551,7 +2551,7 @@ do_mouse (
             && VIsual_mode == 'v'
             && !vim_iswordc(gchar_pos(&end_visual))
             && equalpos(curwin->w_cursor, VIsual)
-            && (pos = findmatch(oap, NUL)) != NULL) {
+            && (pos = findmatch(oap, '\0')) != NULL) {
           curwin->w_cursor = *pos;
           if (oap->motion_type == MLINE)
             VIsual_mode = 'V';
@@ -2572,7 +2572,7 @@ do_mouse (
           find_end_of_word(&VIsual);
         } else {
           find_start_of_word(&VIsual);
-          if (*p_sel == 'e' && *ml_get_cursor() != NUL)
+          if (*p_sel == 'e' && *ml_get_cursor() != '\0')
             curwin->w_cursor.col +=
               (*mb_ptr2len)(ml_get_cursor());
           find_end_of_word(&curwin->w_cursor);
@@ -2635,7 +2635,7 @@ static void find_end_of_word(pos_T *pos)
     pos->col -= (*mb_head_off)(line, line + pos->col);
   }
   cclass = get_mouse_class(line + pos->col);
-  while (line[pos->col] != NUL) {
+  while (line[pos->col] != '\0') {
     col = pos->col + (*mb_ptr2len)(line + pos->col);
     if (get_mouse_class(line + col) != cclass) {
       if (*p_sel == 'e')
@@ -2673,7 +2673,7 @@ static int get_mouse_class(char_u *p)
    * "->", "/ *", "*=", "+=", "&=", "<=", ">=", "!=" etc.  Otherwise, each
    * character is in its own class.
    */
-  if (c != NUL && vim_strchr((char_u *)"-+*/%<>&|^!=", c) != NULL)
+  if (c != '\0' && vim_strchr((char_u *)"-+*/%<>&|^!=", c) != NULL)
     return 1;
   return c;
 }
@@ -2797,14 +2797,14 @@ int find_ident_at_pos(win_T *wp, linenr_T lnum, colnr_T startcol, char_u **strin
      */
     col = startcol;
     if (has_mbyte) {
-      while (ptr[col] != NUL) {
+      while (ptr[col] != '\0') {
         this_class = mb_get_class(ptr + col);
         if (this_class != 0 && (i == 1 || this_class != 1))
           break;
         col += (*mb_ptr2len)(ptr + col);
       }
     } else
-      while (ptr[col] != NUL
+      while (ptr[col] != '\0'
              && (i == 0 ? !vim_iswordc(ptr[col]) : vim_iswhite(ptr[col]))
              )
         ++col;
@@ -2851,7 +2851,7 @@ int find_ident_at_pos(win_T *wp, linenr_T lnum, colnr_T startcol, char_u **strin
     }
   }
 
-  if (ptr[col] == NUL || (i == 0 && (
+  if (ptr[col] == '\0' || (i == 0 && (
                             has_mbyte ? this_class != 2 :
                             !vim_iswordc(ptr[col])))) {
     /*
@@ -2873,14 +2873,14 @@ int find_ident_at_pos(win_T *wp, linenr_T lnum, colnr_T startcol, char_u **strin
   if (has_mbyte) {
     /* Search for point of changing multibyte character class. */
     this_class = mb_get_class(ptr);
-    while (ptr[col] != NUL
+    while (ptr[col] != '\0'
            && ((i == 0 ? mb_get_class(ptr + col) == this_class
                 : mb_get_class(ptr + col) != 0)
                ))
       col += (*mb_ptr2len)(ptr + col);
   } else
     while ((i == 0 ? vim_iswordc(ptr[col])
-            : (ptr[col] != NUL && !vim_iswhite(ptr[col])))
+            : (ptr[col] != '\0' && !vim_iswhite(ptr[col])))
            ) {
       ++col;
     }
@@ -2894,7 +2894,7 @@ int find_ident_at_pos(win_T *wp, linenr_T lnum, colnr_T startcol, char_u **strin
 static void prep_redo_cmd(cmdarg_T *cap)
 {
   prep_redo(cap->oap->regname, cap->count0,
-      NUL, cap->cmdchar, NUL, NUL, cap->nchar);
+      '\0', cap->cmdchar, '\0', '\0', cap->nchar);
 }
 
 /*
@@ -2911,15 +2911,15 @@ static void prep_redo(int regname, long num, int cmd1, int cmd2, int cmd3, int c
   if (num)
     AppendNumberToRedobuff(num);
 
-  if (cmd1 != NUL)
+  if (cmd1 != '\0')
     AppendCharToRedobuff(cmd1);
-  if (cmd2 != NUL)
+  if (cmd2 != '\0')
     AppendCharToRedobuff(cmd2);
-  if (cmd3 != NUL)
+  if (cmd3 != '\0')
     AppendCharToRedobuff(cmd3);
-  if (cmd4 != NUL)
+  if (cmd4 != '\0')
     AppendCharToRedobuff(cmd4);
-  if (cmd5 != NUL)
+  if (cmd5 != '\0')
     AppendCharToRedobuff(cmd5);
 }
 
@@ -2955,7 +2955,7 @@ static void clearop(oparg_T *oap)
 {
   oap->op_type = OP_NOP;
   oap->regname = 0;
-  oap->motion_force = NUL;
+  oap->motion_force = '\0';
   oap->use_reg_one = FALSE;
 }
 
@@ -3057,10 +3057,10 @@ void clear_showcmd(void)
       else
         sprintf((char *)showcmd_buf, "%d-%d", chars, bytes);
     }
-    showcmd_buf[SHOWCMD_COLS] = NUL;            /* truncate */
+    showcmd_buf[SHOWCMD_COLS] = '\0';            /* truncate */
     showcmd_visual = TRUE;
   } else {
-    showcmd_buf[0] = NUL;
+    showcmd_buf[0] = '\0';
     showcmd_visual = FALSE;
 
     /* Don't actually display something if there is nothing to clear. */
@@ -3098,7 +3098,7 @@ int add_to_showcmd(int c)
     return FALSE;
 
   if (showcmd_visual) {
-    showcmd_buf[0] = NUL;
+    showcmd_buf[0] = '\0';
     showcmd_visual = FALSE;
   }
 
@@ -3146,7 +3146,7 @@ static void del_from_showcmd(int len)
   old_len = (int)STRLEN(showcmd_buf);
   if (len > old_len)
     len = old_len;
-  showcmd_buf[old_len - len] = NUL;
+  showcmd_buf[old_len - len] = '\0';
 
   if (!char_avail())
     display_showcmd();
@@ -3459,7 +3459,7 @@ find_decl (
     par_pos = curwin->w_cursor;
   } else {
     par_pos = curwin->w_cursor;
-    while (curwin->w_cursor.lnum > 1 && *skipwhite(ml_get_curline()) != NUL)
+    while (curwin->w_cursor.lnum > 1 && *skipwhite(ml_get_curline()) != '\0')
       --curwin->w_cursor.lnum;
   }
   curwin->w_cursor.col = 0;
@@ -4407,11 +4407,11 @@ static void nv_ident(cmdarg_T *cap)
   /* Allocate buffer to put the command in.  Inserting backslashes can
    * double the length of the word.  p_kp / curbuf->b_p_kp could be added
    * and some numbers. */
-  kp = (*curbuf->b_p_kp == NUL ? p_kp : curbuf->b_p_kp);
-  kp_help = (*kp == NUL || STRCMP(kp, ":he") == 0
+  kp = (*curbuf->b_p_kp == '\0' ? p_kp : curbuf->b_p_kp);
+  kp_help = (*kp == '\0' || STRCMP(kp, ":he") == 0
              || STRCMP(kp, ":help") == 0);
   buf = alloc((unsigned)(n * 2 + 30 + STRLEN(kp)));
-  buf[0] = NUL;
+  buf[0] = '\0';
 
   switch (cmdchar) {
   case '*':
@@ -4528,7 +4528,7 @@ static void nv_ident(cmdarg_T *cap)
       }
       *p++ = *ptr++;
     }
-    *p = NUL;
+    *p = '\0';
   }
 
   /*
@@ -4541,7 +4541,7 @@ static void nv_ident(cmdarg_T *cap)
       STRCAT(buf, "\\>");
     /* put pattern in search history */
     init_history();
-    add_to_history(HIST_SEARCH, buf, TRUE, NUL);
+    add_to_history(HIST_SEARCH, buf, TRUE, '\0');
     normal_search(cap, cmdchar == '*' ? '/' : '?', buf, 0);
   } else
     do_cmdline_cmd(buf);
@@ -4698,7 +4698,7 @@ static void nv_right(cmdarg_T *cap)
 
   for (n = cap->count1; n > 0; --n) {
     if ((!PAST_LINE && oneright() == FAIL)
-        || (PAST_LINE && *ml_get_cursor() == NUL)
+        || (PAST_LINE && *ml_get_cursor() == '\0')
         ) {
       /*
        *	  <Space> wraps to next line if 'whichwrap' has 's'.
@@ -4801,7 +4801,7 @@ static void nv_left(cmdarg_T *cap)
                    && !lineempty(curwin->w_cursor.lnum)) {
           char_u *cp = ml_get_cursor();
 
-          if (*cp != NUL) {
+          if (*cp != '\0') {
             if (has_mbyte) {
               curwin->w_cursor.col += (*mb_ptr2len)(cp);
             } else {
@@ -4933,7 +4933,7 @@ static void nv_dollar(cmdarg_T *cap)
   /* In virtual mode when off the edge of a line and an operator
    * is pending (whew!) keep the cursor where it is.
    * Otherwise, send it to the end of the line. */
-  if (!virtual_active() || gchar_cursor() != NUL
+  if (!virtual_active() || gchar_cursor() != '\0'
       || cap->oap->op_type == OP_NOP)
     curwin->w_curswant = MAXCOL;        /* so we stay at the end */
   if (cursor_down((long)(cap->count1 - 1),
@@ -5269,7 +5269,7 @@ static void nv_brackets(cmdarg_T *cap)
         if (VIsual_mode == 'V') {
           /* delete visually selected lines */
           cap->cmdchar = 'd';
-          cap->nchar = NUL;
+          cap->nchar = '\0';
           cap->oap->regname = regname;
           nv_operator(cap);
           do_pending_operator(cap, 0, FALSE);
@@ -5371,7 +5371,7 @@ static void nv_percent(cmdarg_T *cap)
   } else {                /* "%" : go to matching paren */
     cap->oap->motion_type = MCHAR;
     cap->oap->use_reg_one = TRUE;
-    if ((pos = findmatch(cap->oap, NUL)) == NULL)
+    if ((pos = findmatch(cap->oap, '\0')) == NULL)
       clearopbeep(cap->oap);
     else {
       setpcmark();
@@ -5432,7 +5432,7 @@ static void nv_findpar(cmdarg_T *cap)
   cap->oap->inclusive = FALSE;
   cap->oap->use_reg_one = TRUE;
   curwin->w_set_curswant = TRUE;
-  if (!findpar(&cap->oap->inclusive, cap->arg, cap->count1, NUL, FALSE))
+  if (!findpar(&cap->oap->inclusive, cap->arg, cap->count1, '\0', FALSE))
     clearopbeep(cap->oap);
   else {
     curwin->w_cursor.coladd = 0;
@@ -5486,9 +5486,9 @@ static void nv_replace(cmdarg_T *cap)
     cap->nchar = get_literal();
     /* Don't redo a multibyte character with CTRL-V. */
     if (cap->nchar > DEL)
-      had_ctrl_v = NUL;
+      had_ctrl_v = '\0';
   } else
-    had_ctrl_v = NUL;
+    had_ctrl_v = '\0';
 
   /* Abort if the character is a special key. */
   if (IS_SPECIAL(cap->nchar)) {
@@ -5514,7 +5514,7 @@ static void nv_replace(cmdarg_T *cap)
   if (virtual_active()) {
     if (u_save_cursor() == FAIL)
       return;
-    if (gchar_cursor() == NUL) {
+    if (gchar_cursor() == '\0') {
       /* Add extra space and put the cursor on the first one. */
       coladvance_force((colnr_T)(getviscol() + cap->count1));
       curwin->w_cursor.col -= cap->count1;
@@ -5567,7 +5567,7 @@ static void nv_replace(cmdarg_T *cap)
     invoke_edit(cap, TRUE, 'r', FALSE);
   } else {
     prep_redo(cap->oap->regname, cap->count1,
-        NUL, 'r', NUL, had_ctrl_v, cap->nchar);
+        '\0', 'r', '\0', had_ctrl_v, cap->nchar);
 
     curbuf->b_op_start = curwin->w_cursor;
     if (has_mbyte) {
@@ -5586,7 +5586,7 @@ static void nv_replace(cmdarg_T *cap)
         if (cap->nchar == Ctrl_E || cap->nchar == Ctrl_Y) {
           int c = ins_copychar(curwin->w_cursor.lnum
               + (cap->nchar == Ctrl_Y ? -1 : 1));
-          if (c != NUL)
+          if (c != '\0')
             ins_char(c);
           else
             /* will be decremented further down */
@@ -5613,7 +5613,7 @@ static void nv_replace(cmdarg_T *cap)
         if (cap->nchar == Ctrl_E || cap->nchar == Ctrl_Y) {
           int c = ins_copychar(curwin->w_cursor.lnum
               + (cap->nchar == Ctrl_Y ? -1 : 1));
-          if (c != NUL)
+          if (c != '\0')
             ptr[curwin->w_cursor.col] = c;
         } else
           ptr[curwin->w_cursor.col] = cap->nchar;
@@ -5689,7 +5689,7 @@ static void nv_Replace(cmdarg_T *cap)
 {
   if (VIsual_active) {          /* "R" is replace lines */
     cap->cmdchar = 'c';
-    cap->nchar = NUL;
+    cap->nchar = '\0';
     VIsual_mode_orig = VIsual_mode;     /* remember original area for gv */
     VIsual_mode = 'V';
     nv_operator(cap);
@@ -5754,7 +5754,7 @@ static void n_swapchar(cmdarg_T *cap)
   for (n = cap->count1; n > 0; --n) {
     did_change |= swapchar(cap->oap->op_type, &curwin->w_cursor);
     inc_cursor();
-    if (gchar_cursor() == NUL) {
+    if (gchar_cursor() == '\0') {
       if (vim_strchr(p_ww, '~') != NULL
           && curwin->w_cursor.lnum < curbuf->b_ml.ml_line_count) {
         ++curwin->w_cursor.lnum;
@@ -5972,7 +5972,7 @@ static void nv_regname(cmdarg_T *cap)
     return;
   if (cap->nchar == '=')
     cap->nchar = get_expr_register();
-  if (cap->nchar != NUL && valid_yank_reg(cap->nchar, FALSE)) {
+  if (cap->nchar != '\0' && valid_yank_reg(cap->nchar, FALSE)) {
     cap->oap->regname = cap->nchar;
     cap->opcount = cap->count0;         /* remember count before '"' */
     set_reg_var(cap->oap->regname);
@@ -6011,7 +6011,7 @@ static void nv_visual(cmdarg_T *cap)
     redraw_curbuf_later(INVERTED);          /* update the inversion */
   } else {                /* start Visual mode */
     check_visual_highlight();
-    if (cap->count0 > 0 && resel_VIsual_mode != NUL) {
+    if (cap->count0 > 0 && resel_VIsual_mode != '\0') {
       /* use previously selected part */
       VIsual = curwin->w_cursor;
 
@@ -6136,7 +6136,7 @@ static void n_start_visual_mode(int c)
 static void nv_window(cmdarg_T *cap)
 {
   if (!checkclearop(cap->oap))
-    do_window(cap->nchar, cap->count0, NUL);     /* everything is in window.c */
+    do_window(cap->nchar, cap->count0, '\0');     /* everything is in window.c */
 }
 
 /*
@@ -6365,7 +6365,7 @@ static void nv_g_cmd(cmdarg_T *cap)
       char_u  *ptr = ml_get_curline();
 
       /* In Visual mode we may end up after the line. */
-      if (curwin->w_cursor.col > 0 && ptr[curwin->w_cursor.col] == NUL)
+      if (curwin->w_cursor.col > 0 && ptr[curwin->w_cursor.col] == '\0')
         --curwin->w_cursor.col;
 
       /* Decrease the cursor column until it's on a non-blank. */
@@ -6781,7 +6781,7 @@ static void set_op_var(int optype)
   else {
     opchars[0] = get_op_char(optype);
     opchars[1] = get_extra_op_char(optype);
-    opchars[2] = NUL;
+    opchars[2] = '\0';
     set_vim_var_string(VV_OP, opchars, -1);
   }
 }
@@ -6884,7 +6884,7 @@ static void nv_wordcmd(cmdarg_T *cap)
    */
   if (!word_end && cap->oap->op_type == OP_CHANGE) {
     n = gchar_cursor();
-    if (n != NUL) {                     /* not an empty line */
+    if (n != '\0') {                     /* not an empty line */
       if (vim_iswhite(n)) {
         /*
          * Reproduce a funny Vi behaviour: "cw" on a blank only
@@ -6949,7 +6949,7 @@ static void adjust_cursor(oparg_T *oap)
    * - not in Visual mode or 'selection' is "o"
    * - 'virtualedit' is not "all" and not "onemore".
    */
-  if (curwin->w_cursor.col > 0 && gchar_cursor() == NUL
+  if (curwin->w_cursor.col > 0 && gchar_cursor() == '\0'
       && (!VIsual_active || *p_sel == 'o')
       && !virtual_active() && (ve_flags & VE_ONEMORE) == 0
       ) {
@@ -6982,7 +6982,7 @@ static void nv_beginline(cmdarg_T *cap)
 static void adjust_for_sel(cmdarg_T *cap)
 {
   if (VIsual_active && cap->oap->inclusive && *p_sel == 'e'
-      && gchar_cursor() != NUL && lt(VIsual, curwin->w_cursor)) {
+      && gchar_cursor() != '\0' && lt(VIsual, curwin->w_cursor)) {
     if (has_mbyte)
       inc_cursor();
     else
@@ -7185,10 +7185,10 @@ static void nv_edit(cmdarg_T *cap)
        * column otherwise, also to append after an unprintable char */
       if (virtual_active()
           && (curwin->w_cursor.coladd > 0
-              || *ml_get_cursor() == NUL
+              || *ml_get_cursor() == '\0'
               || *ml_get_cursor() == TAB))
         curwin->w_cursor.coladd++;
-      else if (*ml_get_cursor() != NUL)
+      else if (*ml_get_cursor() != '\0')
         inc_cursor();
       break;
     }
@@ -7339,7 +7339,7 @@ static void nv_at(cmdarg_T *cap)
   if (checkclearop(cap->oap))
     return;
   if (cap->nchar == '=') {
-    if (get_expr_register() == NUL)
+    if (get_expr_register() == '\0')
       return;
   }
   while (cap->count1-- && !got_int) {
@@ -7379,8 +7379,8 @@ static void nv_join(cmdarg_T *cap)
       clearopbeep(cap->oap);        /* beyond last line */
     else {
       prep_redo(cap->oap->regname, cap->count0,
-          NUL, cap->cmdchar, NUL, NUL, cap->nchar);
-      (void)do_join(cap->count0, cap->nchar == NUL, TRUE, TRUE);
+          '\0', cap->cmdchar, '\0', '\0', cap->nchar);
+      (void)do_join(cap->count0, cap->nchar == '\0', TRUE, TRUE);
     }
   }
 }
@@ -7431,8 +7431,8 @@ static void nv_put(cmdarg_T *cap)
 
       /* Now delete the selected text. */
       cap->cmdchar = 'd';
-      cap->nchar = NUL;
-      cap->oap->regname = NUL;
+      cap->nchar = '\0';
+      cap->oap->regname = '\0';
       nv_operator(cap);
       do_pending_operator(cap, 0, FALSE);
       empty = (curbuf->b_ml.ml_flags & ML_EMPTY);
@@ -7482,7 +7482,7 @@ static void nv_put(cmdarg_T *cap)
 
     /* When all lines were selected and deleted do_put() leaves an empty
      * line that needs to be deleted now. */
-    if (empty && *ml_get(curbuf->b_ml.ml_line_count) == NUL) {
+    if (empty && *ml_get(curbuf->b_ml.ml_line_count) == '\0') {
       ml_delete(curbuf->b_ml.ml_line_count, TRUE);
 
       /* If the cursor was in that line, move it to the end of the last
diff --git a/src/ops.c b/src/ops.c
index 3a86432b01..eedbece866 100644
--- a/src/ops.c
+++ b/src/ops.c
@@ -119,34 +119,34 @@ static int fmt_check_par(linenr_T, int *, char_u **, int do_comments);
  */
 static char opchars[][3] =
 {
-  {NUL, NUL, FALSE},    /* OP_NOP */
-  {'d', NUL, FALSE},    /* OP_DELETE */
-  {'y', NUL, FALSE},    /* OP_YANK */
-  {'c', NUL, FALSE},    /* OP_CHANGE */
-  {'<', NUL, TRUE},     /* OP_LSHIFT */
-  {'>', NUL, TRUE},     /* OP_RSHIFT */
-  {'!', NUL, TRUE},     /* OP_FILTER */
-  {'g', '~', FALSE},    /* OP_TILDE */
-  {'=', NUL, TRUE},     /* OP_INDENT */
-  {'g', 'q', TRUE},     /* OP_FORMAT */
-  {':', NUL, TRUE},     /* OP_COLON */
-  {'g', 'U', FALSE},    /* OP_UPPER */
-  {'g', 'u', FALSE},    /* OP_LOWER */
-  {'J', NUL, TRUE},     /* DO_JOIN */
-  {'g', 'J', TRUE},     /* DO_JOIN_NS */
-  {'g', '?', FALSE},    /* OP_ROT13 */
-  {'r', NUL, FALSE},    /* OP_REPLACE */
-  {'I', NUL, FALSE},    /* OP_INSERT */
-  {'A', NUL, FALSE},    /* OP_APPEND */
-  {'z', 'f', TRUE},     /* OP_FOLD */
-  {'z', 'o', TRUE},     /* OP_FOLDOPEN */
-  {'z', 'O', TRUE},     /* OP_FOLDOPENREC */
-  {'z', 'c', TRUE},     /* OP_FOLDCLOSE */
-  {'z', 'C', TRUE},     /* OP_FOLDCLOSEREC */
-  {'z', 'd', TRUE},     /* OP_FOLDDEL */
-  {'z', 'D', TRUE},     /* OP_FOLDDELREC */
-  {'g', 'w', TRUE},     /* OP_FORMAT2 */
-  {'g', '@', FALSE},    /* OP_FUNCTION */
+  {'\0', '\0', FALSE},    /* OP_NOP */
+  {'d', '\0', FALSE},    /* OP_DELETE */
+  {'y', '\0', FALSE},    /* OP_YANK */
+  {'c', '\0', FALSE},    /* OP_CHANGE */
+  {'<', '\0', TRUE},     /* OP_LSHIFT */
+  {'>', '\0', TRUE},     /* OP_RSHIFT */
+  {'!', '\0', TRUE},     /* OP_FILTER */
+  {'g', '~',  FALSE},    /* OP_TILDE */
+  {'=', '\0', TRUE},     /* OP_INDENT */
+  {'g', 'q',  TRUE},     /* OP_FORMAT */
+  {':', '\0', TRUE},     /* OP_COLON */
+  {'g', 'U',  FALSE},    /* OP_UPPER */
+  {'g', 'u',  FALSE},    /* OP_LOWER */
+  {'J', '\0', TRUE},     /* DO_JOIN */
+  {'g', 'J',  TRUE},     /* DO_JOIN_NS */
+  {'g', '?',  FALSE},    /* OP_ROT13 */
+  {'r', '\0', FALSE},    /* OP_REPLACE */
+  {'I', '\0', FALSE},    /* OP_INSERT */
+  {'A', '\0', FALSE},    /* OP_APPEND */
+  {'z', 'f',  TRUE},     /* OP_FOLD */
+  {'z', 'o',  TRUE},     /* OP_FOLDOPEN */
+  {'z', 'O',  TRUE},     /* OP_FOLDOPENREC */
+  {'z', 'c',  TRUE},     /* OP_FOLDCLOSE */
+  {'z', 'C',  TRUE},     /* OP_FOLDCLOSEREC */
+  {'z', 'd',  TRUE},     /* OP_FOLDDEL */
+  {'z', 'D',  TRUE},     /* OP_FOLDDELREC */
+  {'g', 'w',  TRUE},     /* OP_FORMAT2 */
+  {'g', '@',  FALSE},    /* OP_FUNCTION */
 };
 
 /*
@@ -211,7 +211,7 @@ void op_shift(oparg_T *oap, int curs_top, int amount)
 
   for (i = oap->line_count; --i >= 0; ) {
     first_char = *ml_get_curline();
-    if (first_char == NUL)                              /* empty line */
+    if (first_char == '\0')                              /* empty line */
       curwin->w_cursor.col = 0;
     else if (oap->block_mode)
       shift_block(oap, amount);
@@ -309,7 +309,7 @@ shift_line (
 
   /* Set new indent */
   if (State & VREPLACE_FLAG)
-    change_indent(INDENT_SET, count, FALSE, NUL, call_changed_bytes);
+    change_indent(INDENT_SET, count, FALSE, '\0', call_changed_bytes);
   else
     (void)set_indent(count, call_changed_bytes ? SIN_CHANGED : 0);
 }
@@ -377,7 +377,7 @@ static void shift_block(oparg_T *oap, int amount)
     bd.textcol -= bd.pre_whitesp_c - (bd.startspaces != 0);
     len = (int)STRLEN(bd.textstart) + 1;
     newp = (char_u *) xmalloc((size_t)(bd.textcol + i + j + len));
-    memset(newp, NUL, (size_t)(bd.textcol + i + j + len));
+    memset(newp, '\0', (size_t)(bd.textcol + i + j + len));
     memmove(newp, oldp, (size_t)bd.textcol);
     copy_chars(newp + bd.textcol, (size_t)i, TAB);
     copy_spaces(newp + bd.textcol + i, (size_t)j);
@@ -597,7 +597,7 @@ int         (*how)(void);
     if (i != oap->line_count - 1 || oap->line_count == 1
         || how != get_lisp_indent) {
       l = skipwhite(ml_get_curline());
-      if (*l == NUL)                        /* empty or blank line */
+      if (*l == '\0')                        /* empty or blank line */
         count = 0;
       else
         count = how();                      /* get the indent for this line */
@@ -654,8 +654,8 @@ int get_expr_register(void)
 
   new_line = getcmdline('=', 0L, 0);
   if (new_line == NULL)
-    return NUL;
-  if (*new_line == NUL)         /* use previous line */
+    return '\0';
+  if (*new_line == '\0')         /* use previous line */
     vim_free(new_line);
   else
     set_expr_line(new_line);
@@ -927,7 +927,7 @@ static int stuff_yank(int regname, char_u *p)
   return OK;
 }
 
-static int execreg_lastc = NUL;
+static int execreg_lastc = '\0';
 
 /*
  * execute a yank register: copy it into the stuff buffer
@@ -948,7 +948,7 @@ do_execreg (
   int remap;
 
   if (regname == '@') {                 /* repeat previous one */
-    if (execreg_lastc == NUL) {
+    if (execreg_lastc == '\0') {
       EMSG(_("E748: No previously used register"));
       return FAIL;
     }
@@ -1046,17 +1046,17 @@ static void put_reedit_in_typebuf(int silent)
 {
   char_u buf[3];
 
-  if (restart_edit != NUL) {
+  if (restart_edit != '\0') {
     if (restart_edit == 'V') {
       buf[0] = 'g';
       buf[1] = 'R';
-      buf[2] = NUL;
+      buf[2] = '\0';
     } else {
       buf[0] = restart_edit == 'I' ? 'i' : restart_edit;
-      buf[1] = NUL;
+      buf[1] = '\0';
     }
     if (ins_typebuf(buf, REMAP_NONE, 0, TRUE, silent) == OK)
-      restart_edit = NUL;
+      restart_edit = '\0';
   }
 }
 
@@ -1126,12 +1126,12 @@ insert_reg (
     return FAIL;
 
   /* check for valid regname */
-  if (regname != NUL && !valid_yank_reg(regname, FALSE))
+  if (regname != '\0' && !valid_yank_reg(regname, FALSE))
     return FAIL;
 
 
   if (regname == '.')                   /* insert last inserted text */
-    retval = stuff_inserted(NUL, 1L, TRUE);
+    retval = stuff_inserted('\0', 1L, TRUE);
   else if (get_spec_reg(regname, &arg, &allocated, TRUE)) {
     if (arg == NULL)
       return FAIL;
@@ -1167,7 +1167,7 @@ static void stuffescaped(char_u *arg, int literally)
   int c;
   char_u      *start;
 
-  while (*arg != NUL) {
+  while (*arg != '\0') {
     /* Stuff a sequence of normal ASCII characters, that's fast.  Also
      * stuff K_SPECIAL to get the effect of a special key when "literally"
      * is TRUE. */
@@ -1181,7 +1181,7 @@ static void stuffescaped(char_u *arg, int literally)
       stuffReadbuffLen(start, (long)(arg - start));
 
     /* stuff a single special character */
-    if (*arg != NUL) {
+    if (*arg != '\0') {
       if (has_mbyte)
         c = mb_cptr2char_adv(&arg);
       else
@@ -1302,7 +1302,7 @@ cmdline_paste_reg (
         || (i < y_current->y_size - 1
             && !(remcr
                  && i == y_current->y_size - 2
-                 && *y_current->y_array[i + 1] == NUL)))
+                 && *y_current->y_array[i + 1] == '\0')))
       cmdline_paste_str((char_u *)"\r", literally);
 
     /* Check for CTRL-C, in case someone tries to paste a few thousand
@@ -1356,13 +1356,13 @@ int op_delete(oparg_T *oap)
              && !oap->is_VIsual
              && !oap->block_mode
              && oap->line_count > 1
-             && oap->motion_force == NUL
+             && oap->motion_force == '\0'
              && oap->op_type == OP_DELETE) {
     ptr = ml_get(oap->end.lnum) + oap->end.col;
-    if (*ptr != NUL)
+    if (*ptr != '\0')
       ptr += oap->inclusive;
     ptr = skipwhite(ptr);
-    if (*ptr == NUL && inindent(0))
+    if (*ptr == '\0' && inindent(0))
       oap->motion_type = MLINE;
   }
 
@@ -1373,7 +1373,7 @@ int op_delete(oparg_T *oap)
   if (       oap->motion_type == MCHAR
              && oap->line_count == 1
              && oap->op_type == OP_DELETE
-             && *ml_get(oap->start.lnum) == NUL) {
+             && *ml_get(oap->start.lnum) == '\0') {
     /*
      * It's an error to operate on an empty region, when 'E' included in
      * 'cpoptions' (Vi compatible).
@@ -1588,7 +1588,7 @@ int op_delete(oparg_T *oap)
           n = 1;
 
         /* When deleted a char in the line, reset coladd. */
-        if (gchar_cursor() != NUL)
+        if (gchar_cursor() != '\0')
           curwin->w_cursor.coladd = 0;
       }
       if (oap->op_type == OP_DELETE
@@ -1720,7 +1720,7 @@ int op_replace(oparg_T *oap, int c)
        */
       /* If the range starts in virtual space, count the initial
        * coladd offset as part of "startspaces" */
-      if (virtual_op && bd.is_short && *bd.textstart == NUL) {
+      if (virtual_op && bd.is_short && *bd.textstart == '\0') {
         pos_T vpos;
 
         vpos.lnum = curwin->w_cursor.lnum;
@@ -1759,7 +1759,7 @@ int op_replace(oparg_T *oap, int c)
       oldp = ml_get_curline();
       oldlen = STRLEN(oldp);
       newp = (char_u *) xmalloc((size_t)(oldlen + 1 + n));
-      memset(newp, NUL, (size_t)(oldlen + 1 + n));
+      memset(newp, '\0', (size_t)(oldlen + 1 + n));
       /* copy up to deleted part */
       memmove(newp, oldp, (size_t)bd.textcol);
       oldp += bd.textcol + bd.textlen;
@@ -1809,7 +1809,7 @@ int op_replace(oparg_T *oap, int c)
 
     while (ltoreq(curwin->w_cursor, oap->end)) {
       n = gchar_cursor();
-      if (n != NUL) {
+      if (n != '\0') {
         if ((*mb_char2len)(c) > 1 || (*mb_char2len)(n) > 1) {
           /* This is slow, but it handles replacing a single-byte
            * with a multi-byte and the other way around. */
@@ -2082,7 +2082,7 @@ void op_insert(oparg_T *oap, long count1)
         ) {
       /* Move the cursor to the character right of the block. */
       curwin->w_set_curswant = TRUE;
-      while (*ml_get_cursor() != NUL
+      while (*ml_get_cursor() != '\0'
              && (curwin->w_cursor.col < bd.textcol + bd.textlen))
         ++curwin->w_cursor.col;
       if (bd.is_short && !bd.is_MAX) {
@@ -2105,7 +2105,7 @@ void op_insert(oparg_T *oap, long count1)
     }
   }
 
-  edit(NUL, FALSE, (linenr_T)count1);
+  edit('\0', FALSE, (linenr_T)count1);
 
   /* If user has moved off this line, we don't know what to do, so do
    * nothing.
@@ -2225,7 +2225,7 @@ int op_change(oparg_T *oap)
   if (oap->block_mode) {
     /* Add spaces before getting the current line length. */
     if (virtual_op && (curwin->w_cursor.coladd > 0
-                       || gchar_cursor() == NUL))
+                       || gchar_cursor() == '\0'))
       coladvance_force(getviscol());
     firstline = ml_get(oap->start.lnum);
     pre_textlen = (long)STRLEN(firstline);
@@ -2236,7 +2236,7 @@ int op_change(oparg_T *oap)
   if (oap->motion_type == MLINE)
     fix_indent();
 
-  retval = edit(NUL, FALSE, (linenr_T)1);
+  retval = edit('\0', FALSE, (linenr_T)1);
 
   /*
    * In Visual block mode, handle copying the new text to all lines of the
@@ -2458,7 +2458,7 @@ int op_yank(oparg_T *oap, int deleting, int mess)
         endcol = oap->end.col;
         if (virtual_op) {
           getvcol(curwin, &oap->end, &cs, NULL, &ce);
-          if (p[endcol] == NUL || (cs + oap->end.coladd < ce
+          if (p[endcol] == '\0' || (cs + oap->end.coladd < ce
                                    /* Don't add space for double-wide
                                     * char; endcol will be on last byte
                                     * of multi-byte char. */
@@ -2582,7 +2582,7 @@ static void yank_copy_line(struct block_def *bd, long y_idx)
   pnew += bd->textlen;
   copy_spaces(pnew, (size_t)bd->endspaces);
   pnew += bd->endspaces;
-  *pnew = NUL;
+  *pnew = '\0';
 }
 
 
@@ -2646,7 +2646,7 @@ do_put (
                           (count == -1 ? 'O' : 'i')), count, FALSE);
     /* Putting the text is done later, so can't really move the cursor to
      * the next character.  Use "l" to simulate it. */
-    if ((flags & PUT_CURSEND) && gchar_cursor() != NUL)
+    if ((flags & PUT_CURSEND) && gchar_cursor() != '\0')
       stuffcharReadbuff('l');
     return;
   }
@@ -2680,10 +2680,10 @@ do_put (
           ptr = vim_strchr(ptr, '\n');
           if (ptr != NULL) {
             if (y_array != NULL)
-              *ptr = NUL;
+              *ptr = '\0';
             ++ptr;
             /* A trailing '\n' makes the register linewise. */
-            if (*ptr == NUL) {
+            if (*ptr == '\0') {
               y_type = MLINE;
               break;
             }
@@ -2784,7 +2784,7 @@ do_put (
         coladvance_force(getviscol());
       else
         curwin->w_cursor.coladd = 0;
-    } else if (curwin->w_cursor.coladd > 0 || gchar_cursor() == NUL)
+    } else if (curwin->w_cursor.coladd > 0 || gchar_cursor() == '\0')
       coladvance_force(getviscol() + (dir == FORWARD));
   }
 
@@ -2798,7 +2798,7 @@ do_put (
     char c = gchar_cursor();
     colnr_T endcol2 = 0;
 
-    if (dir == FORWARD && c != NUL) {
+    if (dir == FORWARD && c != '\0') {
       if (ve_flags == VE_ALL)
         getvcol(curwin, &curwin->w_cursor, &col, NULL, &endcol2);
       else
@@ -2817,9 +2817,9 @@ do_put (
     if (ve_flags == VE_ALL
         && (curwin->w_cursor.coladd > 0
             || endcol2 == curwin->w_cursor.col)) {
-      if (dir == FORWARD && c == NUL)
+      if (dir == FORWARD && c == '\0')
         ++col;
-      if (dir != FORWARD && c != NUL)
+      if (dir != FORWARD && c != '\0')
         ++curwin->w_cursor.col;
       if (c == TAB) {
         if (dir == BACKWARD && curwin->w_cursor.col)
@@ -2948,7 +2948,7 @@ do_put (
     if (y_type == MCHAR) {
       /* if type is MCHAR, FORWARD is the same as BACKWARD on the next
        * char */
-      if (dir == FORWARD && gchar_cursor() != NUL) {
+      if (dir == FORWARD && gchar_cursor() != '\0') {
         if (has_mbyte) {
           int bytelen = (*mb_ptr2len)(ml_get_cursor());
 
@@ -3060,7 +3060,7 @@ do_put (
               lendiff = (int)STRLEN(ptr);
             if (*ptr == '#' && preprocs_left())
               indent = 0;                   /* Leave # lines at start */
-            else if (*ptr == NUL)
+            else if (*ptr == '\0')
               indent = 0;                   /* Ignore empty lines */
             else if (first_indent) {
               indent_diff = orig_indent - get_indent();
@@ -3153,7 +3153,7 @@ end:
 void adjust_cursor_eol(void)
 {
   if (curwin->w_cursor.col > 0
-      && gchar_cursor() == NUL
+      && gchar_cursor() == '\0'
       && (ve_flags & VE_ONEMORE) == 0
       && !(restart_edit || (State & INSERT))) {
     /* Put the cursor on the last character in the line. */
@@ -3209,7 +3209,7 @@ void ex_display(exarg_T *eap)
   char_u              *arg = eap->arg;
   int clen;
 
-  if (arg != NULL && *arg == NUL)
+  if (arg != NULL && *arg == '\0')
     arg = NULL;
   attr = hl_attr(HLF_8);
 
@@ -3337,8 +3337,8 @@ dis_msg (
   int l;
 
   n = (int)Columns - 6;
-  while (*p != NUL
-         && !(*p == ESC && skip_esc && *(p + 1) == NUL)
+  while (*p != '\0'
+         && !(*p == ESC && skip_esc && *(p + 1) == '\0')
          && (n -= ptr2cells(p)) >= 0) {
     if (has_mbyte && (l = (*mb_ptr2len)(p)) > 1) {
       msg_outtrans_len(p, l);
@@ -3407,7 +3407,7 @@ static char_u *skip_comment(char_u *line, int process, int include_space, int *i
    * starting with a closing part of a three-part comment. That's good,
    * because we don't want to remove those as this would be annoying.
    */
-  if (*comment_flags == ':' || *comment_flags == NUL)
+  if (*comment_flags == ':' || *comment_flags == '\0')
     line += lead_len;
 
   return line;
@@ -3428,8 +3428,8 @@ int do_join(long count, int insert_space, int save_undo, int use_formatoptions)
   char_u      *cend;
   char_u      *newp;
   char_u      *spaces;          /* number of spaces inserted before a line */
-  int endcurr1 = NUL;
-  int endcurr2 = NUL;
+  int endcurr1 = '\0';
+  int endcurr2 = '\0';
   int currsize = 0;             /* size of the current line */
   int sumsize = 0;              /* size of the long new line */
   linenr_T t;
@@ -3501,7 +3501,7 @@ int do_join(long count, int insert_space, int save_undo, int use_formatoptions)
     }
     currsize = (int)STRLEN(curr);
     sumsize += currsize + spaces[t];
-    endcurr1 = endcurr2 = NUL;
+    endcurr1 = endcurr2 = '\0';
     if (insert_space && currsize > 0) {
       if (has_mbyte) {
         cend = curr + currsize;
@@ -3626,7 +3626,7 @@ static int same_leader(linenr_T lnum, int leader1_len, char_u *leader1_flags, in
       if (*p == COM_END)
         return FALSE;
       if (*p == COM_START) {
-        if (*(ml_get(lnum) + leader1_len) == NUL)
+        if (*(ml_get(lnum) + leader1_len) == '\0')
           return FALSE;
         if (leader2_flags == NULL || leader2_len == 0)
           return FALSE;
@@ -3739,7 +3739,7 @@ void op_formatexpr(oparg_T *oap)
     /* When there is no change: need to remove the Visual selection */
     redraw_curbuf_later(INVERTED);
 
-  if (fex_format(oap->start.lnum, oap->line_count, NUL) != 0)
+  if (fex_format(oap->start.lnum, oap->line_count, '\0') != 0)
     /* As documented: when 'formatexpr' returns non-zero fall back to
      * internal formatting. */
     op_format(oap, FALSE);
@@ -3941,7 +3941,7 @@ format_lines (
         State = INSERT;         /* for open_line() */
         smd_save = p_smd;
         p_smd = FALSE;
-        insertchar(NUL, INSCHAR_FORMAT
+        insertchar('\0', INSCHAR_FORMAT
             + (do_comments ? INSCHAR_DO_COM : 0)
             + (do_comments && do_comments_list
                ? INSCHAR_COM_LIST : 0)
@@ -4010,7 +4010,7 @@ static int ends_in_white(linenr_T lnum)
   char_u      *s = ml_get(lnum);
   size_t l;
 
-  if (*s == NUL)
+  if (*s == '\0')
     return FALSE;
   /* Don't use STRLEN() inside vim_iswhite(), SAS/C complains: "macro
    * invocation may call function multiple times". */
@@ -4046,9 +4046,9 @@ static int fmt_check_par(linenr_T lnum, int *leader_len, char_u **leader_flags,
       ++flags;
   }
 
-  return *skipwhite(ptr + *leader_len) == NUL
+  return *skipwhite(ptr + *leader_len) == '\0'
          || (*leader_len > 0 && *flags == COM_END)
-         || startPS(lnum, NUL, FALSE);
+         || startPS(lnum, '\0', FALSE);
 }
 
 /*
@@ -4068,7 +4068,7 @@ int paragraph_start(linenr_T lnum)
     return TRUE;                /* start of the file */
 
   p = ml_get(lnum - 1);
-  if (*p == NUL)
+  if (*p == '\0')
     return TRUE;                /* after empty line */
 
   do_comments = has_format_option(FO_Q_COMS);
@@ -4180,7 +4180,7 @@ static void block_prep(oparg_T *oap, struct block_def *bdp, linenr_T lnum, int i
       }
     } else {
       prev_pend = pend;
-      while (bdp->end_vcol <= oap->end_vcol && *pend != NUL) {
+      while (bdp->end_vcol <= oap->end_vcol && *pend != '\0') {
         /* Count a tab for what it's worth (if list mode not on) */
         prev_pend = pend;
         incr = lbr_chartabsize_adv(&pend, (colnr_T)bdp->end_vcol);
@@ -4292,7 +4292,7 @@ int do_addsub(int command, linenr_T Prenum1)
      */
     col = curwin->w_cursor.col;
 
-    while (ptr[col] != NUL
+    while (ptr[col] != '\0'
            && !vim_isdigit(ptr[col])
            && !(doalp && ASCII_ISALPHA(ptr[col])))
       ++col;
@@ -4452,7 +4452,7 @@ int do_addsub(int command, linenr_T Prenum1)
     if (firstdigit == '0' && !(dooct && hex == 0))
       while (length-- > 0)
         *ptr++ = '0';
-    *ptr = NUL;
+    *ptr = '\0';
     STRCAT(buf1, buf2);
     ins_str(buf1);              /* insert the new number */
     vim_free(buf1);
@@ -4495,7 +4495,7 @@ int read_viminfo_register(vir_T *virp, int force)
 
   if (*str == '@') {
     /* "x@: register x used for @@ */
-    if (force || execreg_lastc == NUL)
+    if (force || execreg_lastc == '\0')
       execreg_lastc = str[-1];
   }
 
@@ -4583,7 +4583,7 @@ void write_viminfo_registers(FILE *fp)
     num_lines = y_regs[i].y_size;
     if (num_lines == 0
         || (num_lines == 1 && y_regs[i].y_type == MCHAR
-            && *y_regs[i].y_array[0] == NUL))
+            && *y_regs[i].y_array[0] == '\0'))
       continue;
 
     if (max_kbyte > 0) {
@@ -4659,7 +4659,7 @@ char_u get_reg_type(int regname, long *reglen)
   }
 
 
-  if (regname != NUL && !valid_yank_reg(regname, FALSE))
+  if (regname != '\0' && !valid_yank_reg(regname, FALSE))
     return MAUTO;
 
   get_yank_register(regname, FALSE);
@@ -4703,7 +4703,7 @@ get_reg_contents (
     regname = '"';
 
   /* check for valid regname */
-  if (regname != NUL && !valid_yank_reg(regname, FALSE))
+  if (regname != '\0' && !valid_yank_reg(regname, FALSE))
     return NULL;
 
 
@@ -4751,7 +4751,7 @@ get_reg_contents (
       if (y_current->y_type == MLINE || i < y_current->y_size - 1)
         retval[len++] = '\n';
     }
-    retval[len] = NUL;
+    retval[len] = '\0';
   }
 
   return retval;
@@ -4911,10 +4911,10 @@ str_to_reg (
     if (i)
       memmove(s + extra, str + start, (size_t)i);
     extra += i;
-    s[extra] = NUL;
+    s[extra] = '\0';
     y_ptr->y_array[lnum++] = s;
     while (--extra >= 0) {
-      if (*s == NUL)
+      if (*s == '\0')
         *s = '\n';                  /* replace NUL with newline */
       ++s;
     }
@@ -4958,7 +4958,7 @@ static long line_count_info(char_u *line, long *wc, long *cc, long limit, int eo
   long chars = 0;
   int is_word = 0;
 
-  for (i = 0; i < limit && line[i] != NUL; ) {
+  for (i = 0; i < limit && line[i] != '\0'; ) {
     if (is_word) {
       if (vim_isspace(line[i])) {
         words++;
@@ -4975,7 +4975,7 @@ static long line_count_info(char_u *line, long *wc, long *cc, long limit, int eo
   *wc += words;
 
   /* Add eol_size if the end of line was reached before hitting limit. */
-  if (i < limit && line[i] == NUL) {
+  if (i < limit && line[i] == '\0') {
     i += eol_size;
     chars += eol_size;
   }
@@ -5127,7 +5127,7 @@ void cursor_pos_info(void)
         vim_snprintf((char *)buf1, sizeof(buf1), _("%" PRId64 " Cols; "),
             (int64_t)(oparg.end_vcol - oparg.start_vcol + 1));
       } else
-        buf1[0] = NUL;
+        buf1[0] = '\0';
 
       if (char_count_cursor == byte_count_cursor
           && char_count == byte_count)
diff --git a/src/option.c b/src/option.c
index 210a747022..7ad6f20d66 100644
--- a/src/option.c
+++ b/src/option.c
@@ -1944,7 +1944,7 @@ void set_init_1(void)
    * Find default value for 'shell' option.
    * Don't use it if it is empty.
    */
-  if (((p = (char_u *)os_getenv("SHELL")) != NULL && *p != NUL)
+  if (((p = (char_u *)os_getenv("SHELL")) != NULL && *p != '\0')
       )
     set_string_default("sh", p);
 
@@ -1966,12 +1966,12 @@ void set_init_1(void)
     for (n = 0; n < (long)(sizeof(names) / sizeof(char *)); ++n) {
       mustfree = FALSE;
 # ifdef UNIX
-      if (*names[n] == NUL)
+      if (*names[n] == '\0')
         p = (char_u *)"/tmp";
       else
 # endif
       p = vim_getenv((char_u *)names[n], &mustfree);
-      if (p != NULL && *p != NUL) {
+      if (p != NULL && *p != '\0') {
         /* First time count the NUL, otherwise count the ','. */
         len = (int)STRLEN(p) + 3;
         ga_grow(&ga, len);
@@ -2036,7 +2036,7 @@ void set_init_1(void)
       {
         buf[0] = ',';               /* start with ",", current dir first */
         j = 1;
-        for (i = 0; cdpath[i] != NUL; ++i) {
+        for (i = 0; cdpath[i] != '\0'; ++i) {
           if (vim_ispathlistsep(cdpath[i]))
             buf[j++] = ',';
           else {
@@ -2045,7 +2045,7 @@ void set_init_1(void)
             buf[j++] = cdpath[i];
           }
         }
-        buf[j] = NUL;
+        buf[j] = '\0';
         opt_idx = findoption((char_u *)"cdpath");
         if (opt_idx >= 0) {
           options[opt_idx].def_val[VI_DEFAULT] = buf;
@@ -2421,7 +2421,7 @@ static char_u *term_bg_default(void)
       || ((p = (char_u *)os_getenv("COLORFGBG")) != NULL
           && (p = vim_strrchr(p, ';')) != NULL
           && ((p[1] >= '0' && p[1] <= '6') || p[1] == '8')
-          && p[2] == NUL))
+          && p[2] == '\0'))
     return (char_u *)"dark";
   return (char_u *)"light";
 }
@@ -2463,7 +2463,7 @@ void set_init_3(void)
    * But don't do that for Windows, it's common to have a space in the path.
    */
   p = skiptowhite(p_sh);
-  if (*p == NUL) {
+  if (*p == '\0') {
     /* No white space, use the tail. */
     p = vim_strsave(path_tail(p_sh));
   } else {
@@ -2540,7 +2540,7 @@ void set_helplang_default(char_u *lang)
         p_hlg[0] = TOLOWER_ASC(p_hlg[3]);
         p_hlg[1] = TOLOWER_ASC(p_hlg[4]);
       }
-      p_hlg[2] = NUL;
+      p_hlg[2] = '\0';
     }
     options[idx].flags |= P_ALLOCED;
   }
@@ -2619,13 +2619,13 @@ do_set (
   int cp_val = 0;
   char_u key_name[2];
 
-  if (*arg == NUL) {
+  if (*arg == '\0') {
     showoptions(0, opt_flags);
     did_show = TRUE;
     goto theend;
   }
 
-  while (*arg != NUL) {         /* loop to process all options */
+  while (*arg != '\0') {         /* loop to process all options */
     errmsg = NULL;
     startarg = arg;             /* remember for error message */
 
@@ -2670,14 +2670,14 @@ do_set (
           len = 5;
         else {
           len = 1;
-          while (arg[len] != NUL && arg[len] != '>')
+          while (arg[len] != '\0' && arg[len] != '>')
             ++len;
         }
         if (arg[len] != '>') {
           errmsg = e_invarg;
           goto skip;
         }
-        arg[len] = NUL;                             /* put NUL after name */
+        arg[len] = '\0';                             /* put NUL after name */
         if (arg[1] == 't' && arg[2] == '_')         /* could be term code */
           opt_idx = findoption(arg + 1);
         arg[len++] = '>';                           /* restore '>' */
@@ -2694,7 +2694,7 @@ do_set (
           while (ASCII_ISALNUM(arg[len]) || arg[len] == '_')
             ++len;
         nextchar = arg[len];
-        arg[len] = NUL;                             /* put NUL after name */
+        arg[len] = '\0';                             /* put NUL after name */
         opt_idx = findoption(arg);
         arg[len] = nextchar;                        /* restore nextchar */
         if (opt_idx == -1)
@@ -2711,7 +2711,7 @@ do_set (
       adding = FALSE;
       prepending = FALSE;
       removing = FALSE;
-      if (arg[len] != NUL && arg[len + 1] == '=') {
+      if (arg[len] != '\0' && arg[len + 1] == '=') {
         if (arg[len] == '+') {
           adding = TRUE;                        /* "+=" */
           ++len;
@@ -2802,7 +2802,7 @@ do_set (
           }
         }
         if (vim_strchr((char_u *)"?!&<", nextchar) != NULL
-            && arg[1] != NUL && !vim_iswhite(arg[1])) {
+            && arg[1] != '\0' && !vim_iswhite(arg[1])) {
           errmsg = e_trailing;
           goto skip;
         }
@@ -2849,7 +2849,7 @@ do_set (
             (void)show_one_termcode(key_name, p, TRUE);
         }
         if (nextchar != '?'
-            && nextchar != NUL && !vim_iswhite(afterchar))
+            && nextchar != '\0' && !vim_iswhite(afterchar))
           errmsg = e_trailing;
       } else {
         if (flags & P_BOOL) {                       /* boolean */
@@ -2882,7 +2882,7 @@ do_set (
              * ":set invopt": invert
              * ":set opt" or ":set noopt": set or reset
              */
-            if (nextchar != NUL && !vim_iswhite(afterchar)) {
+            if (nextchar != '\0' && !vim_iswhite(afterchar)) {
               errmsg = e_trailing;
               goto skip;
             }
@@ -2947,7 +2947,7 @@ do_set (
                 i += 2;
               while (VIM_ISDIGIT(arg[i]))
                 ++i;
-              if (arg[i] != NUL && !vim_iswhite(arg[i])) {
+              if (arg[i] != '\0' && !vim_iswhite(arg[i])) {
                 errmsg = e_invarg;
                 goto skip;
               }
@@ -3021,7 +3021,7 @@ do_set (
                * Misuse errbuf[] for the resulting string.
                */
               if (varp == (char_u *)&p_kp
-                  && (*arg == NUL || *arg == ' ')) {
+                  && (*arg == '\0' || *arg == ' ')) {
                 STRCPY(errbuf, ":help");
                 save_arg = arg;
                 arg = errbuf;
@@ -3056,7 +3056,7 @@ do_set (
                */
               else if (varp == (char_u *)&p_ww
                        && VIM_ISDIGIT(*arg)) {
-                *errbuf = NUL;
+                *errbuf = '\0';
                 i = getdigits(&arg);
                 if (i & 1)
                   STRCAT(errbuf, "b,");
@@ -3068,8 +3068,8 @@ do_set (
                   STRCAT(errbuf, "<,>,");
                 if (i & 16)
                   STRCAT(errbuf, "[,],");
-                if (*errbuf != NUL)                     /* remove trailing , */
-                  errbuf[STRLEN(errbuf) - 1] = NUL;
+                if (*errbuf != '\0')                     /* remove trailing , */
+                  errbuf[STRLEN(errbuf) - 1] = '\0';
                 save_arg = arg;
                 arg = errbuf;
               }
@@ -3113,7 +3113,7 @@ do_set (
                * The reverse is found in ExpandOldSetting().
                */
               while (*arg && !vim_iswhite(*arg)) {
-                if (*arg == '\\' && arg[1] != NUL
+                if (*arg == '\\' && arg[1] != '\0'
 #ifdef BACKSLASH_IN_FILENAME
                     && !((flags & P_EXPAND)
                          && vim_isfilec(arg[1])
@@ -3132,7 +3132,7 @@ do_set (
                 } else
                   *s++ = *arg++;
               }
-              *s = NUL;
+              *s = '\0';
 
               /*
                * Expand environment variables and ~.
@@ -3165,7 +3165,7 @@ do_set (
                       && STRNCMP(s, newval, i) == 0
                       && (!(flags & P_COMMA)
                           || s[i] == ','
-                          || s[i] == NUL))
+                          || s[i] == '\0'))
                     break;
                   /* Count backslashes.  Only a comma with an
                    * even number of backslashes before it is
@@ -3187,8 +3187,8 @@ do_set (
               /* concatenate the two strings; add a ',' if
                * needed */
               if (adding || prepending) {
-                comma = ((flags & P_COMMA) && *origval != NUL
-                         && *newval != NUL);
+                comma = ((flags & P_COMMA) && *origval != '\0'
+                         && *newval != '\0');
                 if (adding) {
                   i = (int)STRLEN(origval);
                   memmove(newval + i + comma, newval,
@@ -3258,10 +3258,10 @@ do_set (
             } else {
               ++arg;               /* jump to after the '=' or ':' */
               for (p = arg; *p && !vim_iswhite(*p); ++p)
-                if (*p == '\\' && p[1] != NUL)
+                if (*p == '\\' && p[1] != '\0')
                   ++p;
               nextchar = *p;
-              *p = NUL;
+              *p = '\0';
               add_termcode(key_name, arg, FALSE);
               *p = nextchar;
             }
@@ -3284,8 +3284,8 @@ skip:
        * - skip one "=val" argument (for hidden options ":set gfn =xx")
        */
       for (i = 0; i < 2; ++i) {
-        while (*arg != NUL && !vim_iswhite(*arg))
-          if (*arg++ == '\\' && *arg != NUL)
+        while (*arg != '\0' && !vim_iswhite(*arg))
+          if (*arg++ == '\\' && *arg != '\0')
             ++arg;
         arg = skipwhite(arg);
         if (*arg != '=')
@@ -3300,7 +3300,7 @@ skip:
         /* append the argument with the error */
         STRCAT(IObuff, ": ");
         memmove(IObuff + i, startarg, (arg - startarg));
-        IObuff[i + (arg - startarg)] = NUL;
+        IObuff[i + (arg - startarg)] = '\0';
       }
       /* make sure all characters are printable */
       trans_characters(IObuff, IOSIZE);
@@ -3389,7 +3389,7 @@ static char_u *check_cedit(void)
 {
   int n;
 
-  if (*p_cedit == NUL)
+  if (*p_cedit == '\0')
     cedit_key = -1;
   else {
     n = string_to_key(p_cedit);
@@ -3893,7 +3893,7 @@ did_set_string_option (
   }
   /* 'term' */
   else if (varp == &T_NAME) {
-    if (T_NAME[0] == NUL)
+    if (T_NAME[0] == '\0')
       errmsg = (char_u *)N_("E529: Cannot set 'term' to empty string");
     else if (set_termname(T_NAME) == FAIL)
       errmsg = (char_u *)N_("E522: Not found in termcap");
@@ -3952,12 +3952,12 @@ did_set_string_option (
   /* 'helplang' */
   else if (varp == &p_hlg) {
     /* Check for "", "ab", "ab,cd", etc. */
-    for (s = p_hlg; *s != NUL; s += 3) {
-      if (s[1] == NUL || ((s[2] != ',' || s[3] == NUL) && s[2] != NUL)) {
+    for (s = p_hlg; *s != '\0'; s += 3) {
+      if (s[1] == '\0' || ((s[2] != ',' || s[3] == '\0') && s[2] != '\0')) {
         errmsg = e_invarg;
         break;
       }
-      if (s[2] == NUL)
+      if (s[2] == '\0')
         break;
     }
   }
@@ -4033,7 +4033,7 @@ did_set_string_option (
   }
   /* 'winaltkeys' */
   else if (varp == &p_wak) {
-    if (*p_wak == NUL
+    if (*p_wak == '\0'
         || check_opt_strings(p_wak, p_wak_values, FALSE) != OK)
       errmsg = e_invarg;
   }
@@ -4075,13 +4075,13 @@ did_set_string_option (
     if (errmsg == NULL) {
       /* When 'keymap' is used and 'encoding' changes, reload the keymap
        * (with another encoding). */
-      if (varp == &p_enc && *curbuf->b_p_keymap != NUL)
+      if (varp == &p_enc && *curbuf->b_p_keymap != '\0')
         (void)keymap_init();
 
       /* When 'termencoding' is not empty and 'encoding' changes or when
        * 'termencoding' changes, need to setup for keyboard input and
        * display output conversion. */
-      if (((varp == &p_enc && *p_tenc != NUL) || varp == &p_tenc)) {
+      if (((varp == &p_enc && *p_tenc != '\0') || varp == &p_tenc)) {
         convert_setup(&input_conv, p_tenc, p_enc);
         convert_setup(&output_conv, p_enc, p_tenc);
       }
@@ -4095,7 +4095,7 @@ did_set_string_option (
       p_penc = p;
     } else {
       /* Ensure lower case and '-' for '_' */
-      for (s = p_penc; *s != NUL; s++) {
+      for (s = p_penc; *s != '\0'; s++) {
         if (*s == '_')
           *s = '-';
         else
@@ -4107,7 +4107,7 @@ did_set_string_option (
     errmsg = keymap_init();
 
     if (errmsg == NULL) {
-      if (*curbuf->b_p_keymap != NUL) {
+      if (*curbuf->b_p_keymap != '\0') {
         /* Installed a new keymap, switch on using it. */
         curbuf->b_p_iminsert = B_IMODE_LMAP;
         if (curbuf->b_p_imsearch != B_IMODE_USE_INSERT)
@@ -4167,7 +4167,7 @@ did_set_string_option (
       errmsg = e_invarg;
     else {
       /* When setting the global value to empty, make it "zip". */
-      if (*p_cm == NUL) {
+      if (*p_cm == '\0') {
         if (new_value_alloced)
           free_string_option(p_cm);
         p_cm = vim_strsave((char_u *)"zip");
@@ -4177,11 +4177,11 @@ did_set_string_option (
       /* Need to update the swapfile when the effective method changed.
        * Set "s" to the effective old value, "p" to the effective new
        * method and compare. */
-      if ((opt_flags & OPT_LOCAL) && *oldval == NUL)
+      if ((opt_flags & OPT_LOCAL) && *oldval == '\0')
         s = p_cm;          /* was previously using the global value */
       else
         s = oldval;
-      if (*curbuf->b_p_cm == NUL)
+      if (*curbuf->b_p_cm == '\0')
         p = p_cm;          /* is now using the global value */
       else
         p = curbuf->b_p_cm;
@@ -4195,7 +4195,7 @@ did_set_string_option (
         buf_T   *buf;
 
         for (buf = firstbuf; buf != NULL; buf = buf->b_next)
-          if (buf != curbuf && *buf->b_p_cm == NUL)
+          if (buf != curbuf && *buf->b_p_cm == '\0')
             ml_set_crypt_key(buf, buf->b_p_key,
                 crypt_method_from_string(oldval));
       }
@@ -4204,33 +4204,33 @@ did_set_string_option (
   /* 'matchpairs' */
   else if (gvarp == &p_mps) {
     if (has_mbyte) {
-      for (p = *varp; *p != NUL; ++p) {
+      for (p = *varp; *p != '\0'; ++p) {
         int x2 = -1;
         int x3 = -1;
 
-        if (*p != NUL)
+        if (*p != '\0')
           p += mb_ptr2len(p);
-        if (*p != NUL)
+        if (*p != '\0')
           x2 = *p++;
-        if (*p != NUL) {
+        if (*p != '\0') {
           x3 = mb_ptr2char(p);
           p += mb_ptr2len(p);
         }
-        if (x2 != ':' || x3 == -1 || (*p != NUL && *p != ',')) {
+        if (x2 != ':' || x3 == -1 || (*p != '\0' && *p != ',')) {
           errmsg = e_invarg;
           break;
         }
-        if (*p == NUL)
+        if (*p == '\0')
           break;
       }
     } else {
       /* Check for "x:y,x:y" */
-      for (p = *varp; *p != NUL; p += 4) {
-        if (p[1] != ':' || p[2] == NUL || (p[3] != NUL && p[3] != ',')) {
+      for (p = *varp; *p != '\0'; p += 4) {
+        if (p[1] != ':' || p[2] == '\0' || (p[3] != '\0' && p[3] != ',')) {
           errmsg = e_invarg;
           break;
         }
-        if (p[3] == NUL)
+        if (p[3] == '\0')
           break;
       }
     }
@@ -4246,14 +4246,14 @@ did_set_string_option (
         }
         ++s;
       }
-      if (*s++ == NUL)
+      if (*s++ == '\0')
         errmsg = (char_u *)N_("E524: Missing colon");
-      else if (*s == ',' || *s == NUL)
+      else if (*s == ',' || *s == '\0')
         errmsg = (char_u *)N_("E525: Zero length string");
       if (errmsg != NULL)
         break;
       while (*s && *s != ',') {
-        if (*s == '\\' && s[1] != NUL)
+        if (*s == '\\' && s[1] != '\0')
           ++s;
         ++s;
       }
@@ -4275,7 +4275,7 @@ did_set_string_option (
   /* 'verbosefile' */
   else if (varp == &p_vfile) {
     verbose_stop();
-    if (*p_vfile != NUL && verbose_open() == FAIL)
+    if (*p_vfile != '\0' && verbose_open() == FAIL)
       errmsg = e_invarg;
   }
   /* 'viminfo' */
@@ -4409,7 +4409,7 @@ did_set_string_option (
 
   /* 'selection' */
   else if (varp == &p_sel) {
-    if (*p_sel == NUL
+    if (*p_sel == '\0'
         || check_opt_strings(p_sel, p_sel_values, FALSE) != OK)
       errmsg = e_invarg;
   }
@@ -4542,7 +4542,7 @@ did_set_string_option (
         errmsg = illegal_char(errbuf, *s);
         break;
       }
-      if (*++s != NUL && *s != ',' && *s != ' ') {
+      if (*++s != '\0' && *s != ',' && *s != ' ') {
         if (s[-1] == 'k' || s[-1] == 's') {
           /* skip optional filename after 'k' and 's' */
           while (*s && *s != ',' && *s != ' ') {
@@ -4583,7 +4583,7 @@ did_set_string_option (
   /* 'backspace' */
   else if (varp == &p_bs) {
     if (VIM_ISDIGIT(*p_bs)) {
-      if (*p_bs >'2' || p_bs[1] != NUL)
+      if (*p_bs >'2' || p_bs[1] != '\0')
         errmsg = e_invarg;
     } else if (check_opt_strings(p_bs, p_bs_values, TRUE) != OK)
       errmsg = e_invarg;
@@ -4601,7 +4601,7 @@ did_set_string_option (
   /* 'foldmethod' */
   else if (gvarp == &curwin->w_allbuf_opt.wo_fdm) {
     if (check_opt_strings(*varp, p_fdm_values, FALSE) != OK
-        || *curwin->w_p_fdm == NUL)
+        || *curwin->w_p_fdm == '\0')
       errmsg = e_invarg;
     else {
       foldUpdateAll(curwin);
@@ -4619,14 +4619,14 @@ did_set_string_option (
     p = vim_strchr(*varp, ',');
     if (p == NULL)
       errmsg = (char_u *)N_("E536: comma required");
-    else if (p == *varp || p[1] == NUL)
+    else if (p == *varp || p[1] == '\0')
       errmsg = e_invarg;
     else if (foldmethodIsMarker(curwin))
       foldUpdateAll(curwin);
   }
   /* 'commentstring' */
   else if (gvarp == &p_cms) {
-    if (**varp != NUL && strstr((char *)*varp, "%s") == NULL)
+    if (**varp != '\0' && strstr((char *)*varp, "%s") == NULL)
       errmsg = (char_u *)N_("E537: 'commentstring' must be empty or contain %s");
   }
   /* 'foldopen' */
@@ -4657,14 +4657,14 @@ did_set_string_option (
   } else if (varp == &p_csqf) {
     if (p_csqf != NULL) {
       p = p_csqf;
-      while (*p != NUL) {
+      while (*p != '\0') {
         if (vim_strchr((char_u *)CSQF_CMDS, *p) == NULL
-            || p[1] == NUL
+            || p[1] == '\0'
             || vim_strchr((char_u *)CSQF_FLAGS, p[1]) == NULL
-            || (p[2] != NUL && p[2] != ',')) {
+            || (p[2] != '\0' && p[2] != ',')) {
           errmsg = e_invarg;
           break;
-        } else if (p[2] == NUL)
+        } else if (p[2] == '\0')
           break;
         else
           p += 3;
@@ -4769,7 +4769,7 @@ did_set_string_option (
        * Use the first name in 'spelllang' up to '_region' or
        * '.encoding'.
        */
-      for (p = q; *p != NUL; ++p)
+      for (p = q; *p != '\0'; ++p)
         if (vim_strchr((char_u *)"_.,", *p) != NULL)
           break;
       vim_snprintf((char *)fname, 200, "spell/%.*s.vim", (int)(p - q), q);
@@ -4778,7 +4778,7 @@ did_set_string_option (
   }
 
   if (varp == &p_mouse) {
-    if (*p_mouse == NUL)
+    if (*p_mouse == '\0')
       mch_setmouse(FALSE);          /* switch mouse off */
     else
       setmouse();                   /* in case 'mouse' changed */
@@ -4817,7 +4817,7 @@ char_u *check_colorcolumn(win_T *wp)
   if (wp->w_buffer == NULL)
     return NULL;      /* buffer was closed */
 
-  for (s = wp->w_p_cc; *s != NUL && count < 255; ) {
+  for (s = wp->w_p_cc; *s != '\0' && count < 255; ) {
     if (*s == '-' || *s == '+') {
       /* -N and +N: add to 'textwidth' */
       col = (*s == '-') ? -1 : 1;
@@ -4836,11 +4836,11 @@ char_u *check_colorcolumn(win_T *wp)
       return e_invarg;
     color_cols[count++] = col - 1;      /* 1-based to 0-based */
 skip:
-    if (*s == NUL)
+    if (*s == '\0')
       break;
     if (*s != ',')
       return e_invarg;
-    if (*++s == NUL)
+    if (*++s == '\0')
       return e_invarg;        /* illegal trailing comma as in "set cc=80," */
   }
 
@@ -4911,9 +4911,9 @@ static char_u *set_chars_option(char_u **varp)
        * 'fillchars', NUL for 'listchars' */
       for (i = 0; i < entries; ++i)
         if (tab[i].cp != NULL)
-          *(tab[i].cp) = (varp == &p_lcs ? NUL : ' ');
+          *(tab[i].cp) = (varp == &p_lcs ? '\0' : ' ');
       if (varp == &p_lcs)
-        lcs_tab1 = NUL;
+        lcs_tab1 = '\0';
       else
         fill_diff = '-';
     }
@@ -4923,19 +4923,19 @@ static char_u *set_chars_option(char_u **varp)
         len = (int)STRLEN(tab[i].name);
         if (STRNCMP(p, tab[i].name, len) == 0
             && p[len] == ':'
-            && p[len + 1] != NUL) {
+            && p[len + 1] != '\0') {
           s = p + len + 1;
           c1 = mb_ptr2char_adv(&s);
           if (mb_char2cells(c1) > 1)
             continue;
           if (tab[i].cp == &lcs_tab2) {
-            if (*s == NUL)
+            if (*s == '\0')
               continue;
             c2 = mb_ptr2char_adv(&s);
             if (mb_char2cells(c2) > 1)
               continue;
           }
-          if (*s == ',' || *s == NUL) {
+          if (*s == ',' || *s == '\0') {
             if (round) {
               if (tab[i].cp == &lcs_tab2) {
                 lcs_tab1 = c1;
@@ -5032,7 +5032,7 @@ static char_u *compile_cap_prog(synblock_T *synblock)
   regprog_T   *rp = synblock->b_cap_prog;
   char_u      *re;
 
-  if (*synblock->b_p_spc == NUL)
+  if (*synblock->b_p_spc == '\0')
     synblock->b_cap_prog = NULL;
   else {
     /* Prepend a ^ so that we only match at one column */
@@ -5292,7 +5292,7 @@ set_bool_option (
       T_XS = (char_u *)"y";
     else if (!p_wiv && old_value)
       T_XS = empty_option;
-    p_wiv = (*T_XS != NUL);
+    p_wiv = (*T_XS != '\0');
   } else if ((int *)varp == &p_acd) {
     /* Change directories when the 'acd' option is set now. */
     do_autochdir();
@@ -5938,7 +5938,7 @@ get_option_value (
     if (stringval != NULL) {
       /* never return the value of the crypt key */
       if ((char_u **)varp == &curbuf->b_p_key
-          && **(char_u **)(varp) != NUL)
+          && **(char_u **)(varp) != '\0')
         *stringval = vim_strsave((char_u *)"*****");
       else
         *stringval = vim_strsave(*(char_u **)(varp));
@@ -6004,7 +6004,7 @@ set_option_value (
            * to zero. */
           for (idx = 0; string[idx] == '0'; ++idx)
             ;
-          if (string[idx] != NUL || idx == 0) {
+          if (string[idx] != '\0' || idx == 0) {
             /* There's another character after zeros or the string
              * is empty.  In both cases, we are trying to set a
              * num option using a string. */
@@ -6036,7 +6036,7 @@ char_u *get_term_code(char_u *tname)
   char_u  *varp;
 
   if (tname[0] != 't' || tname[1] != '_' ||
-      tname[2] == NUL || tname[3] == NUL)
+      tname[2] == '\0' || tname[3] == '\0')
     return NULL;
   if ((opt_idx = findoption(tname)) >= 0) {
     varp = get_varp(&(options[opt_idx]));
@@ -6405,7 +6405,7 @@ static int put_setstring(FILE *fd, char *cmd, char *name, char_u **valuep, int e
      * CTRL-V or backslash */
     if (valuep == &p_pt) {
       s = *valuep;
-      while (*s != NUL)
+      while (*s != '\0')
         if (put_escstr(fd, str2special(&s, FALSE), 2) == FAIL)
           return FAIL;
     } else if (expand) {
@@ -6679,33 +6679,33 @@ static char_u *get_varp(struct vimoption *p)
   case PV_NONE:   return p->var;
 
   /* global option with local value: use local value if it's been set */
-  case PV_EP:     return *curbuf->b_p_ep != NUL
+  case PV_EP:     return *curbuf->b_p_ep != '\0'
            ? (char_u *)&curbuf->b_p_ep : p->var;
-  case PV_KP:     return *curbuf->b_p_kp != NUL
+  case PV_KP:     return *curbuf->b_p_kp != '\0'
            ? (char_u *)&curbuf->b_p_kp : p->var;
-  case PV_PATH:   return *curbuf->b_p_path != NUL
+  case PV_PATH:   return *curbuf->b_p_path != '\0'
            ? (char_u *)&(curbuf->b_p_path) : p->var;
   case PV_AR:     return curbuf->b_p_ar >= 0
            ? (char_u *)&(curbuf->b_p_ar) : p->var;
-  case PV_TAGS:   return *curbuf->b_p_tags != NUL
+  case PV_TAGS:   return *curbuf->b_p_tags != '\0'
            ? (char_u *)&(curbuf->b_p_tags) : p->var;
-  case PV_DEF:    return *curbuf->b_p_def != NUL
+  case PV_DEF:    return *curbuf->b_p_def != '\0'
            ? (char_u *)&(curbuf->b_p_def) : p->var;
-  case PV_INC:    return *curbuf->b_p_inc != NUL
+  case PV_INC:    return *curbuf->b_p_inc != '\0'
            ? (char_u *)&(curbuf->b_p_inc) : p->var;
-  case PV_DICT:   return *curbuf->b_p_dict != NUL
+  case PV_DICT:   return *curbuf->b_p_dict != '\0'
            ? (char_u *)&(curbuf->b_p_dict) : p->var;
-  case PV_TSR:    return *curbuf->b_p_tsr != NUL
+  case PV_TSR:    return *curbuf->b_p_tsr != '\0'
            ? (char_u *)&(curbuf->b_p_tsr) : p->var;
-  case PV_EFM:    return *curbuf->b_p_efm != NUL
+  case PV_EFM:    return *curbuf->b_p_efm != '\0'
            ? (char_u *)&(curbuf->b_p_efm) : p->var;
-  case PV_GP:     return *curbuf->b_p_gp != NUL
+  case PV_GP:     return *curbuf->b_p_gp != '\0'
            ? (char_u *)&(curbuf->b_p_gp) : p->var;
-  case PV_MP:     return *curbuf->b_p_mp != NUL
+  case PV_MP:     return *curbuf->b_p_mp != '\0'
            ? (char_u *)&(curbuf->b_p_mp) : p->var;
-  case PV_CM:     return *curbuf->b_p_cm != NUL
+  case PV_CM:     return *curbuf->b_p_cm != '\0'
            ? (char_u *)&(curbuf->b_p_cm) : p->var;
-  case PV_STL:    return *curwin->w_p_stl != NUL
+  case PV_STL:    return *curwin->w_p_stl != '\0'
            ? (char_u *)&(curwin->w_p_stl) : p->var;
   case PV_UL:     return curbuf->b_p_ul != NO_LOCAL_UNDOLEVEL
            ? (char_u *)&(curbuf->b_p_ul) : p->var;
@@ -6813,7 +6813,7 @@ static char_u *get_varp(struct vimoption *p)
  */
 char_u *get_equalprg(void)
 {
-  if (*curbuf->b_p_ep == NUL)
+  if (*curbuf->b_p_ep == '\0')
     return p_ep;
   return curbuf->b_p_ep;
 }
@@ -7134,7 +7134,7 @@ void set_imsearch_global(void)
 }
 
 static int expand_option_idx = -1;
-static char_u expand_option_name[5] = {'t', '_', NUL, NUL, NUL};
+static char_u expand_option_name[5] = {'t', '_', '\0', '\0', '\0'};
 static int expand_option_flags = 0;
 
 void 
@@ -7155,7 +7155,7 @@ set_context_in_set_cmd (
   expand_option_flags = opt_flags;
 
   xp->xp_context = EXPAND_SETTINGS;
-  if (*arg == NUL) {
+  if (*arg == '\0') {
     xp->xp_pattern = arg;
     return;
   }
@@ -7189,7 +7189,7 @@ set_context_in_set_cmd (
   xp->xp_pattern = arg = p;
   if (*arg == '<') {
     while (*p != '>')
-      if (*p++ == NUL)              /* expand terminal option name */
+      if (*p++ == '\0')              /* expand terminal option name */
         return;
     key = get_special_key_code(arg + 1);
     if (key == 0) {                 /* unknown name */
@@ -7203,9 +7203,9 @@ set_context_in_set_cmd (
   } else {
     if (p[0] == 't' && p[1] == '_') {
       p += 2;
-      if (*p != NUL)
+      if (*p != '\0')
         ++p;
-      if (*p == NUL)
+      if (*p == '\0')
         return;                 /* expand option name */
       nextchar = *++p;
       is_term_option = TRUE;
@@ -7215,10 +7215,10 @@ set_context_in_set_cmd (
       /* Allow * wildcard */
       while (ASCII_ISALNUM(*p) || *p == '_' || *p == '*')
         p++;
-      if (*p == NUL)
+      if (*p == '\0')
         return;
       nextchar = *p;
-      *p = NUL;
+      *p = '\0';
       opt_idx = findoption(arg);
       *p = nextchar;
       if (opt_idx == -1 || options[opt_idx].var == NULL) {
@@ -7242,7 +7242,7 @@ set_context_in_set_cmd (
     xp->xp_context = EXPAND_UNSUCCESSFUL;
     return;
   }
-  if (xp->xp_context != EXPAND_BOOL_SETTINGS && p[1] == NUL) {
+  if (xp->xp_context != EXPAND_BOOL_SETTINGS && p[1] == '\0') {
     xp->xp_context = EXPAND_OLD_SETTING;
     if (is_term_option)
       expand_option_idx = -1;
@@ -7362,7 +7362,7 @@ int ExpandSettings(expand_T *xp, regmatch_T *regmatch, int *num_file, char_u ***
         name_buf[3] = str[2];
         name_buf[4] = str[3];
         name_buf[5] = '>';
-        name_buf[6] = NUL;
+        name_buf[6] = '\0';
         if (vim_regexec(regmatch, name_buf, (colnr_T)0)) {
           match = TRUE;
           str = name_buf;
@@ -7390,7 +7390,7 @@ int ExpandSettings(expand_T *xp, regmatch_T *regmatch, int *num_file, char_u ***
         name_buf[1] = '_';
         name_buf[2] = str[0];
         name_buf[3] = str[1];
-        name_buf[4] = NUL;
+        name_buf[4] = '\0';
 
         match = FALSE;
         if (vim_regexec(regmatch, name_buf, (colnr_T)0))
@@ -7402,7 +7402,7 @@ int ExpandSettings(expand_T *xp, regmatch_T *regmatch, int *num_file, char_u ***
           name_buf[3] = str[0];
           name_buf[4] = str[1];
           name_buf[5] = '>';
-          name_buf[6] = NUL;
+          name_buf[6] = '\0';
 
           if (vim_regexec(regmatch, name_buf, (colnr_T)0))
             match = TRUE;
@@ -7486,7 +7486,7 @@ int ExpandOldSetting(int *num_file, char_u ***file)
 #ifdef BACKSLASH_IN_FILENAME
   /* For MS-Windows et al. we don't double backslashes at the start and
    * before a file name character. */
-  for (var = buf; *var != NUL; mb_ptr_adv(var))
+  for (var = buf; *var != '\0'; mb_ptr_adv(var))
     if (var[0] == '\\' && var[1] == '\\'
         && expand_option_idx >= 0
         && (options[expand_option_idx].flags & P_EXPAND)
@@ -7526,7 +7526,7 @@ option_value2string (
   } else { /* P_STRING */
     varp = *(char_u **)(varp);
     if (varp == NULL)                       /* just in case */
-      NameBuff[0] = NUL;
+      NameBuff[0] = '\0';
     /* don't show the actual value of 'key', only that it's set */
     else if (opp->var == (char_u *)&p_key && *varp)
       STRCPY(NameBuff, "*****");
@@ -7660,10 +7660,10 @@ static void langmap_set(void)
   ga_clear(&langmap_mapga);                 /* clear the previous map first */
   langmap_init();                           /* back to one-to-one map */
 
-  for (p = p_langmap; p[0] != NUL; ) {
-    for (p2 = p; p2[0] != NUL && p2[0] != ',' && p2[0] != ';';
+  for (p = p_langmap; p[0] != '\0'; ) {
+    for (p2 = p; p2[0] != '\0' && p2[0] != ',' && p2[0] != ';';
          mb_ptr_adv(p2)) {
-      if (p2[0] == '\\' && p2[1] != NUL)
+      if (p2[0] == '\\' && p2[1] != '\0')
         ++p2;
     }
     if (p2[0] == ';')
@@ -7675,10 +7675,10 @@ static void langmap_set(void)
         ++p;
         break;
       }
-      if (p[0] == '\\' && p[1] != NUL)
+      if (p[0] == '\\' && p[1] != '\0')
         ++p;
       from = (*mb_ptr2char)(p);
-      to = NUL;
+      to = '\0';
       if (p2 == NULL) {
         mb_ptr_adv(p);
         if (p[0] != ',') {
@@ -7693,7 +7693,7 @@ static void langmap_set(void)
           to = (*mb_ptr2char)(p2);
         }
       }
-      if (to == NUL) {
+      if (to == '\0') {
         EMSG2(_("E357: 'langmap': Matching character missing for %s"),
             transchar(from));
         return;
@@ -7710,7 +7710,7 @@ static void langmap_set(void)
         mb_ptr_adv(p2);
         if (*p == ';') {
           p = p2;
-          if (p[0] != NUL) {
+          if (p[0] != '\0') {
             if (p[0] != ',') {
               EMSG2(_(
                       "E358: 'langmap': Extra characters after semicolon: %s"),
@@ -7997,7 +7997,7 @@ opt_strings_flags (
 
       len = (int)STRLEN(values[i]);
       if (STRNCMP(values[i], val, len) == 0
-          && ((list && val[len] == ',') || val[len] == NUL)) {
+          && ((list && val[len] == ',') || val[len] == '\0')) {
         val += len + (val[len] == ',');
         new_flags |= (1 << i);
         break;                  /* check next item in val list */
@@ -8026,7 +8026,7 @@ static int check_opt_wim(void)
   for (p = p_wim; *p; ++p) {
     for (i = 0; ASCII_ISALPHA(p[i]); ++i)
       ;
-    if (p[i] != NUL && p[i] != ',' && p[i] != ':')
+    if (p[i] != '\0' && p[i] != ',' && p[i] != ':')
       return FAIL;
     if (i == 7 && STRNCMP(p, "longest", 7) == 0)
       new_wim_flags[idx] |= WIM_LONGEST;
@@ -8037,7 +8037,7 @@ static int check_opt_wim(void)
     else
       return FAIL;
     p += i;
-    if (*p == NUL)
+    if (*p == '\0')
       break;
     if (*p == ',') {
       if (idx == 3)
@@ -8108,7 +8108,7 @@ int file_ff_differs(buf_T *buf, int ignore_empty)
   if (ignore_empty
       && (buf->b_flags & BF_NEW)
       && buf->b_ml.ml_line_count == 1
-      && *ml_get_buf(buf, (linenr_T)1, FALSE) == NUL)
+      && *ml_get_buf(buf, (linenr_T)1, FALSE) == '\0')
     return FALSE;
   if (buf->b_start_ffc != *buf->b_p_ff)
     return TRUE;
@@ -8117,7 +8117,7 @@ int file_ff_differs(buf_T *buf, int ignore_empty)
   if (!buf->b_p_bin && buf->b_start_bomb != buf->b_p_bomb)
     return TRUE;
   if (buf->b_start_fenc == NULL)
-    return *buf->b_p_fenc != NUL;
+    return *buf->b_p_fenc != '\0';
   return STRCMP(buf->b_start_fenc, buf->b_p_fenc) != 0;
 }
 
@@ -8158,7 +8158,7 @@ void find_mps_values(int *initc, int *findc, int *backwards, int switchit)
   char_u      *ptr;
 
   ptr = curbuf->b_p_mps;
-  while (*ptr != NUL) {
+  while (*ptr != '\0') {
     if (has_mbyte) {
       char_u *prev;
 
diff --git a/src/os/env.c b/src/os/env.c
index e6cdb92ea8..174f142854 100644
--- a/src/os/env.c
+++ b/src/os/env.c
@@ -42,7 +42,7 @@ char *os_getenvname_at_index(size_t index)
     return NULL;
   }
   int namesize = 0;
-  while (str[namesize] != '=' && str[namesize] != NUL) {
+  while (str[namesize] != '=' && str[namesize] != '\0') {
     namesize++;
   }
   char *name = (char *)vim_strnsave((char_u *)str, namesize);
diff --git a/src/os/fs.c b/src/os/fs.c
index f9b02375e1..461913433f 100644
--- a/src/os/fs.c
+++ b/src/os/fs.c
@@ -84,7 +84,7 @@ static bool is_executable_in_path(const char_u *name)
 {
   const char *path = getenv("PATH");
   // PATH environment variable does not exist or is empty.
-  if (path == NULL || *path == NUL) {
+  if (path == NULL || *path == '\0') {
     return false;
   }
 
diff --git a/src/os/shell.c b/src/os/shell.c
index d14e355d19..7e5f5fd41e 100644
--- a/src/os/shell.c
+++ b/src/os/shell.c
@@ -251,14 +251,14 @@ static int tokenize(char_u *str, char **argv)
   int argc = 0, len;
   char_u *p = str;
 
-  while (*p != NUL) {
+  while (*p != '\0') {
     len = word_length(p);
 
     if (argv != NULL) {
       // Fill the slot
       argv[argc] = xmalloc(len + 1);
       memcpy(argv[argc], p, len);
-      argv[argc][len] = NUL;
+      argv[argc][len] = '\0';
     }
 
     argc++;
@@ -323,7 +323,7 @@ static void write_selection(uv_write_t *req)
         buflen *= 2;
         pdata->wbuffer = xrealloc(pdata->wbuffer, buflen);
       }
-      pdata->wbuffer[off++] = NUL;
+      pdata->wbuffer[off++] = '\0';
     } else {
       char_u  *s = vim_strchr(lp + written, NL);
       len = s == NULL ? l : s - (lp + written);
@@ -405,7 +405,7 @@ static void read_cb(uv_stream_t *stream, ssize_t cnt, const uv_buf_t *buf)
     if (pdata->rbuffer[i] == NL) {
       // Insert the line
       append_ga_line(&pdata->ga);
-    } else if (pdata->rbuffer[i] == NUL) {
+    } else if (pdata->rbuffer[i] == '\0') {
       // Translate NUL to NL
       ga_append(&pdata->ga, NL);
     } else {
diff --git a/src/os/users.c b/src/os/users.c
index e7b362637b..8a52f905be 100644
--- a/src/os/users.c
+++ b/src/os/users.c
@@ -56,7 +56,7 @@ int os_get_uname(uid_t uid, char *s, size_t len)
   struct passwd *pw;
 
   if ((pw = getpwuid(uid)) != NULL
-      && pw->pw_name != NULL && *(pw->pw_name) != NUL) {
+      && pw->pw_name != NULL && *(pw->pw_name) != '\0') {
     vim_strncpy((char_u *)s, (char_u *)pw->pw_name, len - 1);
     return OK;
   }
diff --git a/src/os_unix.c b/src/os_unix.c
index b2f0df988b..d2de9b8c4c 100644
--- a/src/os_unix.c
+++ b/src/os_unix.c
@@ -250,22 +250,22 @@ void mch_settitle(char_u *title, char_u *icon)
    * Note: if "t_ts" is set, title is set with escape sequence rather
    *	     than x11 calls, because the x11 calls don't always work
    */
-  if ((type || *T_TS != NUL) && title != NULL) {
+  if ((type || *T_TS != '\0') && title != NULL) {
     if (oldtitle == NULL
         )                       /* first call but not in GUI, save title */
       (void)get_x11_title(FALSE);
 
-    if (*T_TS != NUL)                   /* it's OK if t_fs is empty */
+    if (*T_TS != '\0')                   /* it's OK if t_fs is empty */
       term_settitle(title);
     did_set_title = TRUE;
   }
 
-  if ((type || *T_CIS != NUL) && icon != NULL) {
+  if ((type || *T_CIS != '\0') && icon != NULL) {
     if (oldicon == NULL
         )                       /* first call, save icon */
       get_x11_icon(FALSE);
 
-    if (*T_CIS != NUL) {
+    if (*T_CIS != '\0') {
       out_str(T_CIS);                           /* set icon start */
       out_str_nf(icon);
       out_str(T_CIE);                           /* set icon end */
@@ -396,7 +396,7 @@ int len;              /* buffer size, only used when name gets longer */
       dirp = opendir(".");
       tail = name;
     } else {
-      *slash = NUL;
+      *slash = '\0';
       dirp = opendir((char *)name);
       *slash = '/';
       tail = slash + 1;
@@ -757,7 +757,7 @@ void get_stty()
     buf[0] = keys.sg_erase;
     intr_char = keys.sg_kill;
 #endif
-    buf[1] = NUL;
+    buf[1] = '\0';
     add_termcode((char_u *)"kb", buf, FALSE);
 
     /*
@@ -832,7 +832,7 @@ void check_mouse_termcode()
     set_mouse_termcode(KS_MOUSE, (char_u *)(term_is_8bit(T_NAME)
                                             ? IF_EB("\233M", CSI_STR "M")
                                             : IF_EB("\033[M", ESC_STR "[M")));
-    if (*p_mouse != NUL) {
+    if (*p_mouse != '\0') {
       /* force mouse off and maybe on to send possibly new mouse
        * activation sequence to the xterm, with(out) drag tracing. */
       mch_setmouse(FALSE);
@@ -867,7 +867,7 @@ void check_mouse_termcode()
                                                   ? IF_EB("\233", CSI_STR)
                                                   : IF_EB("\033[", ESC_STR "[")));
 
-    if (*p_mouse != NUL) {
+    if (*p_mouse != '\0') {
       mch_setmouse(FALSE);
       setmouse();
     }
@@ -880,7 +880,7 @@ void check_mouse_termcode()
                                                 ? IF_EB("\233<", CSI_STR "<")
                                                 : IF_EB("\033[<", ESC_STR "[<")));
 
-    if (*p_mouse != NUL) {
+    if (*p_mouse != '\0') {
       mch_setmouse(FALSE);
       setmouse();
     }
@@ -1115,7 +1115,7 @@ int flags;                      /* EW_* flags */
     /* Count the length of the patterns in the same way as they are put in
      * "command" below. */
     ++len;                              /* add space */
-    for (j = 0; pat[i][j] != NUL; ++j) {
+    for (j = 0; pat[i][j] != '\0'; ++j) {
       if (vim_strchr(SHELL_SPECIAL, pat[i][j]) != NULL)
         ++len;                  /* may add a backslash */
       ++len;
@@ -1169,10 +1169,10 @@ int flags;                      /* EW_* flags */
 
       p = command + STRLEN(command);
       *p++ = ' ';
-      for (j = 0; pat[i][j] != NUL; ++j) {
+      for (j = 0; pat[i][j] != '\0'; ++j) {
         if (pat[i][j] == '`')
           intick = !intick;
-        else if (pat[i][j] == '\\' && pat[i][j + 1] != NUL) {
+        else if (pat[i][j] == '\\' && pat[i][j + 1] != '\0') {
           /* Remove a backslash, take char literally.  But keep
            * backslash inside backticks, before a special character
            * and before a backtick. */
@@ -1190,7 +1190,7 @@ int flags;                      /* EW_* flags */
         /* Copy one character. */
         *p++ = pat[i][j];
       }
-      *p = NUL;
+      *p = '\0';
     }
 
   if (flags & EW_SILENT) {
@@ -1297,12 +1297,12 @@ int flags;                      /* EW_* flags */
   }
   /* file names are separated with NL */
   else if (shell_style == STYLE_BT || shell_style == STYLE_VIMGLOB) {
-    buffer[len] = NUL;                  /* make sure the buffer ends in NUL */
+    buffer[len] = '\0';                  /* make sure the buffer ends in NUL */
     p = buffer;
-    for (i = 0; *p != NUL; ++i) {       /* count number of entries */
-      while (*p != '\n' && *p != NUL)
+    for (i = 0; *p != '\0'; ++i) {       /* count number of entries */
+      while (*p != '\n' && *p != '\0')
         ++p;
-      if (*p != NUL)
+      if (*p != '\0')
         ++p;
       p = skipwhite(p);                 /* skip leading white space */
     }
@@ -1320,7 +1320,7 @@ int flags;                      /* EW_* flags */
     check_spaces = FALSE;
     if (shell_style == STYLE_PRINT && !did_find_nul) {
       /* If there is a NUL, set did_find_nul, else set check_spaces */
-      buffer[len] = NUL;
+      buffer[len] = '\0';
       if (len && (int)STRLEN(buffer) < (int)len)
         did_find_nul = TRUE;
       else
@@ -1331,15 +1331,15 @@ int flags;                      /* EW_* flags */
      * Make sure the buffer ends with a NUL.  For STYLE_PRINT there
      * already is one, for STYLE_GLOB it needs to be added.
      */
-    if (len && buffer[len - 1] == NUL)
+    if (len && buffer[len - 1] == '\0')
       --len;
     else
-      buffer[len] = NUL;
+      buffer[len] = '\0';
     i = 0;
     for (p = buffer; p < buffer + len; ++p)
-      if (*p == NUL || (*p == ' ' && check_spaces)) {       /* count entry */
+      if (*p == '\0' || (*p == ' ' && check_spaces)) {       /* count entry */
         ++i;
-        *p = NUL;
+        *p = '\0';
       }
     if (len)
       ++i;                              /* count last entry */
@@ -1366,12 +1366,12 @@ int flags;                      /* EW_* flags */
     if (shell_style == STYLE_ECHO || shell_style == STYLE_BT
         || shell_style == STYLE_VIMGLOB) {
       while (!(shell_style == STYLE_ECHO && *p == ' ')
-             && *p != '\n' && *p != NUL)
+             && *p != '\n' && *p != '\0')
         ++p;
       if (p == buffer + len)                    /* last entry */
-        *p = NUL;
+        *p = '\0';
       else {
-        *p++ = NUL;
+        *p++ = '\0';
         p = skipwhite(p);                       /* skip to next entry */
       }
     } else {          /* NUL separates */
@@ -1455,7 +1455,7 @@ int mch_has_exp_wildcard(p)
 char_u  *p;
 {
   for (; *p; mb_ptr_adv(p)) {
-    if (*p == '\\' && p[1] != NUL)
+    if (*p == '\\' && p[1] != '\0')
       ++p;
     else if (vim_strchr((char_u *)
                  "*?[{'"
@@ -1473,12 +1473,12 @@ int mch_has_wildcard(p)
 char_u  *p;
 {
   for (; *p; mb_ptr_adv(p)) {
-    if (*p == '\\' && p[1] != NUL)
+    if (*p == '\\' && p[1] != '\0')
       ++p;
     else if (vim_strchr((char_u *)
                  "*?[{`'$"
                  , *p) != NULL
-             || (*p == '~' && p[1] != NUL))
+             || (*p == '~' && p[1] != '\0'))
       return TRUE;
   }
   return FALSE;
diff --git a/src/path.c b/src/path.c
index 6e736f1daa..292659a4c3 100644
--- a/src/path.c
+++ b/src/path.c
@@ -72,7 +72,7 @@ char_u *path_tail(char_u *fname)
   char_u *tail = get_past_head(fname);
   char_u *p = tail;
   // Find last part of path.
-  while (*p != NUL) {
+  while (*p != '\0') {
     if (vim_ispathsep_nocolon(*p)) {
       tail = p + 1;
     }
@@ -97,10 +97,10 @@ char_u *path_tail_with_sep(char_u *fname)
 char_u *path_next_component(char_u *fname)
 {
   assert(fname != NULL);
-  while (*fname != NUL && !vim_ispathsep(*fname)) {
+  while (*fname != '\0' && !vim_ispathsep(*fname)) {
     mb_ptr_adv(fname);
   }
-  if (*fname != NUL) {
+  if (*fname != '\0') {
     fname++;
   }
   return fname;
@@ -180,7 +180,7 @@ void shorten_dir(char_u *str)
   for (s = str;; ++s) {
     if (s >= tail) {                /* copy the whole tail */
       *d++ = *s;
-      if (*s == NUL)
+      if (*s == '\0')
         break;
     } else if (vim_ispathsep(*s)) {       /* copy '/' and next char */
       *d++ = *s;
@@ -215,7 +215,7 @@ int dir_of_file_exists(char_u *fname)
   if (p == fname)
     return TRUE;
   c = *p;
-  *p = NUL;
+  *p = '\0';
   retval = os_isdir(fname);
   *p = c;
   return retval;
@@ -241,13 +241,13 @@ int vim_fnamencmp(char_u *x, char_u *y, size_t len)
 #ifdef BACKSLASH_IN_FILENAME
   char_u      *px = x;
   char_u      *py = y;
-  int cx = NUL;
-  int cy = NUL;
+  int cx = '\0';
+  int cy = '\0';
 
   while (len > 0) {
     cx = PTR2CHAR(px);
     cy = PTR2CHAR(py);
-    if (cx == NUL || cy == NUL
+    if (cx == '\0' || cy == '\0'
         || ((p_fic ? vim_tolower(cx) != vim_tolower(cy) : cx != cy)
             && !(cx == '/' && cy == '\\')
             && !(cx == '\\' && cy == '/')))
@@ -301,7 +301,7 @@ char_u *concat_str(char_u *str1, char_u *str2)
  */
 void add_pathsep(char_u *p)
 {
-  if (*p != NUL && !after_pathsep(p, p + STRLEN(p)))
+  if (*p != '\0' && !after_pathsep(p, p + STRLEN(p)))
     STRCAT(p, PATHSEPSTR);
 }
 
@@ -402,7 +402,7 @@ unix_expandpath (
   s = buf;
   e = NULL;
   path_end = path;
-  while (*path_end != NUL) {
+  while (*path_end != '\0') {
     /* May ignore a wildcard that has a backslash before it; it will
      * be removed by rem_backslash() or file_pat_to_reg_pat() below. */
     if (path_end >= path + wildoff && rem_backslash(path_end))
@@ -425,7 +425,7 @@ unix_expandpath (
       *p++ = *path_end++;
   }
   e = p;
-  *e = NUL;
+  *e = '\0';
 
   /* Now we have one wildcard component between "s" and "e". */
   /* Remove backslashes between "wildoff" and the start of the wildcard
@@ -478,8 +478,8 @@ unix_expandpath (
   }
 
   /* open the directory for scanning */
-  *s = NUL;
-  dirp = opendir(*buf == NUL ? "." : (char *)buf);
+  *s = '\0';
+  dirp = opendir(*buf == '\0' ? "." : (char *)buf);
 
   /* Find all matching entries */
   if (dirp != NULL) {
@@ -513,7 +513,7 @@ unix_expandpath (
         } else {
           /* no more wildcards, check if there is a match */
           /* remove backslashes for the remaining components only */
-          if (*path_end != NUL)
+          if (*path_end != '\0')
             backslash_halve(buf + len + 1);
           if (os_file_exists(buf)) {          /* add existing file */
 #ifdef MACOS_CONVERT
@@ -614,7 +614,7 @@ static int is_unique(char_u *maybe_unique, garray_T *gap, int i)
  */
 static void expand_path_option(char_u *curdir, garray_T *gap)
 {
-  char_u      *path_option = *curbuf->b_p_path == NUL
+  char_u      *path_option = *curbuf->b_p_path == '\0'
                              ? p_path : curbuf->b_p_path;
   char_u      *buf;
   char_u      *p;
@@ -622,10 +622,10 @@ static void expand_path_option(char_u *curdir, garray_T *gap)
 
   buf = alloc((int)MAXPATHL);
 
-  while (*path_option != NUL) {
+  while (*path_option != '\0') {
     copy_option_part(&path_option, buf, MAXPATHL, " ,");
 
-    if (buf[0] == '.' && (buf[1] == NUL || vim_ispathsep(buf[1]))) {
+    if (buf[0] == '.' && (buf[1] == '\0' || vim_ispathsep(buf[1]))) {
       /* Relative to current buffer:
        * "/path/file" + "." -> "/path/"
        * "/path/file"  + "./subdir" -> "/path/subdir" */
@@ -635,13 +635,13 @@ static void expand_path_option(char_u *curdir, garray_T *gap)
       len = (int)(p - curbuf->b_ffname);
       if (len + (int)STRLEN(buf) >= MAXPATHL)
         continue;
-      if (buf[1] == NUL)
-        buf[len] = NUL;
+      if (buf[1] == '\0')
+        buf[len] = '\0';
       else
         STRMOVE(buf + len, buf + 2);
       memmove(buf, curbuf->b_ffname, len);
       simplify_filename(buf);
-    } else if (buf[0] == NUL)
+    } else if (buf[0] == '\0')
       /* relative to current directory */
       STRCPY(buf, curdir);
     else if (path_with_url(buf))
@@ -688,7 +688,7 @@ static char_u *get_path_cutoff(char_u *fname, garray_T *gap)
     int j = 0;
 
     while ((fname[j] == path_part[i][j]
-            ) && fname[j] != NUL && path_part[i][j] != NUL)
+            ) && fname[j] != '\0' && path_part[i][j] != '\0')
       j++;
     if (j > maxlen) {
       maxlen = j;
@@ -736,7 +736,7 @@ static void uniquefy_paths(garray_T *gap, char_u *pattern)
   len = (int)STRLEN(pattern);
   file_pattern = alloc(len + 2);
   file_pattern[0] = '*';
-  file_pattern[1] = NUL;
+  file_pattern[1] = '\0';
   STRCAT(file_pattern, pattern);
   pat = file_pat_to_reg_pat(file_pattern, NULL, NULL, TRUE);
   vim_free(file_pattern);
@@ -764,7 +764,7 @@ static void uniquefy_paths(garray_T *gap, char_u *pattern)
 
     len = (int)STRLEN(path);
     is_in_curdir = fnamencmp(curdir, path, dir_end - path) == 0
-                   && curdir[dir_end - path] == NUL;
+                   && curdir[dir_end - path] == '\0';
     if (is_in_curdir)
       in_curdir[i] = vim_strsave(path);
 
@@ -862,7 +862,7 @@ static char_u *gettail_dir(char_u *fname)
   int look_for_sep = TRUE;
   char_u      *p;
 
-  for (p = fname; *p != NUL; ) {
+  for (p = fname; *p != '\0'; ) {
     if (vim_ispathsep(*p)) {
       if (look_for_sep) {
         next_dir_end = p;
@@ -917,15 +917,15 @@ expand_in_path (
 
   /* Copy each path in files into gap */
   s = e = files;
-  while (*s != NUL) {
-    while (*e != '\n' && *e != NUL)
+  while (*s != '\0') {
+    while (*e != '\n' && *e != '\0')
       e++;
-    if (*e == NUL) {
+    if (*e == '\0') {
       addfile(gap, s, flags);
       break;
     } else {
       /* *e is '\n' */
-      *e = NUL;
+      *e = '\0';
       addfile(gap, s, flags);
       e++;
       s = e;
@@ -946,7 +946,7 @@ static int has_env_var(char_u *p);
 static int has_env_var(char_u *p)
 {
   for (; *p; mb_ptr_adv(p)) {
-    if (*p == '\\' && p[1] != NUL)
+    if (*p == '\\' && p[1] != '\0')
       ++p;
     else if (vim_strchr((char_u *)
                  "$"
@@ -966,7 +966,7 @@ static int has_special_wildchar(char_u *p);
 static int has_special_wildchar(char_u *p)
 {
   for (; *p; mb_ptr_adv(p)) {
-    if (*p == '\\' && p[1] != NUL)
+    if (*p == '\\' && p[1] != '\0')
       ++p;
     else if (vim_strchr((char_u *)SPECIAL_WILDCHAR, *p) != NULL)
       return TRUE;
@@ -1125,7 +1125,7 @@ gen_expand_wildcards (
  */
 static int vim_backtick(char_u *p)
 {
-  return *p == '`' && *(p + 1) != NUL && *(p + STRLEN(p) - 1) == '`';
+  return *p == '`' && *(p + 1) != '\0' && *(p + STRLEN(p) - 1) == '`';
 }
 
 /*
@@ -1161,21 +1161,21 @@ expand_backtick (
     return 0;
 
   cmd = buffer;
-  while (*cmd != NUL) {
+  while (*cmd != '\0') {
     cmd = skipwhite(cmd);               /* skip over white space */
     p = cmd;
-    while (*p != NUL && *p != '\r' && *p != '\n')     /* skip over entry */
+    while (*p != '\0' && *p != '\r' && *p != '\n')     /* skip over entry */
       ++p;
     /* add an entry if it is not empty */
     if (p > cmd) {
       i = *p;
-      *p = NUL;
+      *p = '\0';
       addfile(gap, cmd, flags);
       *p = i;
       ++cnt;
     }
     cmd = p;
-    while (*cmd != NUL && (*cmd == '\r' || *cmd == '\n'))
+    while (*cmd != '\0' && (*cmd == '\r' || *cmd == '\n'))
       ++cmd;
   }
 
@@ -1271,16 +1271,16 @@ void simplify_filename(char_u *filename)
      * or "p" is at the "start" of the (absolute or relative) path name. */
     if (vim_ispathsep(*p))
       STRMOVE(p, p + 1);                /* remove duplicate "/" */
-    else if (p[0] == '.' && (vim_ispathsep(p[1]) || p[1] == NUL)) {
+    else if (p[0] == '.' && (vim_ispathsep(p[1]) || p[1] == '\0')) {
       if (p == start && relative)
-        p += 1 + (p[1] != NUL);         /* keep single "." or leading "./" */
+        p += 1 + (p[1] != '\0');         /* keep single "." or leading "./" */
       else {
         /* Strip "./" or ".///".  If we are at the end of the file name
          * and there is no trailing path separator, either strip "/." if
          * we are after "start", or strip "." if we are at the beginning
          * of an absolute path name . */
         tail = p + 1;
-        if (p[1] != NUL)
+        if (p[1] != '\0')
           while (vim_ispathsep(*tail))
             mb_ptr_adv(tail);
         else if (p > start)
@@ -1288,7 +1288,7 @@ void simplify_filename(char_u *filename)
         STRMOVE(p, tail);
       }
     } else if (p[0] == '.' && p[1] == '.' &&
-               (vim_ispathsep(p[2]) || p[2] == NUL)) {
+               (vim_ispathsep(p[2]) || p[2] == '\0')) {
       /* Skip to after ".." or "../" or "..///". */
       tail = p + 2;
       while (vim_ispathsep(*tail))
@@ -1305,7 +1305,7 @@ void simplify_filename(char_u *filename)
            * system, we strip it.  On Unix, we don't accept a symbolic
            * link that refers to a non-existent file. */
           saved_char = p[-1];
-          p[-1] = NUL;
+          p[-1] = '\0';
 #ifdef UNIX
           if (mch_lstat((char *)filename, &st) < 0)
 #else
@@ -1331,7 +1331,7 @@ void simplify_filename(char_u *filename)
              * a valid one, and we disable stripping of later
              * components. */
             saved_char = *tail;
-            *tail = NUL;
+            *tail = '\0';
             if (mch_stat((char *)filename, &st) >= 0)
               do_strip = TRUE;
             else
@@ -1352,7 +1352,7 @@ void simplify_filename(char_u *filename)
                 (void)mch_stat(".", &new_st);
               else {
                 saved_char = *p;
-                *p = NUL;
+                *p = '\0';
                 (void)mch_stat((char *)filename, &new_st);
                 *p = saved_char;
               }
@@ -1383,7 +1383,7 @@ void simplify_filename(char_u *filename)
            * path separator as well. */
           if (p == start && relative && tail[-1] == '.') {
             *p++ = '.';
-            *p = NUL;
+            *p = '\0';
           } else {
             if (p > start && tail[-1] == '.')
               --p;
@@ -1405,7 +1405,7 @@ void simplify_filename(char_u *filename)
       ++components;                     /* simple path component */
       p = path_next_component(p);
     }
-  } while (*p != NUL);
+  } while (*p != '\0');
 }
 
 static char_u *eval_includeexpr(char_u *ptr, int len);
@@ -1438,7 +1438,7 @@ find_file_name_in_path (
   int c;
   char_u      *tofree = NULL;
 
-  if ((options & FNAME_INCL) && *curbuf->b_p_inex != NUL) {
+  if ((options & FNAME_INCL) && *curbuf->b_p_inex != '\0') {
     tofree = eval_includeexpr(ptr, len);
     if (tofree != NULL) {
       ptr = tofree;
@@ -1455,7 +1455,7 @@ find_file_name_in_path (
      * 'includeexpr' (unless done already).
      */
     if (file_name == NULL
-        && !(options & FNAME_INCL) && *curbuf->b_p_inex != NUL) {
+        && !(options & FNAME_INCL) && *curbuf->b_p_inex != '\0') {
       tofree = eval_includeexpr(ptr, len);
       if (tofree != NULL) {
         ptr = tofree;
@@ -1466,7 +1466,7 @@ find_file_name_in_path (
     }
     if (file_name == NULL && (options & FNAME_MESS)) {
       c = ptr[len];
-      ptr[len] = NUL;
+      ptr[len] = '\0';
       EMSG2(_("E447: Can't find file \"%s\" in path"), ptr);
       ptr[len] = c;
     }
@@ -1537,7 +1537,7 @@ vim_FullName (
   int retval = OK;
   int url;
 
-  *buf = NUL;
+  *buf = '\0';
   if (fname == NULL)
     return FAIL;
 
@@ -1642,15 +1642,15 @@ int pathcmp(const char *p, const char *q, int maxlen)
     c2 = PTR2CHAR((char_u *)q + i);
 
     /* End of "p": check if "q" also ends or just has a slash. */
-    if (c1 == NUL) {
-      if (c2 == NUL)        /* full match */
+    if (c1 == '\0') {
+      if (c2 == '\0')        /* full match */
         return 0;
       s = q;
       break;
     }
 
     /* End of "q": check if "p" just has a slash. */
-    if (c2 == NUL) {
+    if (c2 == '\0') {
       s = p;
       break;
     }
@@ -1676,7 +1676,7 @@ int pathcmp(const char *p, const char *q, int maxlen)
   c1 = PTR2CHAR((char_u *)s + i);
   c2 = PTR2CHAR((char_u *)s + i + MB_PTR2LEN((char_u *)s + i));
   /* ignore a trailing slash, but not "//" or ":/" */
-  if (c2 == NUL
+  if (c2 == '\0'
       && i > 0
       && !after_pathsep((char_u *)s, (char_u *)s + i)
 #ifdef BACKSLASH_IN_FILENAME
@@ -1715,7 +1715,7 @@ char_u *path_shorten_fname_if_possible(char_u *full_path)
 
   if (os_dirname(dirname, MAXPATHL) == OK) {
     p = path_shorten_fname(full_path, dirname);
-    if (p == NULL || *p == NUL) {
+    if (p == NULL || *p == '\0') {
       p = full_path;
     }
   }
@@ -1970,7 +1970,7 @@ int append_path(char *path, const char *to_append, int max_len)
 static int path_get_absolute_path(char_u *fname, char_u *buf, int len, int force)
 {
   char_u *p;
-  *buf = NUL;
+  *buf = '\0';
 
   char relative_directory[len];
   char *end_of_path = (char *) fname;
@@ -1979,10 +1979,10 @@ static int path_get_absolute_path(char_u *fname, char_u *buf, int len, int force
   if (force || !path_is_absolute_path(fname)) {
     if ((p = vim_strrchr(fname, '/')) != NULL) {
       STRNCPY(relative_directory, fname, p-fname);
-      relative_directory[p-fname] = NUL;
+      relative_directory[p-fname] = '\0';
       end_of_path = (char *) (p + 1);
     } else {
-      relative_directory[0] = NUL;
+      relative_directory[0] = '\0';
       end_of_path = (char *) fname;
     }
 
diff --git a/src/popupmnu.c b/src/popupmnu.c
index bfb6b693e1..283d6a5e83 100644
--- a/src/popupmnu.c
+++ b/src/popupmnu.c
@@ -333,13 +333,13 @@ void pum_redraw(void)
           }
           w = ptr2cells(p);
 
-          if ((*p == NUL) || (*p == TAB) || (totwidth + w > pum_width)) {
+          if ((*p == '\0') || (*p == TAB) || (totwidth + w > pum_width)) {
             // Display the text that fits or comes before a Tab.
             // First convert it to printable characters.
             char_u *st;
             int saved = *p;
 
-            *p = NUL;
+            *p = '\0';
             st = transstr(s);
             *p = saved;
 
@@ -573,13 +573,13 @@ static int pum_set_selected(int n, int repeat)
           char_u *p, *e;
           linenr_T lnum = 0;
 
-          for (p = pum_array[pum_selected].pum_info; *p != NUL;) {
+          for (p = pum_array[pum_selected].pum_info; *p != '\0';) {
             e = vim_strchr(p, '\n');
             if (e == NULL) {
               ml_append(lnum++, p, 0, FALSE);
               break;
             } else {
-              *e = NUL;
+              *e = '\0';
               ml_append(lnum++, p, (int)(e - p + 1), FALSE);
               *e = '\n';
               p = e + 1;
diff --git a/src/quickfix.c b/src/quickfix.c
index c98b43fff6..a97415ebc6 100644
--- a/src/quickfix.c
+++ b/src/quickfix.c
@@ -305,7 +305,7 @@ qf_init_ext (
    * regex prog.  Only a few % characters are allowed.
    */
   /* Use the local value of 'errorformat' if it's set. */
-  if (errorformat == p_efm && tv == NULL && *buf->b_p_efm != NUL)
+  if (errorformat == p_efm && tv == NULL && *buf->b_p_efm != '\0')
     efm = buf->b_p_efm;
   else
     efm = errorformat;
@@ -322,7 +322,7 @@ qf_init_ext (
 #endif
   fmtstr = alloc(i);
 
-  while (efm[0] != NUL) {
+  while (efm[0] != '\0') {
     /*
      * Allocate a new eformat structure and put it at the end of the list
      */
@@ -336,8 +336,8 @@ qf_init_ext (
     /*
      * Isolate one part in the 'errorformat' option
      */
-    for (len = 0; efm[len] != NUL && efm[len] != ','; ++len)
-      if (efm[len] == '\\' && efm[len + 1] != NUL)
+    for (len = 0; efm[len] != '\0' && efm[len] != ','; ++len)
+      if (efm[len] == '\\' && efm[len + 1] != '\0')
         ++len;
 
     /*
@@ -383,7 +383,7 @@ qf_init_ext (
             ptr += 10;
           }
 #endif
-          if (*efmp == 'f' && efmp[1] != NUL) {
+          if (*efmp == 'f' && efmp[1] != '\0') {
             if (efmp[1] != '\\' && efmp[1] != '%') {
               /* A file name may contain spaces, but this isn't
                * in "\f".  For "%f:%l:%m" there may be a ":" in
@@ -400,7 +400,7 @@ qf_init_ext (
             }
           } else {
             srcptr = (char_u *)fmt_pat[idx].pattern;
-            while ((*ptr = *srcptr++) != NUL)
+            while ((*ptr = *srcptr++) != '\0')
               ++ptr;
           }
           *ptr++ = '\\';
@@ -464,7 +464,7 @@ qf_init_ext (
       }
     }
     *ptr++ = '$';
-    *ptr = NUL;
+    *ptr = '\0';
     if ((fmt_ptr->prog = vim_regcomp(fmtstr, RE_MAGIC + RE_STRING)) == NULL)
       goto error2;
     /*
@@ -546,14 +546,14 @@ qf_init_ext (
     } else if (fgets((char *)IObuff, CMDBUFFSIZE - 2, fd) == NULL)
       break;
 
-    IObuff[CMDBUFFSIZE - 2] = NUL;      /* for very long lines */
+    IObuff[CMDBUFFSIZE - 2] = '\0';      /* for very long lines */
     remove_bom(IObuff);
 
     if ((efmp = vim_strrchr(IObuff, '\n')) != NULL)
-      *efmp = NUL;
+      *efmp = '\0';
 #ifdef USE_CRNL
     if ((efmp = vim_strrchr(IObuff, '\r')) != NULL)
-      *efmp = NUL;
+      *efmp = '\0';
 #endif
 
     /* If there was no %> item start at the first pattern */
@@ -574,10 +574,10 @@ restofline:
       idx = fmt_ptr->prefix;
       if (multiscan && vim_strchr((char_u *)"OPQ", idx) == NULL)
         continue;
-      namebuf[0] = NUL;
-      pattern[0] = NUL;
+      namebuf[0] = '\0';
+      pattern[0] = '\0';
       if (!multiscan)
-        errmsg[0] = NUL;
+        errmsg[0] = '\0';
       lnum = 0;
       col = 0;
       use_viscol = FALSE;
@@ -606,7 +606,7 @@ restofline:
 
           /* Expand ~/file and $HOME/file to full path. */
           c = *regmatch.endp[i];
-          *regmatch.endp[i] = NUL;
+          *regmatch.endp[i] = '\0';
           expand_env(regmatch.startp[i], namebuf, CMDBUFFSIZE);
           *regmatch.endp[i] = c;
 
@@ -680,7 +680,7 @@ restofline:
           STRNCAT(pattern, regmatch.startp[i], len);
           pattern[len + 3] = '\\';
           pattern[len + 4] = '$';
-          pattern[len + 5] = NUL;
+          pattern[len + 5] = '\0';
         }
         break;
       }
@@ -690,7 +690,7 @@ restofline:
     if (fmt_ptr == NULL || idx == 'D' || idx == 'X') {
       if (fmt_ptr != NULL) {
         if (idx == 'D') {                               /* enter directory */
-          if (*namebuf == NUL) {
+          if (*namebuf == '\0') {
             EMSG(_("E379: Missing or empty directory name"));
             goto error2;
           }
@@ -699,7 +699,7 @@ restofline:
         } else if (idx == 'X')                          /* leave directory */
           directory = qf_pop_dir(&dir_stack);
       }
-      namebuf[0] = NUL;                 /* no match found, remove file name */
+      namebuf[0] = '\0';                 /* no match found, remove file name */
       lnum = 0;                         /* don't jump to this line */
       valid = FALSE;
       STRCPY(errmsg, IObuff);           /* copy whole line to error message */
@@ -746,12 +746,12 @@ restofline:
       } else if (vim_strchr((char_u *)"OPQ", idx) != NULL) {
         /* global file names */
         valid = FALSE;
-        if (*namebuf == NUL || os_file_exists(namebuf)) {
+        if (*namebuf == '\0' || os_file_exists(namebuf)) {
           if (*namebuf && idx == 'P')
             currfile = qf_push_dir(namebuf, &file_stack);
           else if (idx == 'Q')
             currfile = qf_pop_dir(&file_stack);
-          *namebuf = NUL;
+          *namebuf = '\0';
           if (tail && *tail) {
             STRMOVE(IObuff, skipwhite(tail));
             multiscan = TRUE;
@@ -935,7 +935,7 @@ qf_add_entry (
   qfp->qf_lnum = lnum;
   qfp->qf_col = col;
   qfp->qf_viscol = vis_col;
-  if (pattern == NULL || *pattern == NUL)
+  if (pattern == NULL || *pattern == '\0')
     qfp->qf_pattern = NULL;
   else if ((qfp->qf_pattern = vim_strsave(pattern)) == NULL) {
     vim_free(qfp->qf_text);
@@ -1102,7 +1102,7 @@ void copy_loclist(win_T *from, win_T *to)
  */
 static int qf_get_fnum(char_u *directory, char_u *fname)
 {
-  if (fname == NULL || *fname == NUL)           /* no file name */
+  if (fname == NULL || *fname == '\0')           /* no file name */
     return 0;
   {
     char_u      *ptr;
@@ -1487,7 +1487,7 @@ void qf_jump(qf_info_T *qi, int dir, int errornr, int forceit)
     if (!usable_win) {
       /* Locate a window showing a normal buffer */
       FOR_ALL_WINDOWS(win)
-      if (win->w_buffer->b_p_bt[0] == NUL) {
+      if (win->w_buffer->b_p_bt[0] == '\0') {
         usable_win = 1;
         break;
       }
@@ -1545,7 +1545,7 @@ win_found:
             /* Find a previous usable window */
             win = curwin;
             do {
-              if (win->w_buffer->b_p_bt[0] == NUL)
+              if (win->w_buffer->b_p_bt[0] == '\0')
                 break;
               if (win->w_prev == NULL)
                 win = lastwin;                  /* wrap around the top */
@@ -1592,7 +1592,7 @@ win_found:
 
           /* Remember a usable window. */
           if (altwin == NULL && !win->w_p_pvw
-              && win->w_buffer->b_p_bt[0] == NUL)
+              && win->w_buffer->b_p_bt[0] == '\0')
             altwin = win;
         }
 
@@ -1651,7 +1651,7 @@ win_found:
           line = ml_get_curline();
           screen_col = 0;
           for (char_col = 0; char_col < curwin->w_cursor.col; ++char_col) {
-            if (*line == NUL)
+            if (*line == '\0')
               break;
             if (*line++ == '\t') {
               curwin->w_cursor.col -= 7 - (screen_col % 8);
@@ -1757,7 +1757,7 @@ void qf_list(exarg_T *eap)
     EMSG(_(e_quickfix));
     return;
   }
-  if (!get_list_range(&arg, &idx1, &idx2) || *arg != NUL) {
+  if (!get_list_range(&arg, &idx1, &idx2) || *arg != '\0') {
     EMSG(_(e_trailing));
     return;
   }
@@ -1791,7 +1791,7 @@ void qf_list(exarg_T *eap)
       msg_outtrans_attr(IObuff, i == qi->qf_lists[qi->qf_curlist].qf_index
           ? hl_attr(HLF_L) : hl_attr(HLF_D));
       if (qfp->qf_lnum == 0)
-        IObuff[0] = NUL;
+        IObuff[0] = '\0';
       else if (qfp->qf_col == 0)
         sprintf((char *)IObuff, ":%" PRId64, (int64_t)qfp->qf_lnum);
       else
@@ -1832,16 +1832,16 @@ static void qf_fmt_text(char_u *text, char_u *buf, int bufsize)
   int i;
   char_u      *p = text;
 
-  for (i = 0; *p != NUL && i < bufsize - 1; ++i) {
+  for (i = 0; *p != '\0' && i < bufsize - 1; ++i) {
     if (*p == '\n') {
       buf[i] = ' ';
-      while (*++p != NUL)
+      while (*++p != '\0')
         if (!vim_iswhite(*p) && *p != '\n')
           break;
     } else
       buf[i] = *p++;
   }
-  buf[i] = NUL;
+  buf[i] = '\0';
 }
 
 /*
@@ -1984,7 +1984,7 @@ static char_u *qf_types(int c, int nr)
   else {
     cc[0] = ' ';
     cc[1] = c;
-    cc[2] = NUL;
+    cc[2] = '\0';
     p = cc;
   }
 
@@ -2466,7 +2466,7 @@ int grep_internal(cmdidx_T cmdidx)
           || cmdidx == CMD_grepadd
           || cmdidx == CMD_lgrepadd)
          && STRCMP("internal",
-      *curbuf->b_p_gp == NUL ? p_gp : curbuf->b_p_gp) == 0;
+      *curbuf->b_p_gp == '\0' ? p_gp : curbuf->b_p_gp) == 0;
 }
 
 /*
@@ -2518,12 +2518,12 @@ void ex_make(exarg_T *eap)
    * If 'shellpipe' empty: don't redirect to 'errorfile'.
    */
   len = (unsigned)STRLEN(p_shq) * 2 + (unsigned)STRLEN(eap->arg) + 1;
-  if (*p_sp != NUL)
+  if (*p_sp != '\0')
     len += (unsigned)STRLEN(p_sp) + (unsigned)STRLEN(fname) + 3;
   cmd = alloc(len);
   sprintf((char *)cmd, "%s%s%s", (char *)p_shq, (char *)eap->arg,
       (char *)p_shq);
-  if (*p_sp != NUL)
+  if (*p_sp != '\0')
     append_redir(cmd, len, p_sp, fname);
   /*
    * Output a newline if there's something else than the :make command that
@@ -2536,7 +2536,7 @@ void ex_make(exarg_T *eap)
   msg_outtrans(cmd);            /* show what we are doing */
 
   /* let the shell know if we are redirecting output or not */
-  do_shell(cmd, *p_sp != NUL ? kShellOptDoOut : 0);
+  do_shell(cmd, *p_sp != '\0' ? kShellOptDoOut : 0);
 
 
   res = qf_init(wp, fname, (eap->cmdidx != CMD_make
@@ -2577,7 +2577,7 @@ static char_u *get_mef_name(void)
   struct stat sb;
 #endif
 
-  if (*p_mef == NUL) {
+  if (*p_mef == '\0') {
     name = vim_tempname('e');
     if (name == NULL)
       EMSG(_(e_notmp));
@@ -2588,7 +2588,7 @@ static char_u *get_mef_name(void)
     if (p[0] == '#' && p[1] == '#')
       break;
 
-  if (*p == NUL)
+  if (*p == '\0')
     return vim_strsave(p_mef);
 
   /* Keep trying until the name doesn't exist yet. */
@@ -2702,7 +2702,7 @@ void ex_cfile(exarg_T *eap)
   }
   if (au_name != NULL)
     apply_autocmds(EVENT_QUICKFIXCMDPRE, au_name, NULL, FALSE, curbuf);
-  if (*eap->arg != NUL)
+  if (*eap->arg != '\0')
     set_string_option_direct((char_u *)"ef", -1, eap->arg, OPT_FREE, 0);
 
   /*
@@ -2806,7 +2806,7 @@ void ex_vimgrep(exarg_T *eap)
     goto theend;
   }
 
-  if (s != NULL && *s == NUL) {
+  if (s != NULL && *s == '\0') {
     /* Pattern is empty, use last search pattern. */
     if (last_search_pat() == NULL) {
       EMSG(_(e_noprevre));
@@ -2822,7 +2822,7 @@ void ex_vimgrep(exarg_T *eap)
   regmatch.rmm_maxcol = 0;
 
   p = skipwhite(p);
-  if (*p == NUL) {
+  if (*p == '\0') {
     EMSG(_("E683: File name missing or invalid pattern"));
     goto theend;
   }
@@ -3079,8 +3079,8 @@ char_u *skip_vimgrep_pat(char_u *p, char_u **s, int *flags)
     if (s != NULL)
       *s = p;
     p = skiptowhite(p);
-    if (s != NULL && *p != NUL)
-      *p++ = NUL;
+    if (s != NULL && *p != '\0')
+      *p++ = '\0';
   } else {
     /* ":vimgrep /pattern/[g][j] fname" */
     if (s != NULL)
@@ -3092,7 +3092,7 @@ char_u *skip_vimgrep_pat(char_u *p, char_u **s, int *flags)
 
     /* Truncate the pattern. */
     if (s != NULL)
-      *p = NUL;
+      *p = '\0';
     ++p;
 
     /* Find the flags */
@@ -3290,7 +3290,7 @@ int get_errorlist(win_T *wp, list_T *list)
     list_append_dict(list, dict);
 
     buf[0] = qfp->qf_type;
-    buf[1] = NUL;
+    buf[1] = '\0';
     if ( dict_add_nr_str(dict, "bufnr", (long)bufnum, NULL) == FAIL
          || dict_add_nr_str(dict, "lnum",  (long)qfp->qf_lnum, NULL) == FAIL
          || dict_add_nr_str(dict, "col",   (long)qfp->qf_col, NULL) == FAIL
@@ -3388,7 +3388,7 @@ int set_errorlist(win_T *wp, list_T *list, int action, char_u *title)
         vcol,                               /* vis_col */
         pattern,                            /* search pattern */
         nr,
-        type == NULL ? NUL : *type,
+        type == NULL ? '\0' : *type,
         valid);
 
     vim_free(filename);
@@ -3433,9 +3433,9 @@ void ex_cbuffer(exarg_T *eap)
     qi = ll_get_or_alloc_list(curwin);
   }
 
-  if (*eap->arg == NUL)
+  if (*eap->arg == '\0')
     buf = curbuf;
-  else if (*skipwhite(skipdigits(eap->arg)) == NUL)
+  else if (*skipwhite(skipdigits(eap->arg)) == '\0')
     buf = buflist_findnr(atoi((char *)eap->arg));
   if (buf == NULL)
     EMSG(_(e_invarg));
@@ -3576,7 +3576,7 @@ void ex_helpgrep(exarg_T *eap)
 
     /* Go through all directories in 'runtimepath' */
     p = p_rtp;
-    while (*p != NUL && !got_int) {
+    while (*p != '\0' && !got_int) {
       copy_option_part(&p, NameBuff, MAXPATHL, ",");
 
       /* Find all "*.txt" and "*.??x" files in the "doc" directory. */
@@ -3613,7 +3613,7 @@ void ex_helpgrep(exarg_T *eap)
 
                 /* remove trailing CR, LF, spaces, etc. */
                 while (l > 0 && line[l - 1] <= ' ')
-                  line[--l] = NUL;
+                  line[--l] = '\0';
 
                 if (qf_add_entry(qi, &prevp,
                         NULL,                           /* dir */
diff --git a/src/regexp.c b/src/regexp.c
index b9a111caf1..49afd52e66 100644
--- a/src/regexp.c
+++ b/src/regexp.c
@@ -1092,12 +1092,12 @@ static char_u *skip_anyof(char_u *p)
     ++p;
   if (*p == ']' || *p == '-')
     ++p;
-  while (*p != NUL && *p != ']') {
+  while (*p != '\0' && *p != ']') {
     if (has_mbyte && (l = (*mb_ptr2len)(p)) > 1)
       p += l;
     else if (*p == '-')  {
       ++p;
-      if (*p != ']' && *p != NUL)
+      if (*p != ']' && *p != '\0')
         mb_ptr_adv(p);
     } else if (*p == '\\'
                && !reg_cpo_bsl
@@ -1136,15 +1136,15 @@ char_u *skip_regexp(char_u *startp, int dirc, int magic, char_u **newp)
     mymagic = MAGIC_OFF;
   get_cpo_flags();
 
-  for (; p[0] != NUL; mb_ptr_adv(p)) {
+  for (; p[0] != '\0'; mb_ptr_adv(p)) {
     if (p[0] == dirc)           /* found end of regexp */
       break;
     if ((p[0] == '[' && mymagic >= MAGIC_ON)
         || (p[0] == '\\' && p[1] == '[' && mymagic <= MAGIC_OFF)) {
       p = skip_anyof(p + 1);
-      if (p[0] == NUL)
+      if (p[0] == '\0')
         break;
-    } else if (p[0] == '\\' && p[1] != NUL)   {
+    } else if (p[0] == '\\' && p[1] != '\0')   {
       if (dirc == '?' && newp != NULL && p[1] == '?') {
         /* change "\?" to "?", make a copy first. */
         if (*newp == NULL) {
@@ -1236,7 +1236,7 @@ static regprog_T *bt_regcomp(char_u *expr, int re_flags)
   }
 
   /* Dig out information for optimizations. */
-  r->regstart = NUL;            /* Worst-case defaults. */
+  r->regstart = '\0';            /* Worst-case defaults. */
   r->reganch = 0;
   r->regmust = NULL;
   r->regmlen = 0;
@@ -1439,7 +1439,7 @@ reg (
       EMSG2_RET_NULL(_(e_unmatchedpp), reg_magic == MAGIC_ALL);
     else
       EMSG2_RET_NULL(_(e_unmatchedp), reg_magic == MAGIC_ALL);
-  } else if (paren == REG_NOPAREN && peekchr() != NUL) {
+  } else if (paren == REG_NOPAREN && peekchr() != '\0') {
     if (curchr == Magic(')'))
       EMSG2_RET_NULL(_(e_unmatchedpar), reg_magic == MAGIC_ALL);
     else
@@ -1511,7 +1511,7 @@ static char_u *regconcat(int *flagp)
 
   while (cont) {
     switch (peekchr()) {
-    case NUL:
+    case '\0':
     case Magic('|'):
     case Magic('&'):
     case Magic(')'):
@@ -1823,7 +1823,7 @@ static char_u *regatom(int *flagp)
       /* In a string "\n" matches a newline character. */
       ret = regnode(EXACTLY);
       regc(NL);
-      regc(NUL);
+      regc('\0');
       *flagp |= HASWIDTH | SIMPLE;
     } else {
       /* In buffer text "\n" matches the end of a line. */
@@ -1841,7 +1841,7 @@ static char_u *regatom(int *flagp)
     *flagp |= flags & (HASWIDTH | SPSTART | HASNL | HASLOOKBH);
     break;
 
-  case NUL:
+  case '\0':
   case Magic('|'):
   case Magic('&'):
   case Magic(')'):
@@ -1869,10 +1869,10 @@ static char_u *regatom(int *flagp)
 
       ret = regnode(EXACTLY);
       lp = reg_prev_sub;
-      while (*lp != NUL)
+      while (*lp != '\0')
         regc(*lp++);
-      regc(NUL);
-      if (*reg_prev_sub != NUL) {
+      regc('\0');
+      if (*reg_prev_sub != '\0') {
         *flagp |= HASWIDTH;
         if ((lp - reg_prev_sub) == 1)
           *flagp |= SIMPLE;
@@ -1904,11 +1904,11 @@ static char_u *regatom(int *flagp)
     if (!had_endbrace[refnum]) {
       /* Trick: check if "@<=" or "@<!" follows, in which case
        * the \1 can appear before the referenced match. */
-      for (p = regparse; *p != NUL; ++p)
+      for (p = regparse; *p != '\0'; ++p)
         if (p[0] == '@' && p[1] == '<'
             && (p[2] == '!' || p[2] == '='))
           break;
-      if (*p == NUL)
+      if (*p == '\0')
         EMSG_RET_NULL(_("E65: Illegal back reference"));
     }
     ret = regnode(BACKREF + refnum);
@@ -1999,7 +1999,7 @@ static char_u *regatom(int *flagp)
 
         ret = NULL;
         while ((c = getchr()) != ']') {
-          if (c == NUL)
+          if (c == '\0')
             EMSG2_RET_NULL(_(e_missing_sb),
                 reg_magic == MAGIC_ALL);
           br = regnode(BRANCH);
@@ -2066,7 +2066,7 @@ static char_u *regatom(int *flagp)
         regc(0x0a);
       else
         regmbc(i);
-      regc(NUL);
+      regc('\0');
       *flagp |= HASWIDTH;
       break;
     }
@@ -2150,12 +2150,12 @@ collection:
           regc(*regparse++);
         }
 
-        while (*regparse != NUL && *regparse != ']') {
+        while (*regparse != '\0' && *regparse != ']') {
           if (*regparse == '-') {
             ++regparse;
             /* The '-' is not used for a range at the end and
              * after or before a '\n'. */
-            if (*regparse == ']' || *regparse == NUL
+            if (*regparse == ']' || *regparse == '\0'
                 || startc == -1
                 || (regparse[0] == '\\' && regparse[1] == 'n')) {
               regc('-');
@@ -2345,7 +2345,7 @@ collection:
             }
           }
         }
-        regc(NUL);
+        regc('\0');
         prevchr_len = 1;                /* last char was the ']' */
         if (*regparse != ']')
           EMSG_RET_NULL(_(e_toomsbra));                 /* Cannot happen? */
@@ -2382,7 +2382,7 @@ do_multibyte:
      * But always emit at least one character.  Might be a Multi,
      * e.g., a "[" without matching "]".
      */
-    for (len = 0; c != NUL && (len == 0
+    for (len = 0; c != '\0' && (len == 0
                                || (re_multi_type(peekchr()) == NOT_MULTI
                                    && !one_exactly
                                    && !is_Magic(c))); ++len) {
@@ -2407,7 +2407,7 @@ do_multibyte:
     }
     ungetchr();
 
-    regc(NUL);
+    regc('\0');
     *flagp |= HASWIDTH;
     if (len == 1)
       *flagp |= SIMPLE;
@@ -2442,8 +2442,8 @@ static char_u *regnode(int op)
     regsize += 3;
   else {
     *regcode++ = op;
-    *regcode++ = NUL;                   /* Null "next" pointer. */
-    *regcode++ = NUL;
+    *regcode++ = '\0';                   /* Null "next" pointer. */
+    *regcode++ = '\0';
   }
   return ret;
 }
@@ -2495,8 +2495,8 @@ static void reginsert(int op, char_u *opnd)
 
   place = opnd;                 /* Op node, where operand used to be. */
   *place++ = op;
-  *place++ = NUL;
-  *place = NUL;
+  *place++ = '\0';
+  *place = '\0';
 }
 
 /*
@@ -2521,8 +2521,8 @@ static void reginsert_nr(int op, long val, char_u *opnd)
 
   place = opnd;                 /* Op node, where operand used to be. */
   *place++ = op;
-  *place++ = NUL;
-  *place++ = NUL;
+  *place++ = '\0';
+  *place++ = '\0';
   re_put_long(place, (long_u)val);
 }
 
@@ -2550,8 +2550,8 @@ static void reginsert_limits(int op, long minval, long maxval, char_u *opnd)
 
   place = opnd;                 /* Op node, where operand used to be. */
   *place++ = op;
-  *place++ = NUL;
-  *place++ = NUL;
+  *place++ = '\0';
+  *place++ = '\0';
   place = re_put_long(place, (long_u)minval);
   place = re_put_long(place, (long_u)maxval);
   regtail(opnd, place);
@@ -2753,7 +2753,7 @@ static int peekchr(void)
         while (p[0] == '\\' && (p[1] == 'c' || p[1] == 'C'
                                 || p[1] == 'm' || p[1] == 'M' || p[1] == 'Z'))
           p += 2;
-        if (p[0] == NUL
+        if (p[0] == '\0'
             || (p[0] == '\\'
                 && (p[1] == '|' || p[1] == '&' || p[1] == ')'
                     || p[1] == 'n'))
@@ -2765,7 +2765,7 @@ static int peekchr(void)
     {
       int c = regparse[1];
 
-      if (c == NUL)
+      if (c == '\0')
         curchr = '\\';                  /* trailing '\' */
       else if (
         c <= '~' && META_flags[c]
@@ -2825,7 +2825,7 @@ static void skipchr(void)
     prevchr_len = 1;
   else
     prevchr_len = 0;
-  if (regparse[prevchr_len] != NUL) {
+  if (regparse[prevchr_len] != '\0') {
     if (enc_utf8)
       /* exclude composing chars that mb_ptr2len does include */
       prevchr_len += utf_ptr2len(regparse + prevchr_len);
@@ -3479,7 +3479,7 @@ proftime_T  *tm;         /* timeout limit or NULL */
       c = (*mb_ptr2char)(regline + col);
     else
       c = regline[col];
-    if (prog->regstart == NUL
+    if (prog->regstart == '\0'
         || prog->regstart == c
         || (ireg_ic && ((
                           (enc_utf8 && utf_fold(prog->regstart) == utf_fold(c)))
@@ -3492,7 +3492,7 @@ proftime_T  *tm;         /* timeout limit or NULL */
     int tm_count = 0;
     /* Messy cases:  unanchored match. */
     while (!got_int) {
-      if (prog->regstart != NUL) {
+      if (prog->regstart != '\0') {
         /* Skip until the char we know it must start with.
          * Used often, do some work to avoid call overhead. */
         if (!ireg_ic
@@ -3523,7 +3523,7 @@ proftime_T  *tm;         /* timeout limit or NULL */
         reglnum = 0;
         regline = reg_getline((linenr_T)0);
       }
-      if (regline[col] == NUL)
+      if (regline[col] == '\0')
         break;
       if (has_mbyte)
         col += (*mb_ptr2len)(regline + col);
@@ -3827,7 +3827,7 @@ regmatch (
       op = OP(scan);
       /* Check for character class with NL added. */
       if (!reg_line_lbr && WITH_NL(op) && REG_MULTI
-          && *reginput == NUL && reglnum <= reg_maxline) {
+          && *reginput == '\0' && reglnum <= reg_maxline) {
         reg_nextline();
       } else if (reg_line_lbr && WITH_NL(op) && *reginput == '\n') {
         ADVANCE_REGINPUT();
@@ -3845,7 +3845,7 @@ regmatch (
           break;
 
         case EOL:
-          if (c != NUL)
+          if (c != '\0')
             status = RA_NOMATCH;
           break;
 
@@ -3859,7 +3859,7 @@ regmatch (
           break;
 
         case RE_EOF:
-          if (reglnum != reg_maxline || c != NUL)
+          if (reglnum != reg_maxline || c != '\0')
             status = RA_NOMATCH;
           break;
 
@@ -3919,7 +3919,7 @@ regmatch (
           break;
 
         case BOW:       /* \<word; reginput points to w */
-          if (c == NUL)         /* Can't match at end of line */
+          if (c == '\0')         /* Can't match at end of line */
             status = RA_NOMATCH;
           else if (has_mbyte) {
             int this_class;
@@ -3952,14 +3952,14 @@ regmatch (
               status = RA_NOMATCH;
           } else {
             if (!vim_iswordc_buf(reginput[-1], reg_buf)
-                || (reginput[0] != NUL && vim_iswordc_buf(c, reg_buf)))
+                || (reginput[0] != '\0' && vim_iswordc_buf(c, reg_buf)))
               status = RA_NOMATCH;
           }
           break;   /* Matched with EOW */
 
         case ANY:
           /* ANY does not match new lines. */
-          if (c == NUL)
+          if (c == '\0')
             status = RA_NOMATCH;
           else
             ADVANCE_REGINPUT();
@@ -4029,7 +4029,7 @@ regmatch (
           break;
 
         case NWHITE:
-          if (c == NUL || vim_iswhite(c))
+          if (c == '\0' || vim_iswhite(c))
             status = RA_NOMATCH;
           else
             ADVANCE_REGINPUT();
@@ -4043,7 +4043,7 @@ regmatch (
           break;
 
         case NDIGIT:
-          if (c == NUL || ri_digit(c))
+          if (c == '\0' || ri_digit(c))
             status = RA_NOMATCH;
           else
             ADVANCE_REGINPUT();
@@ -4057,7 +4057,7 @@ regmatch (
           break;
 
         case NHEX:
-          if (c == NUL || ri_hex(c))
+          if (c == '\0' || ri_hex(c))
             status = RA_NOMATCH;
           else
             ADVANCE_REGINPUT();
@@ -4071,7 +4071,7 @@ regmatch (
           break;
 
         case NOCTAL:
-          if (c == NUL || ri_octal(c))
+          if (c == '\0' || ri_octal(c))
             status = RA_NOMATCH;
           else
             ADVANCE_REGINPUT();
@@ -4085,7 +4085,7 @@ regmatch (
           break;
 
         case NWORD:
-          if (c == NUL || ri_word(c))
+          if (c == '\0' || ri_word(c))
             status = RA_NOMATCH;
           else
             ADVANCE_REGINPUT();
@@ -4099,7 +4099,7 @@ regmatch (
           break;
 
         case NHEAD:
-          if (c == NUL || ri_head(c))
+          if (c == '\0' || ri_head(c))
             status = RA_NOMATCH;
           else
             ADVANCE_REGINPUT();
@@ -4113,7 +4113,7 @@ regmatch (
           break;
 
         case NALPHA:
-          if (c == NUL || ri_alpha(c))
+          if (c == '\0' || ri_alpha(c))
             status = RA_NOMATCH;
           else
             ADVANCE_REGINPUT();
@@ -4127,7 +4127,7 @@ regmatch (
           break;
 
         case NLOWER:
-          if (c == NUL || ri_lower(c))
+          if (c == '\0' || ri_lower(c))
             status = RA_NOMATCH;
           else
             ADVANCE_REGINPUT();
@@ -4141,7 +4141,7 @@ regmatch (
           break;
 
         case NUPPER:
-          if (c == NUL || ri_upper(c))
+          if (c == '\0' || ri_upper(c))
             status = RA_NOMATCH;
           else
             ADVANCE_REGINPUT();
@@ -4159,10 +4159,10 @@ regmatch (
                     !enc_utf8 &&
                     vim_tolower(*opnd) != vim_tolower(*reginput))))
             status = RA_NOMATCH;
-          else if (*opnd == NUL) {
+          else if (*opnd == '\0') {
             /* match empty string always works; happens when "~" is
              * empty. */
-          } else if (opnd[1] == NUL
+          } else if (opnd[1] == '\0'
                      && !(enc_utf8 && ireg_ic)
                      )
             ++reginput;                 /* matched a single char */
@@ -4187,7 +4187,7 @@ regmatch (
 
         case ANYOF:
         case ANYBUT:
-          if (c == NUL)
+          if (c == '\0')
             status = RA_NOMATCH;
           else if ((cstrchr(OPERAND(scan), c) == NULL) == (op == ANYOF))
             status = RA_NOMATCH;
@@ -4214,7 +4214,7 @@ regmatch (
               /* When only a composing char is given match at any
                * position where that composing char appears. */
               status = RA_NOMATCH;
-              for (i = 0; reginput[i] != NUL; i += utf_char2len(inpc)) {
+              for (i = 0; reginput[i] != '\0'; i += utf_char2len(inpc)) {
                 inpc = mb_ptr2char(reginput + i);
                 if (!utf_iscomposing(inpc)) {
                   if (i > 0)
@@ -4573,8 +4573,8 @@ regmatch (
             } else
               rst.nextb_ic = rst.nextb;
           } else {
-            rst.nextb = NUL;
-            rst.nextb_ic = NUL;
+            rst.nextb = '\0';
+            rst.nextb_ic = '\0';
           }
           if (op != BRACE_SIMPLE) {
             rst.minval = (op == STAR) ? 0 : 1;
@@ -4670,7 +4670,7 @@ regmatch (
           break;
 
         case NEWL:
-          if ((c != NUL || !REG_MULTI || reglnum > reg_maxline
+          if ((c != '\0' || !REG_MULTI || reglnum > reg_maxline
                || reg_line_lbr) && (c != '\n' || !reg_line_lbr))
             status = RA_NOMATCH;
           else if (reg_line_lbr)
@@ -4998,7 +4998,7 @@ regmatch (
             status = RA_NOMATCH;
 
           /* If it could match, try it. */
-          if (rst->nextb == NUL || *reginput == rst->nextb
+          if (rst->nextb == '\0' || *reginput == rst->nextb
               || *reginput == rst->nextb_ic) {
             reg_save(&rp->rs_un.regsave, &backpos);
             scan = regnext(rp->rs_scan);
@@ -5110,7 +5110,7 @@ regrepeat (
     while (count < maxcount) {
       /* Matching anything means we continue until end-of-line (or
        * end-of-file for ANY + ADD_NL), only limited by maxcount. */
-      while (*scan != NUL && count < maxcount) {
+      while (*scan != '\0' && count < maxcount) {
         ++count;
         mb_ptr_adv(scan);
       }
@@ -5134,7 +5134,7 @@ regrepeat (
     while (count < maxcount) {
       if (vim_isIDc(PTR2CHAR(scan)) && (testval || !VIM_ISDIGIT(*scan))) {
         mb_ptr_adv(scan);
-      } else if (*scan == NUL) {
+      } else if (*scan == '\0') {
         if (!REG_MULTI || !WITH_NL(OP(p)) || reglnum > reg_maxline
             || reg_line_lbr)
           break;
@@ -5160,7 +5160,7 @@ regrepeat (
       if (vim_iswordp_buf(scan, reg_buf)
           && (testval || !VIM_ISDIGIT(*scan))) {
         mb_ptr_adv(scan);
-      } else if (*scan == NUL) {
+      } else if (*scan == '\0') {
         if (!REG_MULTI || !WITH_NL(OP(p)) || reglnum > reg_maxline
             || reg_line_lbr)
           break;
@@ -5185,7 +5185,7 @@ regrepeat (
     while (count < maxcount) {
       if (vim_isfilec(PTR2CHAR(scan)) && (testval || !VIM_ISDIGIT(*scan))) {
         mb_ptr_adv(scan);
-      } else if (*scan == NUL) {
+      } else if (*scan == '\0') {
         if (!REG_MULTI || !WITH_NL(OP(p)) || reglnum > reg_maxline
             || reg_line_lbr)
           break;
@@ -5208,7 +5208,7 @@ regrepeat (
   case SPRINT:
   case SPRINT + ADD_NL:
     while (count < maxcount) {
-      if (*scan == NUL) {
+      if (*scan == '\0') {
         if (!REG_MULTI || !WITH_NL(OP(p)) || reglnum > reg_maxline
             || reg_line_lbr)
           break;
@@ -5233,7 +5233,7 @@ regrepeat (
 do_class:
     while (count < maxcount) {
       int l;
-      if (*scan == NUL) {
+      if (*scan == '\0') {
         if (!REG_MULTI || !WITH_NL(OP(p)) || reglnum > reg_maxline
             || reg_line_lbr)
           break;
@@ -5380,7 +5380,7 @@ do_class:
   case ANYBUT + ADD_NL:
     while (count < maxcount) {
       int len;
-      if (*scan == NUL) {
+      if (*scan == '\0') {
         if (!REG_MULTI || !WITH_NL(OP(p)) || reglnum > reg_maxline
             || reg_line_lbr)
           break;
@@ -5405,7 +5405,7 @@ do_class:
 
   case NEWL:
     while (count < maxcount
-           && ((*scan == NUL && reglnum <= reg_maxline && !reg_line_lbr
+           && ((*scan == '\0' && reglnum <= reg_maxline && !reg_line_lbr
                 && REG_MULTI) || (*scan == '\n' && reg_line_lbr))) {
       count++;
       if (reg_line_lbr)
@@ -5760,7 +5760,7 @@ static void regdump(char_u *pattern, bt_regprog_T *r)
         || op == EXACTLY) {
       /* Literal string, where present. */
       fprintf(f, "\nxxxxxxxxx\n");
-      while (*s != NUL)
+      while (*s != '\0')
         fprintf(f, "%c", *s++);
       fprintf(f, "\nxxxxxxxxx\n");
       s++;
@@ -5769,7 +5769,7 @@ static void regdump(char_u *pattern, bt_regprog_T *r)
   }
 
   /* Header fields of interest. */
-  if (r->regstart != NUL)
+  if (r->regstart != '\0')
     fprintf(f, "start `%s' 0x%x; ", r->regstart < 256
         ? (char *)transchar(r->regstart)
         : "multibyte", r->regstart);
@@ -6314,7 +6314,7 @@ static char_u *cstrchr(char_u *s, int c)
     return vim_strchr(s, c);
 
   if (has_mbyte) {
-    for (p = s; *p != NUL; p += (*mb_ptr2len)(p)) {
+    for (p = s; *p != '\0'; p += (*mb_ptr2len)(p)) {
       if (enc_utf8 && c > 0x80) {
         if (utf_fold(utf_ptr2char(p)) == cc)
           return p;
@@ -6323,7 +6323,7 @@ static char_u *cstrchr(char_u *s, int c)
     }
   } else
     /* Faster version for when there are no multi-byte characters. */
-    for (p = s; *p != NUL; ++p)
+    for (p = s; *p != '\0'; ++p)
       if (*p == c || *p == cc)
         return p;
 
@@ -6564,13 +6564,13 @@ static int vim_regsub_both(char_u *source, char_u *dest, int copy, int magic, in
       if (eval_result != NULL) {
         int had_backslash = FALSE;
 
-        for (s = eval_result; *s != NUL; mb_ptr_adv(s)) {
+        for (s = eval_result; *s != '\0'; mb_ptr_adv(s)) {
           /* Change NL to CR, so that it becomes a line break,
            * unless called from vim_regexec_nl().
            * Skip over a backslashed character. */
           if (*s == NL && !submatch_line_lbr)
             *s = CAR;
-          else if (*s == '\\' && s[1] != NUL) {
+          else if (*s == '\\' && s[1] != '\0') {
             ++s;
             /* Change NL to CR here too, so that this works:
              * :s/abc\\\ndef/\="aaa\\\nbbb"/  on text:
@@ -6605,10 +6605,10 @@ static int vim_regsub_both(char_u *source, char_u *dest, int copy, int magic, in
       can_f_submatch = FALSE;
     }
   } else
-    while ((c = *src++) != NUL) {
+    while ((c = *src++) != '\0') {
       if (c == '&' && magic)
         no = 0;
-      else if (c == '\\' && *src != NUL) {
+      else if (c == '\\' && *src != '\0') {
         if (*src == '&' && !magic) {
           ++src;
           no = 0;
@@ -6631,7 +6631,7 @@ static int vim_regsub_both(char_u *source, char_u *dest, int copy, int magic, in
         }
       }
       if (no < 0) {           /* Ordinary character. */
-        if (c == K_SPECIAL && src[0] != NUL && src[1] != NUL) {
+        if (c == K_SPECIAL && src[0] != '\0' && src[1] != '\0') {
           /* Copy a special key as-is. */
           if (copy) {
             *dst++ = c;
@@ -6644,7 +6644,7 @@ static int vim_regsub_both(char_u *source, char_u *dest, int copy, int magic, in
           continue;
         }
 
-        if (c == '\\' && *src != NUL) {
+        if (c == '\\' && *src != '\0') {
           /* Check for abbreviations -- webb */
           switch (*src) {
           case 'r':   c = CAR;        ++src;  break;
@@ -6734,7 +6734,7 @@ static int vim_regsub_both(char_u *source, char_u *dest, int copy, int magic, in
                   len = (int)STRLEN(s);
               } else
                 break;
-            } else if (*s == NUL) {   /* we hit NUL. */
+            } else if (*s == '\0') {   /* we hit NUL. */
               if (copy)
                 EMSG(_(e_re_damg));
               goto exit;
@@ -6795,7 +6795,7 @@ static int vim_regsub_both(char_u *source, char_u *dest, int copy, int magic, in
       }
     }
   if (copy)
-    *dst = NUL;
+    *dst = '\0';
 
 exit:
   return (int)((dst - dest) + 1);
@@ -6884,7 +6884,7 @@ char_u *reg_submatch(int no)
               submatch_mmatch->endpos[no].col);
         len += submatch_mmatch->endpos[no].col;
         if (round == 2)
-          retval[len] = NUL;
+          retval[len] = '\0';
         ++len;
       }
 
diff --git a/src/regexp_nfa.c b/src/regexp_nfa.c
index 52dc228bb2..6d87b0b183 100644
--- a/src/regexp_nfa.c
+++ b/src/regexp_nfa.c
@@ -527,7 +527,7 @@ static char_u *nfa_get_match_text(nfa_state_T *start)
       *s++ = p->c;
     p = p->out;
   }
-  *s = NUL;
+  *s = '\0';
 
   return ret;
 }
@@ -1060,7 +1060,7 @@ static int nfa_regatom(void)
 
   c = getchr();
   switch (c) {
-  case NUL:
+  case '\0':
     EMSG_RET_FAIL(_(e_nul_found));
 
   case Magic('^'):
@@ -1082,7 +1082,7 @@ static int nfa_regatom(void)
 
   case Magic('_'):
     c = no_Magic(getchr());
-    if (c == NUL)
+    if (c == '\0')
       EMSG_RET_FAIL(_(e_nul_found));
 
     if (c == '^') {             /* "\_^" is start-of-line */
@@ -1201,7 +1201,7 @@ static int nfa_regatom(void)
       EMSG(_(e_nopresub));
       return FAIL;
     }
-    for (lp = reg_prev_sub; *lp != NUL; mb_cptr_adv(lp)) {
+    for (lp = reg_prev_sub; *lp != '\0'; mb_cptr_adv(lp)) {
       EMIT(PTR2CHAR(lp));
       if (lp != reg_prev_sub)
         EMIT(NFA_CONCAT);
@@ -1326,7 +1326,7 @@ static int nfa_regatom(void)
 
       /* \%[abc] */
       for (n = 0; (c = peekchr()) != ']'; ++n) {
-        if (c == NUL)
+        if (c == '\0')
           EMSG2_RET_FAIL(_(e_missing_sb),
               reg_magic == MAGIC_ALL);
         /* recursive call! */
@@ -1915,7 +1915,7 @@ static int nfa_regconcat(void)
 
   while (cont) {
     switch (peekchr()) {
-    case NUL:
+    case '\0':
     case Magic('|'):
     case Magic('&'):
     case Magic(')'):
@@ -2060,7 +2060,7 @@ nfa_reg (
       EMSG2_RET_FAIL(_(e_unmatchedpp), reg_magic == MAGIC_ALL);
     else
       EMSG2_RET_FAIL(_(e_unmatchedp), reg_magic == MAGIC_ALL);
-  } else if (paren == REG_NOPAREN && peekchr() != NUL) {
+  } else if (paren == REG_NOPAREN && peekchr() != '\0') {
     if (peekchr() == Magic(')'))
       EMSG2_RET_FAIL(_(e_unmatchedpar), reg_magic == MAGIC_ALL);
     else
@@ -2415,7 +2415,7 @@ static void nfa_dump(nfa_regprog_T *prog)
 
     if (prog->reganch)
       fprintf(debugf, "reganch: %d\n", prog->reganch);
-    if (prog->regstart != NUL)
+    if (prog->regstart != '\0')
       fprintf(debugf, "regstart: %c (decimal: %d)\n",
           prog->regstart, prog->regstart);
     if (prog->match_text != NULL)
@@ -3473,7 +3473,7 @@ static char *pim_info(nfa_pim_T *pim)
   static char buf[30];
 
   if (pim == NULL || pim->result == NFA_PIM_UNUSED)
-    buf[0] = NUL;
+    buf[0] = '\0';
   else {
     sprintf(buf, " PIM col %d", REG_MULTI ? (int)pim->end.pos.col
         : (int)(pim->end.ptr - reginput));
@@ -3875,7 +3875,7 @@ addstate (
      * Except when at the end of the line, or when we are going to the
      * next line for a look-behind match. */
     if (reginput > regline
-        && *reginput != NUL
+        && *reginput != '\0'
         && (nfa_endp == NULL
             || !REG_MULTI
             || reglnum == nfa_endp->se_u.pos.lnum))
@@ -4773,7 +4773,7 @@ static long find_match_text(colnr_T startcol, int regstart, char_u *match_text)
   for (;; ) {
     match = TRUE;
     len2 = MB_CHAR2LEN(regstart);     /* skip regstart */
-    for (len1 = 0; match_text[len1] != NUL; len1 += MB_CHAR2LEN(c1)) {
+    for (len1 = 0; match_text[len1] != '\0'; len1 += MB_CHAR2LEN(c1)) {
       c1 = PTR2CHAR(match_text + len1);
       c2 = PTR2CHAR(regline + col + len2);
       if (c1 != c2 && (!ireg_ic || vim_tolower(c1) != vim_tolower(c2))) {
@@ -4918,7 +4918,7 @@ static int nfa_regmatch(nfa_regprog_T *prog, nfa_state_T *start, regsubs_T *subm
       curc = *reginput;
       clen = 1;
     }
-    if (curc == NUL) {
+    if (curc == '\0') {
       clen = 0;
       go_to_nextline = FALSE;
     }
@@ -5249,7 +5249,7 @@ static int nfa_regmatch(nfa_regprog_T *prog, nfa_state_T *start, regsubs_T *subm
         break;
 
       case NFA_EOL:
-        if (curc == NUL) {
+        if (curc == '\0') {
           add_here = TRUE;
           add_state = t->state->out;
         }
@@ -5258,7 +5258,7 @@ static int nfa_regmatch(nfa_regprog_T *prog, nfa_state_T *start, regsubs_T *subm
       case NFA_BOW:
         result = TRUE;
 
-        if (curc == NUL)
+        if (curc == '\0')
           result = FALSE;
         else if (has_mbyte) {
           int this_class;
@@ -5293,7 +5293,7 @@ static int nfa_regmatch(nfa_regprog_T *prog, nfa_state_T *start, regsubs_T *subm
               || prev_class == 0 || prev_class == 1)
             result = FALSE;
         } else if (!vim_iswordc_buf(reginput[-1], reg_buf)
-                   || (reginput[0] != NUL
+                   || (reginput[0] != '\0'
                        && vim_iswordc_buf(curc, reg_buf)))
           result = FALSE;
         if (result) {
@@ -5311,7 +5311,7 @@ static int nfa_regmatch(nfa_regprog_T *prog, nfa_state_T *start, regsubs_T *subm
         break;
 
       case NFA_EOF:
-        if (reglnum == reg_maxline && curc == NUL) {
+        if (reglnum == reg_maxline && curc == '\0') {
           add_here = TRUE;
           add_state = t->state->out;
         }
@@ -5385,7 +5385,7 @@ static int nfa_regmatch(nfa_regprog_T *prog, nfa_state_T *start, regsubs_T *subm
       }
 
       case NFA_NEWL:
-        if (curc == NUL && !reg_line_lbr && REG_MULTI
+        if (curc == '\0' && !reg_line_lbr && REG_MULTI
             && reglnum <= reg_maxline) {
           go_to_nextline = TRUE;
           /* Pass -1 for the offset, which means taking the position
@@ -5410,7 +5410,7 @@ static int nfa_regmatch(nfa_regprog_T *prog, nfa_state_T *start, regsubs_T *subm
 
         /* Never match EOL. If it's part of the collection it is added
          * as a separate state with an OR. */
-        if (curc == NUL)
+        if (curc == '\0')
           break;
 
         state = t->state->out;
@@ -5521,7 +5521,7 @@ static int nfa_regmatch(nfa_regprog_T *prog, nfa_state_T *start, regsubs_T *subm
         break;
 
       case NFA_NWHITE:          /*  \S	*/
-        result = curc != NUL && !vim_iswhite(curc);
+        result = curc != '\0' && !vim_iswhite(curc);
         ADD_STATE_IF_MATCH(t->state);
         break;
 
@@ -5531,7 +5531,7 @@ static int nfa_regmatch(nfa_regprog_T *prog, nfa_state_T *start, regsubs_T *subm
         break;
 
       case NFA_NDIGIT:          /*  \D	*/
-        result = curc != NUL && !ri_digit(curc);
+        result = curc != '\0' && !ri_digit(curc);
         ADD_STATE_IF_MATCH(t->state);
         break;
 
@@ -5541,7 +5541,7 @@ static int nfa_regmatch(nfa_regprog_T *prog, nfa_state_T *start, regsubs_T *subm
         break;
 
       case NFA_NHEX:            /*  \X	*/
-        result = curc != NUL && !ri_hex(curc);
+        result = curc != '\0' && !ri_hex(curc);
         ADD_STATE_IF_MATCH(t->state);
         break;
 
@@ -5551,7 +5551,7 @@ static int nfa_regmatch(nfa_regprog_T *prog, nfa_state_T *start, regsubs_T *subm
         break;
 
       case NFA_NOCTAL:          /*  \O	*/
-        result = curc != NUL && !ri_octal(curc);
+        result = curc != '\0' && !ri_octal(curc);
         ADD_STATE_IF_MATCH(t->state);
         break;
 
@@ -5561,7 +5561,7 @@ static int nfa_regmatch(nfa_regprog_T *prog, nfa_state_T *start, regsubs_T *subm
         break;
 
       case NFA_NWORD:           /*  \W	*/
-        result = curc != NUL && !ri_word(curc);
+        result = curc != '\0' && !ri_word(curc);
         ADD_STATE_IF_MATCH(t->state);
         break;
 
@@ -5571,7 +5571,7 @@ static int nfa_regmatch(nfa_regprog_T *prog, nfa_state_T *start, regsubs_T *subm
         break;
 
       case NFA_NHEAD:           /*  \H	*/
-        result = curc != NUL && !ri_head(curc);
+        result = curc != '\0' && !ri_head(curc);
         ADD_STATE_IF_MATCH(t->state);
         break;
 
@@ -5581,7 +5581,7 @@ static int nfa_regmatch(nfa_regprog_T *prog, nfa_state_T *start, regsubs_T *subm
         break;
 
       case NFA_NALPHA:          /*  \A	*/
-        result = curc != NUL && !ri_alpha(curc);
+        result = curc != '\0' && !ri_alpha(curc);
         ADD_STATE_IF_MATCH(t->state);
         break;
 
@@ -5591,7 +5591,7 @@ static int nfa_regmatch(nfa_regprog_T *prog, nfa_state_T *start, regsubs_T *subm
         break;
 
       case NFA_NLOWER:          /*  \L	*/
-        result = curc != NUL && !ri_lower(curc);
+        result = curc != '\0' && !ri_lower(curc);
         ADD_STATE_IF_MATCH(t->state);
         break;
 
@@ -5601,7 +5601,7 @@ static int nfa_regmatch(nfa_regprog_T *prog, nfa_state_T *start, regsubs_T *subm
         break;
 
       case NFA_NUPPER:          /* \U	*/
-        result = curc != NUL && !ri_upper(curc);
+        result = curc != '\0' && !ri_upper(curc);
         ADD_STATE_IF_MATCH(t->state);
         break;
 
@@ -5611,7 +5611,7 @@ static int nfa_regmatch(nfa_regprog_T *prog, nfa_state_T *start, regsubs_T *subm
         break;
 
       case NFA_NLOWER_IC:       /* [^a-z] */
-        result = curc != NUL
+        result = curc != '\0'
                  && !(ri_lower(curc) || (ireg_ic && ri_upper(curc)));
         ADD_STATE_IF_MATCH(t->state);
         break;
@@ -5622,7 +5622,7 @@ static int nfa_regmatch(nfa_regprog_T *prog, nfa_state_T *start, regsubs_T *subm
         break;
 
       case NFA_NUPPER_IC:       /* ^[A-Z] */
-        result = curc != NUL
+        result = curc != '\0'
                  && !(ri_upper(curc) || (ireg_ic && ri_lower(curc)));
         ADD_STATE_IF_MATCH(t->state);
         break;
@@ -5939,7 +5939,7 @@ static int nfa_regmatch(nfa_regprog_T *prog, nfa_state_T *start, regsubs_T *subm
         int add = TRUE;
         int c;
 
-        if (prog->regstart != NUL && clen != 0) {
+        if (prog->regstart != '\0' && clen != 0) {
           if (nextlist->n == 0) {
             colnr_T col = (colnr_T)(reginput - regline) + clen;
 
@@ -6190,7 +6190,7 @@ nfa_regexec_both (
   } else
     nfa_has_zsubexpr = FALSE;
 
-  if (prog->regstart != NUL) {
+  if (prog->regstart != '\0') {
     /* Skip ahead until a character we know the match must start with.
      * When there is none there is no match. */
     if (skip_to_start(prog->regstart, &col) == FAIL)
diff --git a/src/screen.c b/src/screen.c
index 9910f93324..5889a5b6c9 100644
--- a/src/screen.c
+++ b/src/screen.c
@@ -620,7 +620,7 @@ int conceal_cursor_line(win_T *wp)
 {
   int c;
 
-  if (*wp->w_p_cocu == NUL)
+  if (*wp->w_p_cocu == '\0')
     return FALSE;
   if (get_real_state() & VISUAL)
     c = 'v';
@@ -1974,7 +1974,7 @@ static void fold_line(win_T *wp, long fold_count, foldinfo_T *foldinfo, linenr_T
       idx = off + col;
 
     /* Store multibyte characters in ScreenLines[] et al. correctly. */
-    for (p = text; *p != NUL; ) {
+    for (p = text; *p != '\0'; ) {
       cells = (*mb_ptr2cells)(p);
       c_len = (*mb_ptr2len)(p);
       if (col + cells > W_WIDTH(wp)
@@ -2235,7 +2235,7 @@ win_line (
   char_u extra[18];                     /* line number and 'fdc' must fit in here */
   int n_extra = 0;                      /* number of extra chars */
   char_u      *p_extra = NULL;          /* string of extra chars, plus NUL */
-  int c_extra = NUL;                    /* extra chars, all the same */
+  int c_extra = '\0';                    /* extra chars, all the same */
   int extra_attr = 0;                   /* attributes when n_extra != 0 */
   static char_u *at_end_str = (char_u *)"";   /* used for p_extra when
                                                  displaying lcs_eol at end-of-line */
@@ -2383,7 +2383,7 @@ win_line (
     draw_color_col = advance_color_col(VCOL_HLC, &color_cols);
 
   if (wp->w_p_spell
-      && *wp->w_s->b_p_spl != NUL
+      && *wp->w_s->b_p_spl != '\0'
       && wp->w_s->b_langp.ga_len > 0
       && *(char **)(wp->w_s->b_langp.ga_data) != NULL) {
     /* Prepare for spell checking. */
@@ -2393,7 +2393,7 @@ win_line (
     /* Get the start of the next line, so that words that wrap to the next
      * line are found too: "et<line-break>al.".
      * Trick: skip a few chars for C/shell/Vim comments */
-    nextline[SPWORDLEN] = NUL;
+    nextline[SPWORDLEN] = '\0';
     if (lnum < wp->w_buffer->b_ml.ml_line_count) {
       line = ml_get_buf(wp->w_buffer, lnum + 1, FALSE);
       spell_cat_line(nextline + SPWORDLEN, line, SPWORDLEN);
@@ -2443,7 +2443,7 @@ win_line (
           fromcol = 0;
         else {
           getvvcol(wp, top, (colnr_T *)&fromcol, NULL, NULL);
-          if (gchar_pos(top) == NUL)
+          if (gchar_pos(top) == '\0')
             tocol = fromcol + 1;
         }
       }
@@ -2545,7 +2545,7 @@ win_line (
     /* To be able to spell-check over line boundaries copy the end of the
      * current line into nextline[].  Above the start of the next line was
      * copied to nextline[SPWORDLEN]. */
-    if (nextline[SPWORDLEN] == NUL) {
+    if (nextline[SPWORDLEN] == '\0') {
       /* No next line or it is empty. */
       nextlinecol = MAXCOL;
       nextline_idx = 0;
@@ -2586,7 +2586,7 @@ win_line (
     v = wp->w_leftcol;
   if (v > 0) {
     char_u  *prev_ptr = ptr;
-    while (vcol < v && *ptr != NUL) {
+    while (vcol < v && *ptr != '\0') {
       c = win_lbr_chartabsize(wp, ptr, (colnr_T)vcol, NULL);
       vcol += c;
       prev_ptr = ptr;
@@ -2723,7 +2723,7 @@ win_line (
           shl->endcol = MAXCOL;
         /* Highlight one character for an empty match. */
         if (shl->startcol == shl->endcol) {
-          if (has_mbyte && line[shl->endcol] != NUL)
+          if (has_mbyte && line[shl->endcol] != '\0')
             shl->endcol += (*mb_ptr2len)(line + shl->endcol);
           else
             ++shl->endcol;
@@ -2781,8 +2781,8 @@ win_line (
           fill_foldcolumn(extra, wp, FALSE, lnum);
           n_extra = wp->w_p_fdc;
           p_extra = extra;
-          p_extra[n_extra] = NUL;
-          c_extra = NUL;
+          p_extra[n_extra] = '\0';
+          c_extra = '\0';
           char_attr = hl_attr(HLF_FC);
         }
       }
@@ -2804,7 +2804,7 @@ win_line (
                   if (text_sign != 0) {
                       p_extra = sign_get_text(text_sign);
                       if (p_extra != NULL) {
-                          c_extra = NUL;
+                          c_extra = '\0';
                           n_extra = (int)STRLEN(p_extra);
                       }
                       char_attr = sign_get_attr(text_sign, FALSE);
@@ -2849,7 +2849,7 @@ win_line (
             if (wp->w_p_rl)                         /* reverse line numbers */
               rl_mirror(extra);
             p_extra = extra;
-            c_extra = NUL;
+            c_extra = '\0';
           } else
             c_extra = ' ';
           n_extra = number_width(wp) + 1;
@@ -2878,10 +2878,10 @@ win_line (
             n_extra = W_WIDTH(wp) - col;
           char_attr = hl_attr(HLF_DED);
         }
-        if (*p_sbr != NUL && need_showbreak) {
+        if (*p_sbr != '\0' && need_showbreak) {
           /* Draw 'showbreak' at the start of each broken line. */
           p_extra = p_sbr;
-          c_extra = NUL;
+          c_extra = '\0';
           n_extra = (int)STRLEN(p_sbr);
           char_attr = hl_attr(HLF_AT);
           need_showbreak = FALSE;
@@ -3068,7 +3068,7 @@ win_line (
      * For the '$' of the 'list' option, n_extra == 1, p_extra == "".
      */
     if (n_extra > 0) {
-      if (c_extra != NUL) {
+      if (c_extra != '\0') {
         c = c_extra;
         mb_c = c;               /* doesn't handle non-utf-8 multi-byte! */
         if (enc_utf8 && (*mb_char2len)(c) > 1) {
@@ -3191,7 +3191,7 @@ win_line (
             mb_c = mb_ptr2char_adv(&p_extra);
             mb_utf8 = (c >= 0x80);
             n_extra = (int)STRLEN(p_extra);
-            c_extra = NUL;
+            c_extra = '\0';
             if (area_attr == 0 && search_attr == 0) {
               n_attr = n_extra + 1;
               extra_attr = hl_attr(HLF_8);
@@ -3232,7 +3232,7 @@ win_line (
             if (ptr[1] >= 32)
               mb_c = (c << 8) + ptr[1];
             else {
-              if (ptr[1] == NUL) {
+              if (ptr[1] == '\0') {
                 /* head byte at end of line */
                 mb_l = 1;
                 transchar_nonprint(extra, c);
@@ -3243,7 +3243,7 @@ win_line (
               }
               p_extra = extra;
               n_extra = (int)STRLEN(extra) - 1;
-              c_extra = NUL;
+              c_extra = '\0';
               c = *p_extra++;
               if (area_attr == 0 && search_attr == 0) {
                 n_attr = n_extra + 1;
@@ -3269,7 +3269,7 @@ win_line (
           /* Put pointer back so that the character will be
            * displayed at the start of the next line. */
           --ptr;
-        } else if (*ptr != NUL)
+        } else if (*ptr != '\0')
           ptr += mb_l - 1;
 
         /* If a double-width char doesn't fit at the left side display
@@ -3344,7 +3344,7 @@ win_line (
             char_attr = hl_combine_attr(syntax_attr, char_attr);
           /* no concealing past the end of the line, it interferes
            * with line highlighting */
-          if (c == NUL)
+          if (c == '\0')
             syntax_flags = 0;
           else
             syntax_flags = get_syntax_info(&syntax_seqnr);
@@ -3496,7 +3496,7 @@ win_line (
             c_extra = ' ';
             c = ' ';
           }
-        } else if (c == NUL
+        } else if (c == '\0'
                    && ((wp->w_p_list && lcs_eol > 0)
                        || ((fromcol >= 0 || fromcol_prev >= 0)
                            && tocol > vcol
@@ -3529,7 +3529,7 @@ win_line (
             else {
               p_extra = at_end_str;
               n_extra = 1;
-              c_extra = NUL;
+              c_extra = '\0';
             }
           }
           if (wp->w_p_list)
@@ -3549,12 +3549,12 @@ win_line (
             c = 0xc0;
           } else
             mb_utf8 = FALSE;                    /* don't draw as UTF-8 */
-        } else if (c != NUL) {
+        } else if (c != '\0') {
           p_extra = transchar(c);
           if ((dy_flags & DY_UHEX) && wp->w_p_rl)
             rl_mirror(p_extra);                 /* reverse "<12>" */
           n_extra = byte2cells(c) - 1;
-          c_extra = NUL;
+          c_extra = '\0';
           c = *p_extra++;
           if (!attr_pri) {
             n_attr = n_extra + 1;
@@ -3610,13 +3610,13 @@ win_line (
                   && vim_strchr(wp->w_p_cocu, 'v') == NULL)) {
         char_attr = conceal_attr;
         if (prev_syntax_id != syntax_seqnr
-            && (syn_get_sub_char() != NUL || wp->w_p_cole == 1)
+            && (syn_get_sub_char() != '\0' || wp->w_p_cole == 1)
             && wp->w_p_cole != 3) {
           /* First time at this concealed item: display one
            * character. */
-          if (syn_get_sub_char() != NUL)
+          if (syn_get_sub_char() != '\0')
             c = syn_get_sub_char();
-          else if (lcs_conceal != NUL)
+          else if (lcs_conceal != '\0')
             c = lcs_conceal;
           else
             c = ' ';
@@ -3676,13 +3676,13 @@ win_line (
      * character of the line and the user wants us to show us a
      * special character (via 'listchars' option "precedes:<char>".
      */
-    if (lcs_prec_todo != NUL
+    if (lcs_prec_todo != '\0'
         && (wp->w_p_wrap ? wp->w_skipcol > 0 : wp->w_leftcol > 0)
         && filler_todo <= 0
         && draw_state > WL_NR
-        && c != NUL) {
+        && c != '\0') {
       c = lcs_prec;
-      lcs_prec_todo = NUL;
+      lcs_prec_todo = '\0';
       if (has_mbyte && (*mb_char2cells)(mb_c) > 1) {
         /* Double-width character being overwritten by the "precedes"
          * character, need to fill up half the character. */
@@ -3708,12 +3708,12 @@ win_line (
     /*
      * At end of the text line or just after the last character.
      */
-    if (c == NUL
+    if (c == '\0'
 #if defined(LINE_ATTR)
         || did_line_attr == 1
 #endif
         ) {
-      long prevcol = (long)(ptr - line) - (c == NUL);
+      long prevcol = (long)(ptr - line) - (c == '\0');
 
       /* we're not really at that column when skipping some text */
       if ((long)(wp->w_p_wrap ? wp->w_skipcol : wp->w_leftcol) > prevcol)
@@ -3741,7 +3741,7 @@ win_line (
                && (VIsual_mode != Ctrl_V
                    || lnum == VIsual.lnum
                    || lnum == curwin->w_cursor.lnum)
-               && c == NUL)
+               && c == '\0')
               /* highlight 'hlsearch' match at end of line */
               || (prevcol_hl_flag == TRUE
 # if defined(LINE_ATTR)
@@ -3806,7 +3806,7 @@ win_line (
     /*
      * At end of the text line.
      */
-    if (c == NUL) {
+    if (c == '\0') {
       if (eol_hl_off > 0 && vcol - eol_hl_off == (long)wp->w_virtcol
           && lnum == wp->w_cursor.lnum) {
         /* highlight last char after line */
@@ -3899,9 +3899,9 @@ win_line (
         && (
           wp->w_p_rl ? col == 0 :
           col == W_WIDTH(wp) - 1)
-        && (*ptr != NUL
+        && (*ptr != '\0'
             || (wp->w_p_list && lcs_eol_one > 0)
-            || (n_extra && (c_extra != NUL || *p_extra != NUL)))) {
+            || (n_extra && (c_extra != '\0' || *p_extra != '\0')))) {
       c = lcs_ext;
       char_attr = hl_attr(HLF_AT);
       mb_c = c;
@@ -4088,10 +4088,10 @@ win_line (
     if ((
           wp->w_p_rl ? (col < 0) :
           (col >= W_WIDTH(wp)))
-        && (*ptr != NUL
+        && (*ptr != '\0'
             || filler_todo > 0
-            || (wp->w_p_list && lcs_eol != NUL && p_extra != at_end_str)
-            || (n_extra != 0 && (c_extra != NUL || *p_extra != NUL)))
+            || (wp->w_p_list && lcs_eol != '\0' && p_extra != at_end_str)
+            || (n_extra != 0 && (c_extra != '\0' || *p_extra != '\0')))
         ) {
       SCREEN_LINE(screen_row, W_WINCOL(wp), col - boguscols,
           (int)W_WIDTH(wp), wp->w_p_rl);
@@ -4200,7 +4200,7 @@ win_line (
   }     /* for every character in the line */
 
   /* After an empty line check first word for capital. */
-  if (*skipwhite(line) == NUL) {
+  if (*skipwhite(line) == '\0') {
     capcol_lnum = lnum + 1;
     cap_col = 0;
   }
@@ -4629,7 +4629,7 @@ static int status_match_len(expand_T *xp, char_u *s)
   if (emenu && menu_is_separator(s))
     return 1;
 
-  while (*s != NUL) {
+  while (*s != '\0') {
     s += skip_status_match_char(xp, s);
     len += ptr2cells(s);
     mb_ptr_adv(s);
@@ -4647,7 +4647,7 @@ static int skip_status_match_char(expand_T *xp, char_u *s)
   if ((rem_backslash(s) && xp->xp_context != EXPAND_HELP)
       || ((xp->xp_context == EXPAND_MENUS
            || xp->xp_context == EXPAND_MENUNAMES)
-          && (s[0] == '\t' || (s[0] == '\\' && s[1] != NUL)))
+          && (s[0] == '\t' || (s[0] == '\\' && s[1] != '\0')))
       ) {
 #ifndef BACKSLASH_IN_FILENAME
     if (xp->xp_shell && csh_like_shell() && s[1] == '\\' && s[2] == '!')
@@ -4743,7 +4743,7 @@ win_redr_status_matches (
   fillchar = fillchar_status(&attr, TRUE);
 
   if (first_match == 0) {
-    *buf = NUL;
+    *buf = '\0';
     len = 0;
   } else {
     STRCPY(buf, "< ");
@@ -4768,7 +4768,7 @@ win_redr_status_matches (
       len += l;
       clen += l;
     } else
-      for (; *s != NUL; ++s) {
+      for (; *s != '\0'; ++s) {
         s += skip_status_match_char(xp, s);
         clen += ptr2cells(s);
         if (has_mbyte && (l = (*mb_ptr2len)(s)) > 1) {
@@ -4795,7 +4795,7 @@ win_redr_status_matches (
     ++clen;
   }
 
-  buf[len] = NUL;
+  buf[len] = '\0';
 
   row = cmdline_row - 1;
   if (row >= 0) {
@@ -4828,7 +4828,7 @@ win_redr_status_matches (
 
     screen_puts(buf, row, 0, attr);
     if (selstart != NULL && highlight) {
-      *selend = NUL;
+      *selend = '\0';
       screen_puts(selstart, row, selstart_col, hl_attr(HLF_WM));
     }
 
@@ -4871,7 +4871,7 @@ void win_redr_status(win_T *wp)
              ) {
     /* Don't redraw right now, do it later. */
     wp->w_redr_status = TRUE;
-  } else if (*p_stl != NUL || *wp->w_p_stl != NUL) {
+  } else if (*p_stl != '\0' || *wp->w_p_stl != '\0') {
     /* redraw custom status line */
     redraw_custom_statusline(wp);
   } else {
@@ -4917,7 +4917,7 @@ void win_redr_status(win_T *wp)
 
       /* Find first character that will fit.
        * Going from start to end is much faster for DBCS. */
-      for (i = 0; p[i] != NUL && clen >= this_ru_col - 1;
+      for (i = 0; p[i] != '\0' && clen >= this_ru_col - 1;
            i += (*mb_ptr2len)(p + i))
         clen -= (*mb_ptr2cells)(p + i);
       len = clen;
@@ -4982,7 +4982,7 @@ static void redraw_custom_statusline(win_T *wp)
      * display is messed up with errors and a redraw triggers the problem
      * again and again. */
     set_string_option_direct((char_u *)"statusline", -1,
-        (char_u *)"", OPT_FREE | (*wp->w_p_stl != NUL
+        (char_u *)"", OPT_FREE | (*wp->w_p_stl != '\0'
                                   ? OPT_LOCAL : OPT_GLOBAL), SID_ERROR);
   }
   called_emsg |= save_called_emsg;
@@ -5041,7 +5041,7 @@ get_keymap_str (
     --emsg_skip;
     curbuf = old_curbuf;
     curwin = old_curwin;
-    if (p == NULL || *p == NUL) {
+    if (p == NULL || *p == '\0') {
       if (wp->w_buffer->b_kmap_state & KEYMAP_LOADED)
         p = wp->w_buffer->b_p_keymap;
       else
@@ -5050,10 +5050,10 @@ get_keymap_str (
     if ((int)(STRLEN(p) + 3) < len)
       sprintf((char *)buf, "<%s>", p);
     else
-      buf[0] = NUL;
+      buf[0] = '\0';
     vim_free(s);
   }
-  return buf[0] != NUL;
+  return buf[0] != '\0';
 }
 
 /*
@@ -5131,12 +5131,12 @@ win_redr_custom (
 
       use_sandbox = was_set_insecurely((char_u *)"rulerformat", 0);
     } else {
-      if (*wp->w_p_stl != NUL)
+      if (*wp->w_p_stl != '\0')
         stl = wp->w_p_stl;
       else
         stl = p_stl;
       use_sandbox = was_set_insecurely((char_u *)"statusline",
-          *wp->w_p_stl == NUL ? 0 : OPT_LOCAL);
+          *wp->w_p_stl == '\0' ? 0 : OPT_LOCAL);
     }
 
     col += W_WINCOL(wp);
@@ -5171,7 +5171,7 @@ win_redr_custom (
     len += (*mb_char2bytes)(fillchar, buf + len);
     ++width;
   }
-  buf[len] = NUL;
+  buf[len] = '\0';
 
   /*
    * Draw each snippet with the specified highlighting.
@@ -5225,10 +5225,10 @@ void screen_putchar(int c, int row, int col, int attr)
   char_u buf[MB_MAXBYTES + 1];
 
   if (has_mbyte)
-    buf[(*mb_char2bytes)(c, buf)] = NUL;
+    buf[(*mb_char2bytes)(c, buf)] = '\0';
   else {
     buf[0] = c;
-    buf[1] = NUL;
+    buf[1] = '\0';
   }
   screen_puts(buf, row, col, attr);
 }
@@ -5246,17 +5246,17 @@ void screen_getbytes(int row, int col, char_u *bytes, int *attrp)
     off = LineOffset[row] + col;
     *attrp = ScreenAttrs[off];
     bytes[0] = ScreenLines[off];
-    bytes[1] = NUL;
+    bytes[1] = '\0';
 
     if (enc_utf8 && ScreenLinesUC[off] != 0)
-      bytes[utfc_char2bytes(off, bytes)] = NUL;
+      bytes[utfc_char2bytes(off, bytes)] = '\0';
     else if (enc_dbcs == DBCS_JPNU && ScreenLines[off] == 0x8e) {
       bytes[0] = ScreenLines[off];
       bytes[1] = ScreenLines2[off];
-      bytes[2] = NUL;
+      bytes[2] = '\0';
     } else if (enc_dbcs && MB_BYTE2LEN(bytes[0]) > 1) {
       bytes[1] = ScreenLines[off + 1];
-      bytes[2] = NUL;
+      bytes[2] = '\0';
     }
   }
 }
@@ -5337,7 +5337,7 @@ void screen_puts_len(char_u *text, int len, int row, int col, int attr)
   max_off = LineOffset[row] + screen_Columns;
   while (col < screen_Columns
          && (len < 0 || (int)(ptr - text) < len)
-         && *ptr != NUL) {
+         && *ptr != '\0') {
     c = *ptr;
     /* check if this is the first byte of a multibyte */
     if (has_mbyte) {
@@ -5368,8 +5368,8 @@ void screen_puts_len(char_u *text, int len, int row, int col, int attr)
           /* Do Arabic shaping. */
           if (len >= 0 && (int)(ptr - text) + mbyte_blen >= len) {
             /* Past end of string to be displayed. */
-            nc = NUL;
-            nc1 = NUL;
+            nc = '\0';
+            nc1 = '\0';
           } else {
             nc = utfc_ptr2char_len(ptr + mbyte_blen, pcc,
                 (int)((text + len) - ptr - mbyte_blen));
@@ -5435,7 +5435,7 @@ void screen_puts_len(char_u *text, int len, int row, int col, int attr)
       if (clear_next_cell)
         clear_next_cell = FALSE;
       else if (has_mbyte
-               && (len < 0 ? ptr[mbyte_blen] == NUL
+               && (len < 0 ? ptr[mbyte_blen] == '\0'
                    : ptr + mbyte_blen >= text + len)
                && ((mbyte_cells == 1 && (*mb_off2cells)(off, max_off) > 1)
                    || (mbyte_cells == 2
@@ -5669,7 +5669,7 @@ next_search_hl (
 
       matchcol = shl->rm.startpos[0].col;
       ml = ml_get_buf(shl->buf, lnum, FALSE) + matchcol;
-      if (*ml == NUL) {
+      if (*ml == '\0') {
         ++matchcol;
         shl->lnum = 0;
         break;
@@ -5899,7 +5899,7 @@ static void screen_char(unsigned off, int row, int col)
 
     /* Convert UTF-8 character to bytes and write it. */
 
-    buf[utfc_char2bytes(off, buf)] = NUL;
+    buf[utfc_char2bytes(off, buf)] = '\0';
 
     out_str(buf);
     if (utf_char2cells(ScreenLinesUC[off]) > 1)
@@ -6547,8 +6547,8 @@ static void linecopy(int to, int from, win_T *wp)
  */
 int can_clear(char_u *p)
 {
-  return *p != NUL && (t_colors <= 1
-                       || cterm_normal_bg_color == 0 || *T_UT != NUL);
+  return *p != '\0' && (t_colors <= 1
+                       || cterm_normal_bg_color == 0 || *T_UT != '\0');
 }
 
 /*
@@ -6599,7 +6599,7 @@ void windgoto(int row, int col)
       col = screen_Columns - 1;
 
     /* check if no cursor movement is allowed in highlight mode */
-    if (screen_attr && *T_MS == NUL)
+    if (screen_attr && *T_MS == '\0')
       noinvcurs = HIGHL_COST;
     else
       noinvcurs = 0;
@@ -6739,7 +6739,7 @@ void windgoto(int row, int col)
            * removing a line of pixels from the last bold char, when
            * using the bold trick in the GUI.
            */
-          if (T_ND[0] != NUL && T_ND[1] == NUL) {
+          if (T_ND[0] != '\0' && T_ND[1] == '\0') {
             while (i-- > 0)
               out_char(*T_ND);
           } else {
@@ -6766,7 +6766,7 @@ void windgoto(int row, int col)
       if (noinvcurs)
         screen_stop_highlight();
       if (row == screen_cur_row && (col > screen_cur_col) &&
-          *T_CRI != NUL)
+          *T_CRI != '\0')
         term_cursor_right(col - screen_cur_col);
       else
         term_windgoto(row, col);
@@ -6958,7 +6958,7 @@ static int win_do_lines(win_T *wp, int row, int line_count, int mayclear, int de
   if (scroll_region
       || W_WIDTH(wp) != Columns
       ) {
-    if (scroll_region && (wp->w_width == Columns || *T_CSV != NUL))
+    if (scroll_region && (wp->w_width == Columns || *T_CSV != '\0'))
       scroll_region_set(wp, row);
     if (del)
       retval = screen_del_lines(W_WINROW(wp) + row, 0, line_count,
@@ -6966,7 +6966,7 @@ static int win_do_lines(win_T *wp, int row, int line_count, int mayclear, int de
     else
       retval = screen_ins_lines(W_WINROW(wp) + row, 0, line_count,
           wp->w_height - row, wp);
-    if (scroll_region && (wp->w_width == Columns || *T_CSV != NUL))
+    if (scroll_region && (wp->w_width == Columns || *T_CSV != '\0'))
       scroll_region_reset();
     return retval;
   }
@@ -7074,21 +7074,21 @@ screen_ins_lines (
    * exists.
    */
   result_empty = (row + line_count >= end);
-  if (wp != NULL && wp->w_width != Columns && *T_CSV == NUL)
+  if (wp != NULL && wp->w_width != Columns && *T_CSV == '\0')
     type = USE_REDRAW;
   else if (can_clear(T_CD) && result_empty)
     type = USE_T_CD;
-  else if (*T_CAL != NUL && (line_count > 1 || *T_AL == NUL))
+  else if (*T_CAL != '\0' && (line_count > 1 || *T_AL == '\0'))
     type = USE_T_CAL;
-  else if (*T_CDL != NUL && result_empty && (line_count > 1 || !can_ce))
+  else if (*T_CDL != '\0' && result_empty && (line_count > 1 || !can_ce))
     type = USE_T_CDL;
-  else if (*T_AL != NUL)
+  else if (*T_AL != '\0')
     type = USE_T_AL;
   else if (can_ce && result_empty)
     type = USE_T_CE;
-  else if (*T_DL != NUL && result_empty)
+  else if (*T_DL != '\0' && result_empty)
     type = USE_T_DL;
-  else if (*T_SR != NUL && row == 0 && (*T_DA == NUL || can_ce))
+  else if (*T_SR != '\0' && row == 0 && (*T_DA == '\0' || can_ce))
     type = USE_T_SR;
   else
     return FAIL;
@@ -7111,7 +7111,7 @@ screen_ins_lines (
 
 
 
-  if (*T_CCS != NUL)       /* cursor relative to region */
+  if (*T_CCS != '\0')       /* cursor relative to region */
     cursor_row = row;
   else
     cursor_row = row + off;
@@ -7233,7 +7233,7 @@ screen_del_lines (
    * We can delete lines only when 'db' flag not set or when 'ce' option
    * available.
    */
-  can_delete = (*T_DB == NUL || can_clear(T_CE));
+  can_delete = (*T_DB == '\0' || can_clear(T_CE));
 
   /*
    * There are six ways to delete lines:
@@ -7247,7 +7247,7 @@ screen_del_lines (
    * 5. Use T_DL (delete line) if it exists.
    * 6. redraw the characters from ScreenLines[].
    */
-  if (wp != NULL && wp->w_width != Columns && *T_CSV == NUL)
+  if (wp != NULL && wp->w_width != Columns && *T_CSV == '\0')
     type = USE_REDRAW;
   else if (can_clear(T_CD) && result_empty)
     type = USE_T_CD;
@@ -7255,24 +7255,24 @@ screen_del_lines (
              /* On the Amiga, somehow '\n' on the last line doesn't always scroll
               * up, so use delete-line command */
              line_count == 1 ||
-             *T_CDL == NUL))
+             *T_CDL == '\0'))
     type = USE_NL;
-  else if (*T_CDL != NUL && line_count > 1 && can_delete)
+  else if (*T_CDL != '\0' && line_count > 1 && can_delete)
     type = USE_T_CDL;
   else if (can_clear(T_CE) && result_empty
            && (wp == NULL || wp->w_width == Columns)
            )
     type = USE_T_CE;
-  else if (*T_DL != NUL && can_delete)
+  else if (*T_DL != '\0' && can_delete)
     type = USE_T_DL;
-  else if (*T_CDL != NUL && can_delete)
+  else if (*T_CDL != '\0' && can_delete)
     type = USE_T_CDL;
   else
     return FAIL;
 
 
 
-  if (*T_CCS != NUL) {      /* cursor relative to region */
+  if (*T_CCS != '\0') {      /* cursor relative to region */
     cursor_row = row;
     cursor_end = end;
   } else {
@@ -7590,7 +7590,7 @@ static void draw_tabline(void)
     TabPageIdxs[scol] = 0;
 
   /* Use the 'tabline' option if it's set. */
-  if (*p_tal != NUL) {
+  if (*p_tal != '\0') {
     int save_called_emsg = called_emsg;
 
     /* Check for an error.  If there is one we would loop in redrawing the
@@ -7784,7 +7784,7 @@ void showruler(int always)
     curwin->w_redr_status = TRUE;
     return;
   }
-  if ((*p_stl != NUL || *curwin->w_p_stl != NUL) && curwin->w_status_height) {
+  if ((*p_stl != '\0' || *curwin->w_p_stl != '\0') && curwin->w_status_height) {
     redraw_custom_statusline(curwin);
   } else
     win_redr_ruler(curwin, always);
@@ -7853,7 +7853,7 @@ static void win_redr_ruler(win_T *wp, int always)
    * Check if not in Insert mode and the line is empty (will show "0-1").
    */
   if (!(State & INSERT)
-      && *ml_get_buf(wp->w_buffer, wp->w_cursor.lnum, FALSE) == NUL)
+      && *ml_get_buf(wp->w_buffer, wp->w_cursor.lnum, FALSE) == '\0')
     empty_line = TRUE;
 
   /*
@@ -7886,7 +7886,7 @@ static void win_redr_ruler(win_T *wp, int always)
 
     /* In list mode virtcol needs to be recomputed */
     virtcol = wp->w_virtcol;
-    if (wp->w_p_list && lcs_tab1 == NUL) {
+    if (wp->w_p_list && lcs_tab1 == '\0') {
       wp->w_p_list = FALSE;
       getvvcol(wp, &wp->w_cursor, NULL, &virtcol, NULL);
       wp->w_p_list = TRUE;
@@ -7934,15 +7934,15 @@ static void win_redr_ruler(win_T *wp, int always)
     /* Truncate at window boundary. */
     if (has_mbyte) {
       o = 0;
-      for (i = 0; buffer[i] != NUL; i += (*mb_ptr2len)(buffer + i)) {
+      for (i = 0; buffer[i] != '\0'; i += (*mb_ptr2len)(buffer + i)) {
         o += (*mb_ptr2cells)(buffer + i);
         if (this_ru_col + o > WITH_WIDTH(width)) {
-          buffer[i] = NUL;
+          buffer[i] = '\0';
           break;
         }
       }
     } else if (this_ru_col + (int)STRLEN(buffer) > WITH_WIDTH(width))
-      buffer[WITH_WIDTH(width) - this_ru_col] = NUL;
+      buffer[WITH_WIDTH(width) - this_ru_col] = '\0';
 
     screen_puts(buffer, row, this_ru_col + WITH_OFF(off), attr);
     i = redraw_cmdline;
diff --git a/src/search.c b/src/search.c
index 2c33c3f6ab..c9ca8ea170 100644
--- a/src/search.c
+++ b/src/search.c
@@ -161,7 +161,7 @@ search_regcomp (
   /*
    * If no pattern given, use a previously defined pattern.
    */
-  if (pat == NULL || *pat == NUL) {
+  if (pat == NULL || *pat == '\0') {
     if (pat_use == RE_LAST)
       i = last_idx;
     else
@@ -178,7 +178,7 @@ search_regcomp (
     magic = spats[i].magic;
     no_smartcase = spats[i].no_scs;
   } else if (options & SEARCH_HIS)      /* put new pattern in history */
-    add_to_history(HIST_SEARCH, pat, TRUE, NUL);
+    add_to_history(HIST_SEARCH, pat, TRUE, '\0');
 
   if (mr_pattern_alloced) {
     vim_free(mr_pattern);
@@ -256,7 +256,7 @@ char_u *reverse_text(char_u *s)
         rev[--rev_i] = s[s_i];
 
     }
-    rev[len] = NUL;
+    rev[len] = '\0';
   }
   return rev;
 }
@@ -348,7 +348,7 @@ int pat_has_uppercase(char_u *pat)
 {
   char_u *p = pat;
 
-  while (*p != NUL) {
+  while (*p != '\0') {
     int l;
 
     if (has_mbyte && (l = (*mb_ptr2len)(p)) > 1) {
@@ -356,11 +356,11 @@ int pat_has_uppercase(char_u *pat)
         return TRUE;
       p += l;
     } else if (*p == '\\') {
-      if (p[1] == '_' && p[2] != NUL)        /* skip "\_X" */
+      if (p[1] == '_' && p[2] != '\0')        /* skip "\_X" */
         p += 3;
-      else if (p[1] == '%' && p[2] != NUL)        /* skip "\%X" */
+      else if (p[1] == '%' && p[2] != '\0')        /* skip "\%X" */
         p += 3;
-      else if (p[1] != NUL)        /* skip "\X" */
+      else if (p[1] != '\0')        /* skip "\X" */
         p += 2;
       else
         p += 1;
@@ -394,7 +394,7 @@ void set_last_search_pat(char_u *s, int idx, int magic, int setlast)
 {
   vim_free(spats[idx].pat);
   /* An empty string means that nothing should be matched. */
-  if (*s == NUL)
+  if (*s == '\0')
     spats[idx].pat = NULL;
   else
     spats[idx].pat = vim_strsave(s);
@@ -503,7 +503,7 @@ proftime_T  *tm;         /* timeout limit or NULL */
            && pos->lnum >= 1 && pos->lnum <= buf->b_ml.ml_line_count
            && pos->col < MAXCOL - 2) {
     ptr = ml_get_buf(buf, pos->lnum, FALSE) + pos->col;
-    if (*ptr == NUL)
+    if (*ptr == '\0')
       extra_col = 1;
     else
       extra_col = (*mb_ptr2len)(ptr);
@@ -590,7 +590,7 @@ proftime_T  *tm;         /* timeout limit or NULL */
                            && (int)endpos.col - 1
                            < (int)start_pos.col + extra_col)
                        : ((int)matchpos.col
-                          - (ptr[matchpos.col] == NUL)
+                          - (ptr[matchpos.col] == '\0')
                           < (int)start_pos.col + extra_col))) {
               /*
                * If vi-compatible searching, continue at the end
@@ -607,7 +607,7 @@ proftime_T  *tm;         /* timeout limit or NULL */
                 matchcol = endpos.col;
                 /* for empty match: advance one char */
                 if (matchcol == matchpos.col
-                    && ptr[matchcol] != NUL) {
+                    && ptr[matchcol] != '\0') {
                   if (has_mbyte)
                     matchcol +=
                       (*mb_ptr2len)(ptr + matchcol);
@@ -616,7 +616,7 @@ proftime_T  *tm;         /* timeout limit or NULL */
                 }
               } else {
                 matchcol = matchpos.col;
-                if (ptr[matchcol] != NUL) {
+                if (ptr[matchcol] != '\0') {
                   if (has_mbyte)
                     matchcol += (*mb_ptr2len)(ptr
                                               + matchcol);
@@ -626,7 +626,7 @@ proftime_T  *tm;         /* timeout limit or NULL */
               }
               if (matchcol == 0 && (options & SEARCH_START))
                 break;
-              if (ptr[matchcol] == NUL
+              if (ptr[matchcol] == '\0'
                   || (nmatched = vim_regexec_multi(&regmatch,
                           win, buf, lnum + matchpos.lnum,
                           matchcol,
@@ -696,7 +696,7 @@ proftime_T  *tm;         /* timeout limit or NULL */
                 matchcol = endpos.col;
                 /* for empty match: advance one char */
                 if (matchcol == matchpos.col
-                    && ptr[matchcol] != NUL) {
+                    && ptr[matchcol] != '\0') {
                   if (has_mbyte)
                     matchcol +=
                       (*mb_ptr2len)(ptr + matchcol);
@@ -708,7 +708,7 @@ proftime_T  *tm;         /* timeout limit or NULL */
                 if (matchpos.lnum > 0)
                   break;
                 matchcol = matchpos.col;
-                if (ptr[matchcol] != NUL) {
+                if (ptr[matchcol] != '\0') {
                   if (has_mbyte)
                     matchcol +=
                       (*mb_ptr2len)(ptr + matchcol);
@@ -716,7 +716,7 @@ proftime_T  *tm;         /* timeout limit or NULL */
                     ++matchcol;
                 }
               }
-              if (ptr[matchcol] == NUL
+              if (ptr[matchcol] == '\0'
                   || (nmatched = vim_regexec_multi(&regmatch,
                           win, buf, lnum + matchpos.lnum,
                           matchcol,
@@ -978,7 +978,7 @@ proftime_T      *tm;            /* timeout limit or NULL */
     searchstr = pat;
     dircp = NULL;
     /* use previous pattern */
-    if (pat == NULL || *pat == NUL || *pat == dirc) {
+    if (pat == NULL || *pat == '\0' || *pat == dirc) {
       if (spats[RE_SEARCH].pat == NULL) {           /* no previous pattern */
         pat = spats[RE_SUBST].pat;
         if (pat == NULL) {
@@ -993,7 +993,7 @@ proftime_T      *tm;            /* timeout limit or NULL */
       }
     }
 
-    if (pat != NULL && *pat != NUL) {   /* look for (new) offset */
+    if (pat != NULL && *pat != '\0') {   /* look for (new) offset */
       /*
        * Find end of regular expression.
        * If there is a matching '/' or '?', toss it.
@@ -1008,7 +1008,7 @@ proftime_T      *tm;            /* timeout limit or NULL */
       }
       if (*p == dirc) {
         dircp = p;              /* remember where we put the NUL */
-        *p++ = NUL;
+        *p++ = '\0';
       }
       spats[0].off.line = FALSE;
       spats[0].off.end = FALSE;
@@ -1051,7 +1051,7 @@ proftime_T      *tm;            /* timeout limit or NULL */
       char_u      *msgbuf;
       char_u      *trunc;
 
-      if (*searchstr == NUL)
+      if (*searchstr == '\0')
         p = spats[last_idx].pat;
       else
         p = searchstr;
@@ -1076,7 +1076,7 @@ proftime_T      *tm;            /* timeout limit or NULL */
           if (spats[0].off.off != 0 || spats[0].off.line)
             sprintf((char *)p, "%" PRId64, (int64_t)spats[0].off.off);
           else
-            *p = NUL;
+            *p = '\0';
         }
 
         msg_start();
@@ -1277,7 +1277,7 @@ int search_for_exact_line(buf_T *buf, pos_T *pos, int dir, char_u *pat)
         && !(compl_cont_status & CONT_SOL)) {
       if ((p_ic ? MB_STRICMP(p, pat) : STRCMP(p, pat)) == 0)
         return OK;
-    } else if (*p != NUL) {   /* ignore empty lines */
+    } else if (*p != '\0') {   /* ignore empty lines */
       /* expanding lines or words */
       if ((p_ic ? MB_STRNICMP(p, pat, compl_length)
            : STRNCMP(p, pat, compl_length)) == 0)
@@ -1302,7 +1302,7 @@ int searchc(cmdarg_T *cap, int t_cmd)
   int c = cap->nchar;                   /* char to search for */
   int dir = cap->arg;                   /* TRUE for searching forward */
   long count = cap->count1;                     /* repeat count */
-  static int lastc = NUL;               /* last character searched for */
+  static int lastc = '\0';               /* last character searched for */
   static int lastcdir;                  /* last direction of character search */
   static int last_t_cmd;                /* last search t_cmd */
   int col;
@@ -1312,7 +1312,7 @@ int searchc(cmdarg_T *cap, int t_cmd)
   static char_u bytes[MB_MAXBYTES + 1];
   static int bytelen = 1;               /* >1 for multi-byte char */
 
-  if (c != NUL) {       /* normal search: remember args for repeat */
+  if (c != '\0') {       /* normal search: remember args for repeat */
     if (!KeyStuffed) {      /* don't remember when redoing */
       lastc = c;
       lastcdir = dir;
@@ -1325,7 +1325,7 @@ int searchc(cmdarg_T *cap, int t_cmd)
       }
     }
   } else {            /* repeat previous search */
-    if (lastc == NUL)
+    if (lastc == '\0')
       return FAIL;
     if (dir)            /* repeat in opposite direction */
       dir = -lastcdir;
@@ -1500,10 +1500,10 @@ pos_T *findmatchlimit(oparg_T *oap, int initc, int flags, int maxtravel)
     if (initc == '/')
       ignore_cend = TRUE;
     backwards = (dir == FORWARD) ? FALSE : TRUE;
-    initc = NUL;
-  } else if (initc != '#' && initc != NUL) {
+    initc = '\0';
+  } else if (initc != '#' && initc != '\0') {
     find_mps_values(&initc, &findc, &backwards, TRUE);
-    if (findc == NUL)
+    if (findc == '\0')
       return NULL;
   }
   /*
@@ -1561,11 +1561,11 @@ pos_T *findmatchlimit(oparg_T *oap, int initc, int flags, int maxtravel)
          * If beyond the end of the line, use the last character in
          * the line.
          */
-        if (linep[pos.col] == NUL && pos.col)
+        if (linep[pos.col] == '\0' && pos.col)
           --pos.col;
         for (;; ) {
           initc = PTR2CHAR(linep + pos.col);
-          if (initc == NUL)
+          if (initc == '\0')
             break;
 
           find_mps_values(&initc, &findc, &backwards, FALSE);
@@ -1699,7 +1699,7 @@ pos_T *findmatchlimit(oparg_T *oap, int initc, int flags, int maxtravel)
           pos.col -= (*mb_head_off)(linep, linep + pos.col);
       }
     } else {                          /* forward search */
-      if (linep[pos.col] == NUL
+      if (linep[pos.col] == '\0'
           /* at end of line, go to next one */
           /* don't search for match in comment */
           || (lisp && comment_col != MAXCOL
@@ -1796,7 +1796,7 @@ pos_T *findmatchlimit(oparg_T *oap, int initc, int flags, int maxtravel)
         if (*ptr == '"'
             && (ptr == linep || ptr[-1] != '\'' || ptr[1] != '\''))
           ++do_quotes;
-        if (*ptr == '\\' && ptr[1] != NUL)
+        if (*ptr == '\\' && ptr[1] != '\0')
           ++ptr;
       }
       do_quotes &= 1;               /* result is 1 with even number of quotes */
@@ -1849,7 +1849,7 @@ pos_T *findmatchlimit(oparg_T *oap, int initc, int flags, int maxtravel)
      */
     c = PTR2CHAR(linep + pos.col);
     switch (c) {
-    case NUL:
+    case '\0':
       /* at end of line without trailing backslash, reset inquote */
       if (pos.col == 0 || linep[pos.col - 1] != '\\') {
         inquote = FALSE;
@@ -2020,7 +2020,7 @@ showmatch (
    * Only show match for chars in the 'matchpairs' option.
    */
   /* 'matchpairs' is "x:y,x:y" */
-  for (p = curbuf->b_p_mps; *p != NUL; ++p) {
+  for (p = curbuf->b_p_mps; *p != '\0'; ++p) {
     if (PTR2CHAR(p) == c && (curwin->w_p_rl ^ p_ri))
       break;
     p += MB_PTR2LEN(p) + 1;
@@ -2029,11 +2029,11 @@ showmatch (
         )
       break;
     p += MB_PTR2LEN(p);
-    if (*p == NUL)
+    if (*p == '\0')
       return;
   }
 
-  if ((lpos = findmatch(NULL, NUL)) == NULL)        /* no match, so beep */
+  if ((lpos = findmatch(NULL, '\0')) == NULL)        /* no match, so beep */
     vim_beep();
   else if (lpos->lnum >= curwin->w_topline && lpos->lnum < curwin->w_botline) {
     if (!curwin->w_p_wrap)
@@ -2110,11 +2110,11 @@ int findsent(int dir, long count)
     /*
      * if on an empty line, skip upto a non-empty line
      */
-    if (gchar_pos(&pos) == NUL) {
+    if (gchar_pos(&pos) == '\0') {
       do
         if ((*func)(&pos) == -1)
           break;
-      while (gchar_pos(&pos) == NUL);
+      while (gchar_pos(&pos) == '\0');
       if (dir == FORWARD)
         goto found;
     }
@@ -2123,7 +2123,7 @@ int findsent(int dir, long count)
      * go to the next line
      */
     else if (dir == FORWARD && pos.col == 0 &&
-             startPS(pos.lnum, NUL, FALSE)) {
+             startPS(pos.lnum, '\0', FALSE)) {
       if (pos.lnum == curbuf->b_ml.ml_line_count)
         return FAIL;
       ++pos.lnum;
@@ -2156,7 +2156,7 @@ int findsent(int dir, long count)
 
     for (;; ) {                 /* find end of sentence */
       c = gchar_pos(&pos);
-      if (c == NUL || (pos.col == 0 && startPS(pos.lnum, NUL, FALSE))) {
+      if (c == '\0' || (pos.col == 0 && startPS(pos.lnum, '\0', FALSE))) {
         if (dir == BACKWARD && pos.lnum != startlnum)
           ++pos.lnum;
         break;
@@ -2168,11 +2168,11 @@ int findsent(int dir, long count)
             break;
         while (vim_strchr((char_u *)")]\"'", c = gchar_pos(&tpos))
                != NULL);
-        if (c == -1  || (!cpo_J && (c == ' ' || c == '\t')) || c == NUL
+        if (c == -1  || (!cpo_J && (c == ' ' || c == '\t')) || c == '\0'
             || (cpo_J && (c == ' ' && inc(&tpos) >= 0
                           && gchar_pos(&tpos) == ' '))) {
           pos = tpos;
-          if (gchar_pos(&pos) == NUL)           /* skip NUL at EOL */
+          if (gchar_pos(&pos) == '\0')           /* skip NUL at EOL */
             inc(&pos);
           break;
         }
@@ -2227,7 +2227,7 @@ findpar (
   while (count--) {
     did_skip = FALSE;
     for (first = TRUE;; first = FALSE) {
-      if (*ml_get(curr) != NUL)
+      if (*ml_get(curr) != '\0')
         did_skip = TRUE;
 
       /* skip folded lines */
@@ -2241,7 +2241,7 @@ findpar (
        * doesn't match historical Vi: It also stops at a "{" in the
        * first column and at an empty line. */
       if (!first && did_skip && (startPS(curr, what, both)
-                                 || (posix && what == NUL && *ml_get(curr) ==
+                                 || (posix && what == '\0' && *ml_get(curr) ==
                                      '{')))
         break;
 
@@ -2282,16 +2282,16 @@ static int inmacro(char_u *opt, char_u *s)
      * line or the line having ended. */
     if (       (macro[0] == s[0]
                 || (macro[0] == ' '
-                    && (s[0] == NUL || s[0] == ' ')))
+                    && (s[0] == '\0' || s[0] == ' ')))
                && (macro[1] == s[1]
-                   || ((macro[1] == NUL || macro[1] == ' ')
-                       && (s[0] == NUL || s[1] == NUL || s[1] == ' '))))
+                   || ((macro[1] == '\0' || macro[1] == ' ')
+                       && (s[0] == '\0' || s[1] == '\0' || s[1] == ' '))))
       break;
     ++macro;
-    if (macro[0] == NUL)
+    if (macro[0] == '\0')
       break;
   }
-  return macro[0] != NUL;
+  return macro[0] != '\0';
 }
 
 /*
@@ -2344,7 +2344,7 @@ static int cls(void)
   c = gchar_cursor();
   if (p_altkeymap && c == F_BLANK)
     return 0;
-  if (c == ' ' || c == '\t' || c == NUL)
+  if (c == ' ' || c == '\t' || c == '\0')
     return 0;
   if (enc_dbcs != 0 && c > 0xFF) {
     /* If cls_bigword, report multi-byte chars as class 1. */
@@ -2424,7 +2424,7 @@ fwd_word (
       /*
        * We'll stop if we land on a blank line
        */
-      if (curwin->w_cursor.col == 0 && *ml_get_curline() == NUL)
+      if (curwin->w_cursor.col == 0 && *ml_get_curline() == '\0')
         break;
 
       i = inc_cursor();
@@ -3075,7 +3075,7 @@ current_block (
   if (VIsual_active) {
     if (*p_sel == 'e')
       ++curwin->w_cursor.col;
-    if (sol && gchar_cursor() != NUL)
+    if (sol && gchar_cursor() != '\0')
       inc(&curwin->w_cursor);           /* include the line break */
     VIsual = start_pos;
     VIsual_mode = 'v';
@@ -3110,7 +3110,7 @@ static int in_html_tag(int end_tag)
   char_u      *line = ml_get_curline();
   char_u      *p;
   int c;
-  int lc = NUL;
+  int lc = '\0';
   pos_T pos;
 
   if (enc_dbcs) {
@@ -3249,7 +3249,7 @@ again:
    */
   inc_cursor();
   p = ml_get_cursor();
-  for (cp = p; *cp != NUL && *cp != '>' && !vim_iswhite(*cp); mb_ptr_adv(cp))
+  for (cp = p; *cp != '\0' && *cp != '>' && !vim_iswhite(*cp); mb_ptr_adv(cp))
     ;
   len = (int)(cp - p);
   if (len == 0) {
@@ -3523,7 +3523,7 @@ find_next_quote (
 
   for (;; ) {
     c = line[col];
-    if (c == NUL)
+    if (c == '\0')
       return -1;
     else if (escape != NULL && vim_strchr(escape, c))
       ++col;
@@ -3605,14 +3605,14 @@ current_quote (
     if (vis_bef_curs) {
       inside_quotes = VIsual.col > 0
                       && line[VIsual.col - 1] == quotechar
-                      && line[curwin->w_cursor.col] != NUL
+                      && line[curwin->w_cursor.col] != '\0'
                       && line[curwin->w_cursor.col + 1] == quotechar;
       i = VIsual.col;
       col_end = curwin->w_cursor.col;
     } else {
       inside_quotes = curwin->w_cursor.col > 0
                       && line[curwin->w_cursor.col - 1] == quotechar
-                      && line[VIsual.col] != NUL
+                      && line[VIsual.col] != '\0'
                       && line[VIsual.col + 1] == quotechar;
       i = curwin->w_cursor.col;
       col_end = VIsual.col;
@@ -3760,7 +3760,7 @@ current_quote (
       if (inside_quotes
           || (!selected_quote
               && line[VIsual.col] != quotechar
-              && (line[VIsual.col] == NUL
+              && (line[VIsual.col] == '\0'
                   || line[VIsual.col + 1] != quotechar))) {
         dec_cursor();
         VIsual = curwin->w_cursor;
@@ -3965,7 +3965,7 @@ int linewhite(linenr_T lnum)
   char_u  *p;
 
   p = skipwhite(ml_get(lnum));
-  return *p == NUL;
+  return *p == '\0';
 }
 #endif
 
@@ -4043,15 +4043,15 @@ find_pattern_in_path (
     if (regmatch.regprog == NULL)
       goto fpip_end;
   }
-  inc_opt = (*curbuf->b_p_inc == NUL) ? p_inc : curbuf->b_p_inc;
-  if (*inc_opt != NUL) {
+  inc_opt = (*curbuf->b_p_inc == '\0') ? p_inc : curbuf->b_p_inc;
+  if (*inc_opt != '\0') {
     incl_regmatch.regprog = vim_regcomp(inc_opt, p_magic ? RE_MAGIC : 0);
     if (incl_regmatch.regprog == NULL)
       goto fpip_end;
     incl_regmatch.rm_ic = FALSE;        /* don't ignore case in incl. pat. */
   }
-  if (type == FIND_DEFINE && (*curbuf->b_p_def != NUL || *p_def != NUL)) {
-    def_regmatch.regprog = vim_regcomp(*curbuf->b_p_def == NUL
+  if (type == FIND_DEFINE && (*curbuf->b_p_def != '\0' || *p_def != '\0')) {
+    def_regmatch.regprog = vim_regcomp(*curbuf->b_p_def == '\0'
         ? p_def : curbuf->b_p_def, p_magic ? RE_MAGIC : 0);
     if (def_regmatch.regprog == NULL)
       goto fpip_end;
@@ -4177,7 +4177,7 @@ find_pattern_in_path (
                 ++i;
             }
             save_char = p[i];
-            p[i] = NUL;
+            p[i] = '\0';
             msg_outtrans_attr(p, hl_attr(HLF_D));
             p[i] = save_char;
           }
@@ -4381,7 +4381,7 @@ search_line:
             i += (int)(p - aux);
             reuse |= CONT_S_IPOS;
           }
-          IObuff[i] = NUL;
+          IObuff[i] = '\0';
           aux = IObuff;
 
           if (i == compl_length)
@@ -4480,8 +4480,8 @@ exit_matched:
       if (def_regmatch.regprog == NULL
           && action == ACTION_EXPAND
           && !(compl_cont_status & CONT_SOL)
-          && *startp != NUL
-          && *(p = startp + MB_PTR2LEN(startp)) != NUL)
+          && *startp != '\0'
+          && *(p = startp + MB_PTR2LEN(startp)) != '\0')
         goto search_line;
     }
     line_breakcheck();
@@ -4512,9 +4512,9 @@ exit_matched:
       /* Remove any CR and LF from the line. */
       i = (int)STRLEN(line);
       if (i > 0 && line[i - 1] == '\n')
-        line[--i] = NUL;
+        line[--i] = '\0';
       if (i > 0 && line[i - 1] == '\r')
-        line[--i] = NUL;
+        line[--i] = '\0';
     } else if (!already) {
       if (++lnum > end_lnum)
         break;
@@ -4578,7 +4578,7 @@ static void show_pat_in_path(char_u *line, int type, int did_show, int action, F
         --p;
       if (p >= line && *p == '\r')
         --p;
-      *(p + 1) = NUL;
+      *(p + 1) = '\0';
     }
     if (action == ACTION_SHOW_ALL) {
       sprintf((char *)IObuff, "%3ld: ", count);         /* show match nr */
diff --git a/src/sha256.c b/src/sha256.c
index 0ccb13d763..9c15e9ad29 100644
--- a/src/sha256.c
+++ b/src/sha256.c
@@ -294,7 +294,7 @@ char_u *sha256_bytes(char_u *buf, int buf_len, char_u *salt, int salt_len)
 char_u* sha256_key(char_u *buf, char_u *salt, int salt_len)
 {
   // No passwd means don't encrypt
-  if ((buf == NULL) || (*buf == NUL)) {
+  if ((buf == NULL) || (*buf == '\0')) {
     return (char_u *)"";
   }
 
diff --git a/src/spell.c b/src/spell.c
index b948aed530..59285db5a2 100644
--- a/src/spell.c
+++ b/src/spell.c
@@ -1018,7 +1018,7 @@ spell_check (
   if (spell_iswordp(mi.mi_fend, wp)) {
     do {
       mb_ptr_adv(mi.mi_fend);
-    } while (*mi.mi_fend != NUL && spell_iswordp(mi.mi_fend, wp));
+    } while (*mi.mi_fend != '\0' && spell_iswordp(mi.mi_fend, wp));
 
     if (capcol != NULL && *capcol == 0 && wp->w_s->b_cap_prog != NULL) {
       // Check word starting with capital letter.
@@ -1041,7 +1041,7 @@ spell_check (
 
   // case-fold the word with one non-word character, so that we can check
   // for the word end.
-  if (*mi.mi_fend != NUL)
+  if (*mi.mi_fend != '\0')
     mb_ptr_adv(mi.mi_fend);
 
   (void)spell_casefold(ptr, (int)(mi.mi_fend - ptr), mi.mi_fword,
@@ -1226,7 +1226,7 @@ static void find_word(matchinf_T *mip, int mode)
   // - we reach the end of the tree,
   // - or we reach the end of the line.
   for (;; ) {
-    if (flen <= 0 && *mip->mi_fend != NUL)
+    if (flen <= 0 && *mip->mi_fend != '\0')
       flen = fold_more(mip);
 
     len = byts[arridx++];
@@ -1254,7 +1254,7 @@ static void find_word(matchinf_T *mip, int mode)
     }
 
     // Stop looking at end of the line.
-    if (ptr[wlen] == NUL)
+    if (ptr[wlen] == '\0')
       break;
 
     // Perform a binary search in the list of accepted bytes.
@@ -1288,7 +1288,7 @@ static void find_word(matchinf_T *mip, int mode)
     // checked word.
     if (c == ' ') {
       for (;; ) {
-        if (flen <= 0 && *mip->mi_fend != NUL)
+        if (flen <= 0 && *mip->mi_fend != '\0')
           flen = fold_more(mip);
         if (ptr[wlen] != ' ' && ptr[wlen] != TAB)
           break;
@@ -1459,7 +1459,7 @@ static void find_word(matchinf_T *mip, int mode)
         // words must match with one of the COMPOUNDRULE items and
         // the number of syllables must not be too large.
         mip->mi_compflags[mip->mi_complen] = ((unsigned)flags >> 24);
-        mip->mi_compflags[mip->mi_complen + 1] = NUL;
+        mip->mi_compflags[mip->mi_complen + 1] = '\0';
         if (word_ends) {
           char_u fword[MAXWLEN];
 
@@ -1649,9 +1649,9 @@ static int can_compound(slang_T *slang, char_u *word, char_u *flags)
   if (enc_utf8) {
     // Need to convert the single byte flags to utf8 characters.
     p = uflags;
-    for (i = 0; flags[i] != NUL; ++i)
+    for (i = 0; flags[i] != '\0'; ++i)
       p += mb_char2bytes(flags[i], p);
-    *p = NUL;
+    *p = '\0';
     p = uflags;
   } else
     p = flags;
@@ -1687,9 +1687,9 @@ static int can_be_compound(trystate_T *sp, slang_T *slang, char_u *compflags, in
     int v;
 
     compflags[sp->ts_complen] = flag;
-    compflags[sp->ts_complen + 1] = NUL;
+    compflags[sp->ts_complen + 1] = '\0';
     v = match_compoundrule(slang, compflags + sp->ts_compsplit);
-    compflags[sp->ts_complen] = NUL;
+    compflags[sp->ts_complen] = '\0';
     return v;
   }
 
@@ -1708,22 +1708,22 @@ static int match_compoundrule(slang_T *slang, char_u *compflags)
   int c;
 
   // loop over all the COMPOUNDRULE entries
-  for (p = slang->sl_comprules; *p != NUL; ++p) {
+  for (p = slang->sl_comprules; *p != '\0'; ++p) {
     // loop over the flags in the compound word we have made, match
     // them against the current rule entry
     for (i = 0;; ++i) {
       c = compflags[i];
-      if (c == NUL)
+      if (c == '\0')
         // found a rule that matches for the flags we have so far
         return TRUE;
-      if (*p == '/' || *p == NUL)
+      if (*p == '/' || *p == '\0')
         break;          // end of rule, it's too short
       if (*p == '[') {
         int match = FALSE;
 
         // compare against all the flags in []
         ++p;
-        while (*p != ']' && *p != NUL)
+        while (*p != ']' && *p != '\0')
           if (*p++ == c)
             match = TRUE;
         if (!match)
@@ -1833,7 +1833,7 @@ static void find_prefix(matchinf_T *mip, int mode)
   // - we reach the end of the tree,
   // - or we reach the end of the line.
   for (;; ) {
-    if (flen == 0 && *mip->mi_fend != NUL)
+    if (flen == 0 && *mip->mi_fend != '\0')
       flen = fold_more(mip);
 
     len = byts[arridx++];
@@ -1873,7 +1873,7 @@ static void find_prefix(matchinf_T *mip, int mode)
     }
 
     // Stop looking at end of the line.
-    if (ptr[wlen] == NUL)
+    if (ptr[wlen] == '\0')
       break;
 
     // Perform a binary search in the list of accepted bytes.
@@ -1914,10 +1914,10 @@ static int fold_more(matchinf_T *mip)
   p = mip->mi_fend;
   do {
     mb_ptr_adv(mip->mi_fend);
-  } while (*mip->mi_fend != NUL && spell_iswordp(mip->mi_fend, mip->mi_win));
+  } while (*mip->mi_fend != '\0' && spell_iswordp(mip->mi_fend, mip->mi_win));
 
   // Include the non-word character so that we can check for the word end.
-  if (*mip->mi_fend != NUL)
+  if (*mip->mi_fend != '\0')
     mb_ptr_adv(mip->mi_fend);
 
   (void)spell_casefold(p, (int)(mip->mi_fend - p),
@@ -1945,7 +1945,7 @@ spell_valid_case (
 // Return TRUE if spell checking is not enabled.
 static int no_spell_checking(win_T *wp)
 {
-  if (!wp->w_p_spell || *wp->w_s->b_p_spl == NUL
+  if (!wp->w_p_spell || *wp->w_s->b_p_spl == '\0'
       || wp->w_s->b_langp.ga_len == 0) {
     EMSG(_("E756: Spell checking is not enabled"));
     return TRUE;
@@ -2161,7 +2161,7 @@ spell_move_to (
       --capcol;
 
       // But after empty line check first word in next line
-      if (*skipwhite(line) == NUL)
+      if (*skipwhite(line) == '\0')
         capcol = 0;
     }
 
@@ -2185,7 +2185,7 @@ void spell_cat_line(char_u *buf, char_u *line, int maxlen)
   while (vim_strchr((char_u *)"*#/\"\t", *p) != NULL)
     p = skipwhite(p + 1);
 
-  if (*p != NUL) {
+  if (*p != '\0') {
     // Only worth concatenating if there is something else than spaces to
     // concatenate.
     n = (int)(p - line) + 1;
@@ -2227,14 +2227,14 @@ static void spell_load_lang(char_u *lang)
         lang, spell_enc());
     r = do_in_runtimepath(fname_enc, FALSE, spell_load_cb, &sl);
 
-    if (r == FAIL && *sl.sl_lang != NUL) {
+    if (r == FAIL && *sl.sl_lang != '\0') {
       // Try loading the ASCII version.
       vim_snprintf((char *)fname_enc, sizeof(fname_enc) - 5,
           "spell/%s.ascii.spl",
           lang);
       r = do_in_runtimepath(fname_enc, FALSE, spell_load_cb, &sl);
 
-      if (r == FAIL && *sl.sl_lang != NUL && round == 1
+      if (r == FAIL && *sl.sl_lang != '\0' && round == 1
           && apply_autocmds(EVENT_SPELLFILEMISSING, lang,
               curbuf->b_fname, FALSE, curbuf))
         continue;
@@ -2389,7 +2389,7 @@ static void slang_clear(slang_T *lp)
   lp->sl_compmax = MAXWLEN;
   lp->sl_compminlen = 0;
   lp->sl_compsylmax = MAXWLEN;
-  lp->sl_regions[0] = NUL;
+  lp->sl_regions[0] = '\0';
 }
 
 // Clear the info from the .sug file in "lp".
@@ -2649,7 +2649,7 @@ truncerr:
 endFAIL:
   if (lang != NULL)
     // truncating the name signals the error to spell_load_lang()
-    *lang = NUL;
+    *lang = '\0';
   if (lp != NULL && old_lp == NULL)
     slang_free(lp);
   lp = NULL;
@@ -2701,7 +2701,7 @@ static int read_region_section(FILE *fd, slang_T *lp, int len)
     return SP_FORMERROR;
   for (i = 0; i < len; ++i)
     lp->sl_regions[i] = getc(fd);                       // <regionname>
-  lp->sl_regions[len] = NUL;
+  lp->sl_regions[len] = '\0';
   return 0;
 }
 
@@ -2770,7 +2770,7 @@ static int read_prefcond_section(FILE *fd, slang_T *lp)
       p = buf + 1;
       while (n-- > 0)
         *p++ = getc(fd);                                // <condstr>
-      *p = NUL;
+      *p = '\0';
       lp->sl_prefprog[i] = vim_regcomp(buf, RE_MAGIC + RE_STRING);
     }
   }
@@ -2829,7 +2829,7 @@ static int read_sal_section(FILE *fd, slang_T *slang)
   salitem_T   *smp;
   int ccnt;
   char_u      *p;
-  int c = NUL;
+  int c = '\0';
 
   slang->sl_sofo = FALSE;
 
@@ -2866,7 +2866,7 @@ static int read_sal_section(FILE *fd, slang_T *slang)
       *p++ = c;
     }
     smp->sm_leadlen = (int)(p - smp->sm_lead);
-    *p++ = NUL;
+    *p++ = '\0';
 
     // Put (abc) chars in sm_oneof, if any.
     if (c == '(') {
@@ -2877,7 +2877,7 @@ static int read_sal_section(FILE *fd, slang_T *slang)
           break;
         *p++ = c;
       }
-      *p++ = NUL;
+      *p++ = '\0';
       if (++i < ccnt)
         c = getc(fd);
     } else
@@ -2890,7 +2890,7 @@ static int read_sal_section(FILE *fd, slang_T *slang)
       *p++ = c;
     for (++i; i < ccnt; ++i)
       *p++ = getc(fd);                          // <salfrom>
-    *p++ = NUL;
+    *p++ = '\0';
 
     // <saltolen> <salto>
     smp->sm_to = read_cnt_string(fd, 1, &ccnt);
@@ -2919,7 +2919,7 @@ static int read_sal_section(FILE *fd, slang_T *slang)
     // that we need to check the index every time.
     smp = &((salitem_T *)gap->ga_data)[gap->ga_len];
     p = alloc(1);
-    p[0] = NUL;
+    p[0] = '\0';
     smp->sm_lead = p;
     smp->sm_leadlen = 0;
     smp->sm_oneof = NULL;
@@ -2956,7 +2956,7 @@ static int read_words_section(FILE *fd, slang_T *lp, int len)
       if (c == EOF)
         return SP_TRUNCERROR;
       word[i] = c;
-      if (word[i] == NUL)
+      if (word[i] == '\0')
         break;
       if (i == MAXWLEN - 1)
         return SP_FORMERROR;
@@ -3151,11 +3151,11 @@ static int read_compound(FILE *fd, slang_T *slang, int len)
   // for all flags.
   cp = alloc(todo + 1);
   slang->sl_compstartflags = cp;
-  *cp = NUL;
+  *cp = '\0';
 
   ap = alloc(todo + 1);
   slang->sl_compallflags = ap;
-  *ap = NUL;
+  *ap = '\0';
 
   // And a list of all patterns in their original form, for checking whether
   // compounding may work in match_compoundrule().  This is freed when we
@@ -3180,7 +3180,7 @@ static int read_compound(FILE *fd, slang_T *slang, int len)
     if (vim_strchr((char_u *)"?*+[]/", c) == NULL
         && !byte_in_str(slang->sl_compallflags, c)) {
       *ap++ = c;
-      *ap = NUL;
+      *ap = '\0';
     }
 
     if (atstart != 0) {
@@ -3193,7 +3193,7 @@ static int read_compound(FILE *fd, slang_T *slang, int len)
       else {
         if (!byte_in_str(slang->sl_compstartflags, c)) {
           *cp++ = c;
-          *cp = NUL;
+          *cp = '\0';
         }
         if (atstart == 1)
           atstart = 0;
@@ -3227,10 +3227,10 @@ static int read_compound(FILE *fd, slang_T *slang, int len)
   *pp++ = '\\';
   *pp++ = ')';
   *pp++ = '$';
-  *pp = NUL;
+  *pp = '\0';
 
   if (crp != NULL)
-    *crp = NUL;
+    *crp = '\0';
 
   slang->sl_compprog = vim_regcomp(pat, RE_MAGIC + RE_STRING + RE_STRICT);
   vim_free(pat);
@@ -3246,7 +3246,7 @@ static int byte_in_str(char_u *str, int n)
 {
   char_u      *p;
 
-  for (p = str; *p != NUL; ++p)
+  for (p = str; *p != '\0'; ++p)
     if (*p == n)
       return TRUE;
   return FALSE;
@@ -3270,8 +3270,8 @@ static int init_syl_tab(slang_T *slang)
   ga_init(&slang->sl_syl_items, sizeof(syl_item_T), 4);
   p = vim_strchr(slang->sl_syllable, '/');
   while (p != NULL) {
-    *p++ = NUL;
-    if (*p == NUL)          // trailing slash
+    *p++ = '\0';
+    if (*p == '\0')          // trailing slash
       break;
     s = p;
     p = vim_strchr(p, '/');
@@ -3306,7 +3306,7 @@ static int count_syllables(slang_T *slang, char_u *word)
   if (slang->sl_syllable == NULL)
     return 0;
 
-  for (p = word; *p != NUL; p += len) {
+  for (p = word; *p != '\0'; p += len) {
     // When running into a space reset counter.
     if (*p == ' ') {
       len = 1;
@@ -3365,13 +3365,13 @@ static int set_sofo(slang_T *lp, char_u *from, char_u *to)
 
     // First count the number of items for each list.  Temporarily use
     // sl_sal_first[] for this.
-    for (p = from, s = to; *p != NUL && *s != NUL; ) {
+    for (p = from, s = to; *p != '\0' && *s != '\0'; ) {
       c = mb_cptr2char_adv(&p);
       mb_cptr_adv(s);
       if (c >= 256)
         ++lp->sl_sal_first[c & 0xff];
     }
-    if (*p != NUL || *s != NUL)             // lengths differ
+    if (*p != '\0' || *s != '\0')             // lengths differ
       return SP_FORMERROR;
 
     // Allocate the lists.
@@ -3385,7 +3385,7 @@ static int set_sofo(slang_T *lp, char_u *from, char_u *to)
     // Put the characters up to 255 in sl_sal_first[] the rest in a sl_sal
     // list.
     memset(lp->sl_sal_first, 0, sizeof(salfirst_T) * 256);
-    for (p = from, s = to; *p != NUL && *s != NUL; ) {
+    for (p = from, s = to; *p != '\0' && *s != '\0'; ) {
       c = mb_cptr2char_adv(&p);
       i = mb_cptr2char_adv(&s);
       if (c >= 256) {
@@ -3396,7 +3396,7 @@ static int set_sofo(slang_T *lp, char_u *from, char_u *to)
           ++inp;
         *inp++ = c;                     // from char
         *inp++ = i;                     // to char
-        *inp++ = NUL;                   // NUL at the end
+        *inp++ = '\0';                   // NUL at the end
       } else
         // mapping byte to char is done in sl_sal_first[]
         lp->sl_sal_first[c] = i;
@@ -3406,7 +3406,7 @@ static int set_sofo(slang_T *lp, char_u *from, char_u *to)
     if (STRLEN(from) != STRLEN(to))
       return SP_FORMERROR;
 
-    for (i = 0; to[i] != NUL; ++i)
+    for (i = 0; to[i] != '\0'; ++i)
       lp->sl_sal_first[from[i]] = to[i];
     lp->sl_sal.ga_len = 1;              // indicates we have soundfolding
   }
@@ -3473,9 +3473,9 @@ static int *mb_str2wide(char_u *s)
   int i = 0;
 
   int *res = xmalloc((mb_charlen(s) + 1) * sizeof(int));
-  for (char_u *p = s; *p != NUL; )
+  for (char_u *p = s; *p != '\0'; )
     res[i++] = mb_ptr2char_adv(&p);
-  res[i] = NUL;
+  res[i] = '\0';
 
   return res;
 }
@@ -3675,7 +3675,7 @@ char_u *did_set_spelllang(win_T *wp)
   wp->w_s->b_cjk = 0;
 
   // Loop over comma separated language names.
-  for (splp = spl_copy; *splp != NUL; ) {
+  for (splp = spl_copy; *splp != '\0'; ) {
     // Get one language name.
     copy_option_part(&splp, lang, MAXWLEN, ",");
     region = NULL;
@@ -3712,7 +3712,7 @@ char_u *did_set_spelllang(win_T *wp)
       if (len > 3 && lang[len - 3] == '_') {
         region = lang + len - 2;
         len -= 3;
-        lang[len] = NUL;
+        lang[len] = '\0';
       } else
         dont_use_region = TRUE;
 
@@ -3756,7 +3756,7 @@ char_u *did_set_spelllang(win_T *wp)
           c = find_region(slang->sl_regions, region);
           if (c == REGION_ALL) {
             if (slang->sl_add) {
-              if (*slang->sl_regions != NUL)
+              if (*slang->sl_regions != '\0')
                 // This addition file is for other regions.
                 region_mask = 0;
             } else
@@ -3786,7 +3786,7 @@ char_u *did_set_spelllang(win_T *wp)
   // round 2: load second name in 'spellfile.
   // etc.
   spf = curwin->w_s->b_p_spf;
-  for (round = 0; round == 0 || *spf != NUL; ++round) {
+  for (round = 0; round == 0 || *spf != '\0'; ++round) {
     if (round == 0) {
       // Internal wordlist, if there is one.
       if (int_wordlist == NULL)
@@ -3821,7 +3821,7 @@ char_u *did_set_spelllang(win_T *wp)
         vim_strncpy(lang, path_tail(spf_name), MAXWLEN);
         p = vim_strchr(lang, '.');
         if (p != NULL)
-          *p = NUL;             // truncate at ".encoding.add"
+          *p = '\0';             // truncate at ".encoding.add"
       }
       slang = spell_load_file(spf_name, lang, NULL, TRUE);
 
@@ -3838,7 +3838,7 @@ char_u *did_set_spelllang(win_T *wp)
         c = find_region(slang->sl_regions, use_region);
         if (c != REGION_ALL)
           region_mask = 1 << c;
-        else if (*slang->sl_regions != NUL)
+        else if (*slang->sl_regions != '\0')
           // This spell file is for other regions.
           region_mask = 0;
       }
@@ -3920,7 +3920,7 @@ static void use_midword(slang_T *lp, win_T *wp)
   if (lp->sl_midword == NULL)       // there aren't any
     return;
 
-  for (p = lp->sl_midword; *p != NUL; )
+  for (p = lp->sl_midword; *p != '\0'; )
     if (has_mbyte) {
       int c, l, n;
       char_u  *bp;
@@ -3955,7 +3955,7 @@ static int find_region(char_u *rp, char_u *region)
   int i;
 
   for (i = 0;; i += 2) {
-    if (rp[i] == NUL)
+    if (rp[i] == '\0')
       return REGION_ALL;
     if (rp[i] == region[0] && rp[i + 1] == region[1])
       break;
@@ -3982,7 +3982,7 @@ captype (
 
   // find first letter
   for (p = word; !spell_iswordp_nmw(p, curwin); mb_ptr_adv(p))
-    if (end == NULL ? *p == NUL : p >= end)
+    if (end == NULL ? *p == '\0' : p >= end)
       return 0;             // only non-word characters, illegal word
   if (has_mbyte)
     c = mb_ptr2char_adv(&p);
@@ -3992,7 +3992,7 @@ captype (
 
   // Need to check all letters to find a word with mixed upper/lower.
   // But a word with an upper char only at start is a ONECAP.
-  for (; end == NULL ? *p != NUL : p < end; mb_ptr_adv(p))
+  for (; end == NULL ? *p != '\0' : p < end; mb_ptr_adv(p))
     if (spell_iswordp_nmw(p, curwin)) {
       c = PTR2CHAR(p);
       if (!SPELL_ISUPPER(c)) {
@@ -4107,7 +4107,7 @@ void spell_reload(void)
   for (wp = firstwin; wp != NULL; wp = wp->w_next) {
     // Only load the wordlists when 'spelllang' is set and there is a
     // window for this buffer in which 'spell' is set.
-    if (*wp->w_s->b_p_spl != NUL) {
+    if (*wp->w_s->b_p_spl != '\0') {
       if (wp->w_p_spell) {
         (void)did_set_spelllang(wp);
         break;
@@ -4457,7 +4457,7 @@ static void spell_print_node(wordnode_T *node, int depth)
   } else {
     node->wn_u1.index = TRUE;
 
-    if (node->wn_byte != NUL) {
+    if (node->wn_byte != '\0') {
       if (node->wn_child != NULL)
         PRINTSOME(line1, depth, " %c -> ", node->wn_byte, 0);
       else
@@ -4473,14 +4473,14 @@ static void spell_print_node(wordnode_T *node, int depth)
     else
       PRINTSOME(line3, depth, "      ", 0, 0);
 
-    if (node->wn_byte == NUL) {
+    if (node->wn_byte == '\0') {
       msg(line1);
       msg(line2);
       msg(line3);
     }
 
     // do the children
-    if (node->wn_byte != NUL && node->wn_child != NULL)
+    if (node->wn_byte != '\0' && node->wn_child != NULL)
       spell_print_node(node->wn_child, depth + 1);
 
     // do the siblings
@@ -4606,9 +4606,9 @@ static afffile_T *spell_read_aff(spellinfo_T *spin, char_u *fname)
     // item.
     itemcnt = 0;
     for (p = line;; ) {
-      while (*p != NUL && *p <= ' ')        // skip white space and CR/NL
+      while (*p != '\0' && *p <= ' ')        // skip white space and CR/NL
         ++p;
-      if (*p == NUL)
+      if (*p == '\0')
         break;
       if (itemcnt == MAXITEMCNT)            // too many items
         break;
@@ -4620,9 +4620,9 @@ static afffile_T *spell_read_aff(spellinfo_T *spin, char_u *fname)
       else
         while (*p > ' ')            // skip until white space or CR/NL
           ++p;
-      if (*p == NUL)
+      if (*p == '\0')
         break;
-      *p++ = NUL;
+      *p++ = '\0';
     }
 
     // Handle non-empty lines.
@@ -4754,7 +4754,7 @@ static afffile_T *spell_read_aff(spellinfo_T *spin, char_u *fname)
               fname, lnum, items[1]);
       } else if (is_aff_rule(items, itemcnt, "COMPOUNDRULE", 2))   {
         // Don't use the first rule if it is a number.
-        if (compflags != NULL || *skipdigits(items[1]) != NUL) {
+        if (compflags != NULL || *skipdigits(items[1]) != '\0') {
           // Concatenate this string to previously defined ones,
           // using a slash to separate them.
           l = (int)STRLEN(items[1]) + 1;
@@ -4948,7 +4948,7 @@ static afffile_T *spell_read_aff(spellinfo_T *spin, char_u *fname)
           // Recognize flags on the affix: abcd/XYZ
           aff_entry->ae_flags = vim_strchr(aff_entry->ae_add, '/');
           if (aff_entry->ae_flags != NULL) {
-            *aff_entry->ae_flags++ = NUL;
+            *aff_entry->ae_flags++ = '\0';
             aff_process_flags(aff, aff_entry);
           }
         }
@@ -4989,7 +4989,7 @@ static afffile_T *spell_read_aff(spellinfo_T *spin, char_u *fname)
             if (aff_entry->ae_chop != NULL
                 && aff_entry->ae_add != NULL
                 && aff_entry->ae_chop[(*mb_ptr2len)(
-                                        aff_entry->ae_chop)] == NUL
+                                        aff_entry->ae_chop)] == '\0'
                 ) {
               int c, c_up;
 
@@ -5004,7 +5004,7 @@ static afffile_T *spell_read_aff(spellinfo_T *spin, char_u *fname)
                 if (PTR2CHAR(p) == c_up) {
                   upper = TRUE;
                   aff_entry->ae_chop = NULL;
-                  *p = NUL;
+                  *p = '\0';
 
                   // The condition is matched with the
                   // actual word, thus must check for the
@@ -5107,10 +5107,10 @@ static afffile_T *spell_read_aff(spellinfo_T *spin, char_u *fname)
         if (items[0][3] == 'S' ? do_repsal : do_rep) {
           // Replace underscore with space (can't include a space
           // directly).
-          for (p = items[1]; *p != NUL; mb_ptr_adv(p))
+          for (p = items[1]; *p != '\0'; mb_ptr_adv(p))
             if (*p == '_')
               *p = ' ';
-          for (p = items[2]; *p != NUL; mb_ptr_adv(p))
+          for (p = items[2]; *p != '\0'; mb_ptr_adv(p))
             if (*p == '_')
               *p = ' ';
           add_fromto(spin, items[0][3] == 'S'
@@ -5129,7 +5129,7 @@ static afffile_T *spell_read_aff(spellinfo_T *spin, char_u *fname)
           int c;
 
           // Check that every character appears only once.
-          for (p = items[1]; *p != NUL; ) {
+          for (p = items[1]; *p != '\0'; ) {
             c = mb_ptr2char_adv(&p);
             if ((spin->si_map.ga_len > 0
                  && vim_strchr(spin->si_map.ga_data, c)
@@ -5296,7 +5296,7 @@ static void aff_process_flags(afffile_T *affile, affentry_T *entry)
 
   if (entry->ae_flags != NULL
       && (affile->af_compforbid != 0 || affile->af_comppermit != 0)) {
-    for (p = entry->ae_flags; *p != NUL; ) {
+    for (p = entry->ae_flags; *p != '\0'; ) {
       prevp = p;
       flag = get_affitem(affile->af_flagtype, &p);
       if (flag == affile->af_comppermit || flag == affile->af_compforbid) {
@@ -5310,7 +5310,7 @@ static void aff_process_flags(afffile_T *affile, affentry_T *entry)
       if (affile->af_flagtype == AFT_NUM && *p == ',')
         ++p;
     }
-    if (*entry->ae_flags == NUL)
+    if (*entry->ae_flags == '\0')
       entry->ae_flags = NULL;           // nothing left
   }
 }
@@ -5342,7 +5342,7 @@ static unsigned affitem2flag(int flagtype, char_u *item, char_u *fname, int lnum
       smsg((char_u *)_("Illegal flag in %s line %d: %s"),
           fname, lnum, item);
   }
-  if (*p != NUL) {
+  if (*p != '\0') {
     smsg((char_u *)_(e_affname), fname, lnum, item);
     return 0;
   }
@@ -5366,7 +5366,7 @@ static unsigned get_affitem(int flagtype, char_u **pp)
     res = mb_ptr2char_adv(pp);
     if (flagtype == AFT_LONG || (flagtype == AFT_CAPLONG
                                  && res >= 'A' && res <= 'Z')) {
-      if (**pp == NUL)
+      if (**pp == '\0')
         return 0;
       res = mb_ptr2char_adv(pp) + (res << 16);
     }
@@ -5406,7 +5406,7 @@ static void process_compflags(spellinfo_T *spin, afffile_T *aff, char_u *compfla
   spin->si_compflags = p;
   tp = p + STRLEN(p);
 
-  for (p = compflags; *p != NUL; ) {
+  for (p = compflags; *p != '\0'; ) {
     if (vim_strchr((char_u *)"/?*+[]", *p) != NULL)
       // Copy non-flag characters directly.
       *tp++ = *p++;
@@ -5443,7 +5443,7 @@ static void process_compflags(spellinfo_T *spin, afffile_T *aff, char_u *compfla
     }
   }
 
-  *tp = NUL;
+  *tp = '\0';
 }
 
 // Check that the new IDs for postponed affixes and compounding don't overrun
@@ -5470,10 +5470,10 @@ static int flag_in_afflist(int flagtype, char_u *afflist, unsigned flag)
 
   case AFT_CAPLONG:
   case AFT_LONG:
-    for (p = afflist; *p != NUL; ) {
+    for (p = afflist; *p != '\0'; ) {
       n = mb_ptr2char_adv(&p);
       if ((flagtype == AFT_LONG || (n >= 'A' && n <= 'Z'))
-          && *p != NUL)
+          && *p != '\0')
         n = mb_ptr2char_adv(&p) + (n << 16);
       if (n == flag)
         return TRUE;
@@ -5481,11 +5481,11 @@ static int flag_in_afflist(int flagtype, char_u *afflist, unsigned flag)
     break;
 
   case AFT_NUM:
-    for (p = afflist; *p != NUL; ) {
+    for (p = afflist; *p != '\0'; ) {
       n = getdigits(&p);
       if (n == flag)
         return TRUE;
-      if (*p != NUL)            // skip over comma
+      if (*p != '\0')            // skip over comma
         ++p;
     }
     break;
@@ -5634,7 +5634,7 @@ static int spell_read_dic(spellinfo_T *spin, char_u *fname, afffile_T *affile)
       --l;
     if (l == 0)
       continue;         // empty line
-    line[l] = NUL;
+    line[l] = '\0';
 
     // Convert from "SET" to 'encoding' when needed.
     if (spin->si_conv.vc_type != CONV_NONE) {
@@ -5653,11 +5653,11 @@ static int spell_read_dic(spellinfo_T *spin, char_u *fname, afffile_T *affile)
     // Truncate the word at the "/", set "afflist" to what follows.
     // Replace "\/" by "/" and "\\" by "\".
     afflist = NULL;
-    for (p = w; *p != NUL; mb_ptr_adv(p)) {
+    for (p = w; *p != '\0'; mb_ptr_adv(p)) {
       if (*p == '\\' && (p[1] == '\\' || p[1] == '/'))
         STRMOVE(p, p + 1);
       else if (*p == '/') {
-        *p = NUL;
+        *p = '\0';
         afflist = p + 1;
         break;
       }
@@ -5706,7 +5706,7 @@ static int spell_read_dic(spellinfo_T *spin, char_u *fname, afffile_T *affile)
       hash_add_item(&ht, hi, dw, hash);
 
     flags = 0;
-    store_afflist[0] = NUL;
+    store_afflist[0] = '\0';
     pfxlen = 0;
     need_affix = FALSE;
     if (afflist != NULL) {
@@ -5801,7 +5801,7 @@ static int get_pfxlist(afffile_T *affile, char_u *afflist, char_u *store_afflist
   char_u key[AH_KEY_LEN];
   hashitem_T  *hi;
 
-  for (p = afflist; *p != NUL; ) {
+  for (p = afflist; *p != '\0'; ) {
     prevp = p;
     if (get_affitem(affile->af_flagtype, &p) != 0) {
       // A flag is a postponed prefix flag if it appears in "af_pref"
@@ -5818,7 +5818,7 @@ static int get_pfxlist(afffile_T *affile, char_u *afflist, char_u *store_afflist
       ++p;
   }
 
-  store_afflist[cnt] = NUL;
+  store_afflist[cnt] = '\0';
   return cnt;
 }
 
@@ -5833,7 +5833,7 @@ static void get_compflags(afffile_T *affile, char_u *afflist, char_u *store_affl
   char_u key[AH_KEY_LEN];
   hashitem_T  *hi;
 
-  for (p = afflist; *p != NUL; ) {
+  for (p = afflist; *p != '\0'; ) {
     prevp = p;
     if (get_affitem(affile->af_flagtype, &p) != 0) {
       // A flag is a compound flag if it appears in "af_comp".
@@ -5846,7 +5846,7 @@ static void get_compflags(afffile_T *affile, char_u *afflist, char_u *store_affl
       ++p;
   }
 
-  store_afflist[cnt] = NUL;
+  store_afflist[cnt] = '\0';
 }
 
 // Apply affixes to a word and store the resulting words.
@@ -5930,7 +5930,7 @@ store_aff_word (
             if (xht == NULL) {
               // prefix: chop/add at the start of the word
               if (ae->ae_add == NULL)
-                *newword = NUL;
+                *newword = '\0';
               else
                 vim_strncpy(newword, ae->ae_add, MAXWLEN - 1);
               p = word;
@@ -5953,7 +5953,7 @@ store_aff_word (
                 i = (int)MB_CHARLEN(ae->ae_chop);
                 for (; i > 0; --i)
                   mb_ptr_back(newword, p);
-                *p = NUL;
+                *p = '\0';
               }
               if (ae->ae_add != NULL)
                 STRCAT(newword, ae->ae_add);
@@ -6012,14 +6012,14 @@ store_aff_word (
                 // Combine the list of compound flags.
                 // Concatenate them to the prefix IDs list.
                 // Avoid adding the same ID twice.
-                for (i = pfxlen; pfxlist[i] != NUL; ++i) {
+                for (i = pfxlen; pfxlist[i] != '\0'; ++i) {
                   for (j = use_pfxlen;
-                       use_pfxlist[j] != NUL; ++j)
+                       use_pfxlist[j] != '\0'; ++j)
                     if (pfxlist[i] == use_pfxlist[j])
                       break;
-                  if (use_pfxlist[j] == NUL) {
+                  if (use_pfxlist[j] == '\0') {
                     use_pfxlist[j++] = pfxlist[i];
-                    use_pfxlist[j] = NUL;
+                    use_pfxlist[j] = '\0';
                   }
                 }
               }
@@ -6138,7 +6138,7 @@ static int spell_read_wordfile(spellinfo_T *spin, char_u *fname)
       --l;
     if (l == 0)
       continue;         // empty or blank line
-    rline[l] = NUL;
+    rline[l] = '\0';
 
     // Convert from "/encoding={encoding}" to 'encoding' when needed.
     vim_free(pc);
@@ -6214,8 +6214,8 @@ static int spell_read_wordfile(spellinfo_T *spin, char_u *fname)
     // Check for flags and region after a slash.
     p = vim_strchr(line, '/');
     if (p != NULL) {
-      *p++ = NUL;
-      while (*p != NUL) {
+      *p++ = '\0';
+      while (*p != '\0') {
         if (*p == '=')                  // keep-case word
           flags |= WF_KEEPCAP | WF_FIXCAP;
         else if (*p == '!')             // Bad, bad, wicked word.
@@ -6365,20 +6365,20 @@ store_word (
 
   (void)spell_casefold(word, len, foldword, MAXWLEN);
   for (p = pfxlist; res == OK; ++p) {
-    if (!need_affix || (p != NULL && *p != NUL))
+    if (!need_affix || (p != NULL && *p != '\0'))
       res = tree_add_word(spin, foldword, spin->si_foldroot, ct | flags,
           region, p == NULL ? 0 : *p);
-    if (p == NULL || *p == NUL)
+    if (p == NULL || *p == '\0')
       break;
   }
   ++spin->si_foldwcount;
 
   if (res == OK && (ct == WF_KEEPCAP || (flags & WF_KEEPCAP))) {
     for (p = pfxlist; res == OK; ++p) {
-      if (!need_affix || (p != NULL && *p != NUL))
+      if (!need_affix || (p != NULL && *p != '\0'))
         res = tree_add_word(spin, word, spin->si_keeproot, flags,
             region, p == NULL ? 0 : *p);
-      if (p == NULL || *p == NUL)
+      if (p == NULL || *p == '\0')
         break;
     }
     ++spin->si_keepwcount;
@@ -6415,7 +6415,7 @@ static int tree_add_word(spellinfo_T *spin, char_u *word, wordnode_T *root, int
         if (np->wn_child != NULL)
           ++np->wn_child->wn_refs;              // child gets extra ref
         np->wn_byte = copyp->wn_byte;
-        if (np->wn_byte == NUL) {
+        if (np->wn_byte == '\0') {
           np->wn_flags = copyp->wn_flags;
           np->wn_region = copyp->wn_region;
           np->wn_affixID = copyp->wn_affixID;
@@ -6439,7 +6439,7 @@ static int tree_add_word(spellinfo_T *spin, char_u *word, wordnode_T *root, int
     // done on flags and then on affixID.
     while (node != NULL
            && (node->wn_byte < word[i]
-               || (node->wn_byte == NUL
+               || (node->wn_byte == '\0'
                    && (flags < 0
                        ? node->wn_affixID < (unsigned)affixID
                        : (node->wn_flags < (unsigned)(flags & WN_MASK)
@@ -6453,7 +6453,7 @@ static int tree_add_word(spellinfo_T *spin, char_u *word, wordnode_T *root, int
     }
     if (node == NULL
         || node->wn_byte != word[i]
-        || (word[i] == NUL
+        || (word[i] == '\0'
             && (flags < 0
                 || spin->si_sugtree
                 || node->wn_flags != (flags & WN_MASK)
@@ -6480,7 +6480,7 @@ static int tree_add_word(spellinfo_T *spin, char_u *word, wordnode_T *root, int
       node = np;
     }
 
-    if (word[i] == NUL) {
+    if (word[i] == '\0') {
       node->wn_flags = flags;
       node->wn_region |= region;
       node->wn_affixID = affixID;
@@ -6572,7 +6572,7 @@ int spell_check_msm(void)
   if (!VIM_ISDIGIT(*p))
     return FAIL;
   added = getdigits(&p) * 1024;
-  if (*p != NUL)
+  if (*p != '\0')
     return FAIL;
 
   if (start == 0 || incr == 0 || added == 0 || incr > start)
@@ -6741,7 +6741,7 @@ node_compress (
   node->wn_u1.hashkey[0] = len;
   nr = 0;
   for (np = node; np != NULL; np = np->wn_sibling) {
-    if (np->wn_byte == NUL)
+    if (np->wn_byte == '\0')
       // end node: use wn_flags, wn_region and wn_affixID
       n = np->wn_flags + (np->wn_region << 8) + (np->wn_affixID << 16);
     else
@@ -6759,7 +6759,7 @@ node_compress (
   node->wn_u1.hashkey[3] = n == 0 ? 1 : n;
   n = (nr >> 24) & 0xff;
   node->wn_u1.hashkey[4] = n == 0 ? 1 : n;
-  node->wn_u1.hashkey[5] = NUL;
+  node->wn_u1.hashkey[5] = '\0';
 
   // Check for CTRL-C pressed now and then.
   fast_breakcheck();
@@ -6776,7 +6776,7 @@ static int node_equal(wordnode_T *n1, wordnode_T *n2)
   for (p1 = n1, p2 = n2; p1 != NULL && p2 != NULL;
        p1 = p1->wn_sibling, p2 = p2->wn_sibling)
     if (p1->wn_byte != p2->wn_byte
-        || (p1->wn_byte == NUL
+        || (p1->wn_byte == '\0'
             ? (p1->wn_flags != p2->wn_flags
                || p1->wn_region != p2->wn_region
                || p1->wn_affixID != p2->wn_affixID)
@@ -7181,7 +7181,7 @@ static void clear_node(wordnode_T *node)
       np->wn_u1.index = 0;
       np->wn_u2.wnode = NULL;
 
-      if (np->wn_byte != NUL)
+      if (np->wn_byte != '\0')
         clear_node(np->wn_child);
     }
 }
@@ -7448,7 +7448,7 @@ static int sug_filltree(spellinfo_T *spin, slang_T *slang)
       c = byts[n];
       if (c == 0) {
         // Sound-fold the word.
-        tword[depth] = NUL;
+        tword[depth] = '\0';
         spell_soundfold(slang, tword, TRUE, tsalword);
 
         // We use the "flags" field for the MSB of the wordnr,
@@ -7529,10 +7529,10 @@ sug_filltable (
   int prev_nr;
 
   for (p = node; p != NULL; p = p->wn_sibling) {
-    if (p->wn_byte == NUL) {
+    if (p->wn_byte == '\0') {
       gap->ga_len = 0;
       prev_nr = 0;
-      for (np = p; np != NULL && np->wn_byte == NUL; np = np->wn_sibling) {
+      for (np = p; np != NULL && np->wn_byte == '\0'; np = np->wn_sibling) {
         ga_grow(gap, 10);
 
         nr = (np->wn_flags << 16) + (np->wn_region & 0xffff);
@@ -7547,7 +7547,7 @@ sug_filltable (
       }
 
       // add the NUL byte
-      ((char_u *)gap->ga_data)[gap->ga_len++] = NUL;
+      ((char_u *)gap->ga_data)[gap->ga_len++] = '\0';
 
       if (ml_append_buf(spin->si_spellbuf, (linenr_T)wordnr,
               gap->ga_data, gap->ga_len, TRUE) == FAIL)
@@ -7556,7 +7556,7 @@ sug_filltable (
 
       // Remove extra NUL entries, we no longer need them. We don't
       // bother freeing the nodes, the won't be reused anyway.
-      while (p->wn_sibling != NULL && p->wn_sibling->wn_byte == NUL)
+      while (p->wn_sibling != NULL && p->wn_sibling->wn_byte == '\0')
         p->wn_sibling = p->wn_sibling->wn_sibling;
 
       // Clear the flags on the remaining NUL node, so that compression
@@ -8025,22 +8025,22 @@ spell_add_word (
     fname = int_wordlist;
   } else {
     // If 'spellfile' isn't set figure out a good default value.
-    if (*curwin->w_s->b_p_spf == NUL) {
+    if (*curwin->w_s->b_p_spf == '\0') {
       init_spellfile();
       new_spf = TRUE;
     }
 
-    if (*curwin->w_s->b_p_spf == NUL) {
+    if (*curwin->w_s->b_p_spf == '\0') {
       EMSG2(_(e_notset), "spellfile");
       return;
     }
     fnamebuf = alloc(MAXPATHL);
 
-    for (spf = curwin->w_s->b_p_spf, i = 1; *spf != NUL; ++i) {
+    for (spf = curwin->w_s->b_p_spf, i = 1; *spf != '\0'; ++i) {
       copy_option_part(&spf, fnamebuf, MAXPATHL, ",");
       if (i == idx)
         break;
-      if (*spf == NUL) {
+      if (*spf == '\0') {
         EMSGN(_("E765: 'spellfile' does not have %" PRId64 " entries"), idx);
         vim_free(fnamebuf);
         return;
@@ -8107,7 +8107,7 @@ spell_add_word (
 
         // The directory doesn't exist.  Try creating it and opening
         // the file again.
-        *p = NUL;
+        *p = '\0';
         os_mkdir((char *)fname, 0755);
         *p = c;
         fd = mch_fopen((char *)fname, "a");
@@ -8152,12 +8152,12 @@ static void init_spellfile(void)
   int aspath = FALSE;
   char_u      *lstart = curbuf->b_s.b_p_spl;
 
-  if (*curwin->w_s->b_p_spl != NUL && curwin->w_s->b_langp.ga_len > 0) {
+  if (*curwin->w_s->b_p_spl != '\0' && curwin->w_s->b_langp.ga_len > 0) {
     buf = alloc(MAXPATHL);
 
     // Find the end of the language name.  Exclude the region.  If there
     // is a path separator remember the start of the tail.
-    for (lend = curwin->w_s->b_p_spl; *lend != NUL
+    for (lend = curwin->w_s->b_p_spl; *lend != '\0'
          && vim_strchr((char_u *)",._", *lend) == NULL; ++lend)
       if (vim_ispathsep(*lend)) {
         aspath = TRUE;
@@ -8167,7 +8167,7 @@ static void init_spellfile(void)
     // Loop over all entries in 'runtimepath'.  Use the first one where we
     // are allowed to write.
     rtp = p_rtp;
-    while (*rtp != NUL) {
+    while (*rtp != '\0') {
       if (aspath)
         // Use directory of an entry with path, e.g., for
         // "/dir/lg.utf-8.spl" use "/dir".
@@ -8295,8 +8295,8 @@ static int set_spell_chartab(char_u *fol, char_u *low, char_u *upp)
 
   clear_spell_chartab(&new_st);
 
-  while (*pf != NUL) {
-    if (*pl == NUL || *pu == NUL) {
+  while (*pf != '\0') {
+    if (*pl == '\0' || *pu == '\0') {
       EMSG(_(e_affform));
       return FAIL;
     }
@@ -8335,7 +8335,7 @@ static int set_spell_chartab(char_u *fol, char_u *low, char_u *upp)
     }
   }
 
-  if (*pl != NUL || *pu != NUL) {
+  if (*pl != '\0' || *pu != '\0') {
     EMSG(_(e_affform));
     return FAIL;
   }
@@ -8366,7 +8366,7 @@ set_spell_charflags (
       new_st.st_isu[i + 128] = (flags[i] & CF_UPPER) != 0;
     }
 
-    if (*p != NUL) {
+    if (*p != '\0') {
       c = mb_ptr2char_adv(&p);
       new_st.st_fold[i + 128] = c;
       if (i + 128 != c && new_st.st_isu[i + 128] && c < 256)
@@ -8529,7 +8529,7 @@ static int spell_casefold(char_u *str, int len, char_u *buf, int buflen)
   int i;
 
   if (len >= buflen) {
-    buf[0] = NUL;
+    buf[0] = '\0';
     return FAIL;                // result will not fit
   }
 
@@ -8541,18 +8541,18 @@ static int spell_casefold(char_u *str, int len, char_u *buf, int buflen)
     // Fold one character at a time.
     for (p = str; p < str + len; ) {
       if (outi + MB_MAXBYTES > buflen) {
-        buf[outi] = NUL;
+        buf[outi] = '\0';
         return FAIL;
       }
       c = mb_cptr2char_adv(&p);
       outi += mb_char2bytes(SPELL_TOFOLD(c), buf + outi);
     }
-    buf[outi] = NUL;
+    buf[outi] = '\0';
   } else {
     // Be quick for non-multibyte encodings.
     for (i = 0; i < len; ++i)
       buf[i] = spelltab.st_fold[str[i]];
-    buf[i] = NUL;
+    buf[i] = '\0';
   }
 
   return OK;
@@ -8578,14 +8578,14 @@ int spell_check_sps(void)
   sps_flags = 0;
   sps_limit = 9999;
 
-  for (p = p_sps; *p != NUL; ) {
+  for (p = p_sps; *p != '\0'; ) {
     copy_option_part(&p, buf, MAXPATHL, ",");
 
     f = 0;
     if (VIM_ISDIGIT(*buf)) {
       s = buf;
       sps_limit = getdigits(&s);
-      if (*s != NUL && !VIM_ISDIGIT(*s))
+      if (*s != '\0' && !VIM_ISDIGIT(*s))
         f = -1;
     } else if (STRCMP(buf, "best") == 0)
       f = SPS_BEST;
@@ -8663,7 +8663,7 @@ void spell_suggest(int count)
     while (p > line && spell_iswordp_nmw(p, curwin))
       mb_ptr_back(line, p);
     // Forward to start of word.
-    while (*p != NUL && !spell_iswordp_nmw(p, curwin))
+    while (*p != '\0' && !spell_iswordp_nmw(p, curwin))
       mb_ptr_adv(p);
 
     if (!spell_iswordp_nmw(p, curwin)) {                // No word found.
@@ -8847,7 +8847,7 @@ static int check_need_cap(linenr_T lnum, colnr_T col)
       need_cap = TRUE;
     else {
       line = ml_get(lnum - 1);
-      if (*skipwhite(line) == NUL)
+      if (*skipwhite(line) == '\0')
         need_cap = TRUE;
       else {
         // Append a space in place of the line break.
@@ -9008,7 +9008,7 @@ spell_find_suggest (
   memset(su, 0, sizeof(suginfo_T));
   ga_init(&su->su_ga, (int)sizeof(suggest_T), 10);
   ga_init(&su->su_sga, (int)sizeof(suggest_T), 10);
-  if (*badptr == NUL)
+  if (*badptr == '\0')
     return;
   hash_init(&su->su_banned);
 
@@ -9069,7 +9069,7 @@ spell_find_suggest (
     return;
 
   // Loop over the items in 'spellsuggest'.
-  for (p = sps_copy; *p != NUL; ) {
+  for (p = sps_copy; *p != '\0'; ) {
     copy_option_part(&p, buf, MAXPATHL, ",");
 
     if (STRNCMP(buf, "expr:", 5) == 0) {
@@ -9152,12 +9152,12 @@ static void spell_suggest_file(suginfo_T *su, char_u *fname)
     p = vim_strchr(line, '/');
     if (p == NULL)
       continue;             // No Tab found, just skip the line.
-    *p++ = NUL;
+    *p++ = '\0';
     if (STRICMP(su->su_badword, line) == 0) {
       // Match!  Isolate the good word, until CR or NL.
       for (len = 0; p[len] >= ' '; ++len)
         ;
-      p[len] = NUL;
+      p[len] = '\0';
 
       // If the suggestion doesn't have specific case duplicate the case
       // of the bad word.
@@ -9349,7 +9349,7 @@ someerror:
           }
           ga_grow(&ga, 1);
           ((char_u *)ga.ga_data)[ga.ga_len++] = c;
-          if (c == NUL)
+          if (c == '\0')
             break;
         }
         if (ml_append_buf(slang->sl_sugbuf, (linenr_T)wordnr,
@@ -9480,7 +9480,7 @@ static void allcap_copy(char_u *word, char_u *wcopy)
   int c;
 
   d = wcopy;
-  for (s = word; *s != NUL; ) {
+  for (s = word; *s != '\0'; ) {
     if (has_mbyte)
       c = mb_cptr2char_adv(&s);
     else
@@ -9506,7 +9506,7 @@ static void allcap_copy(char_u *word, char_u *wcopy)
       *d++ = c;
     }
   }
-  *d = NUL;
+  *d = '\0';
 }
 
 // Try finding suggestions by recognizing specific situations.
@@ -9525,7 +9525,7 @@ static void suggest_try_special(suginfo_T *su)
     // Include badflags: if the badword is onecap or allcap
     // use that for the goodword too: "The the" -> "The".
     c = su->su_fbadword[len];
-    su->su_fbadword[len] = NUL;
+    su->su_fbadword[len] = '\0';
     make_case_word(su->su_fbadword, word, su->su_badflags);
     su->su_fbadword[len] = c;
 
@@ -9724,7 +9724,7 @@ static void suggest_trie_walk(suginfo_T *su, langp_T *lp, char_u *fword, int sou
 
           // Move the prefix to preword[] with the right case
           // and make find_keepcap_word() works.
-          tword[sp->ts_twordlen] = NUL;
+          tword[sp->ts_twordlen] = '\0';
           make_case_word(tword + sp->ts_splitoff,
               preword + sp->ts_prewordlen, flags);
           sp->ts_prewordlen = (char_u)STRLEN(preword);
@@ -9749,11 +9749,11 @@ static void suggest_trie_walk(suginfo_T *su, langp_T *lp, char_u *fword, int sou
       if (flags & WF_NOSUGGEST)
         break;
 
-      fword_ends = (fword[sp->ts_fidx] == NUL
+      fword_ends = (fword[sp->ts_fidx] == '\0'
                     || (soundfold
                         ? vim_iswhite(fword[sp->ts_fidx])
                         : !spell_iswordp(fword + sp->ts_fidx, curwin)));
-      tword[sp->ts_twordlen] = NUL;
+      tword[sp->ts_twordlen] = '\0';
 
       if (sp->ts_prefixdepth <= PFD_NOTSPECIAL
           && (sp->ts_flags & TSF_PREFIXOK) == 0) {
@@ -9804,7 +9804,7 @@ static void suggest_trie_walk(suginfo_T *su, langp_T *lp, char_u *fword, int sou
               && STRNCMP(fword + sp->ts_splitfidx,
                   tword + sp->ts_splitoff,
                   sp->ts_fidx - sp->ts_splitfidx) == 0) {
-            preword[sp->ts_prewordlen] = NUL;
+            preword[sp->ts_prewordlen] = '\0';
             newscore = score_wordcount_adj(slang, sp->ts_score,
                 preword + sp->ts_prewordlen,
                 sp->ts_prewordlen > 0);
@@ -9834,7 +9834,7 @@ static void suggest_trie_walk(suginfo_T *su, langp_T *lp, char_u *fword, int sou
             break;
 
           compflags[sp->ts_complen] = ((unsigned)flags >> 24);
-          compflags[sp->ts_complen + 1] = NUL;
+          compflags[sp->ts_complen + 1] = '\0';
           vim_strncpy(preword + sp->ts_prewordlen,
               tword + sp->ts_splitoff,
               sp->ts_twordlen - sp->ts_splitoff);
@@ -9846,7 +9846,7 @@ static void suggest_trie_walk(suginfo_T *su, langp_T *lp, char_u *fword, int sou
 
           if (compound_ok) {
             p = preword;
-            while (*skiptowhite(p) != NUL)
+            while (*skiptowhite(p) != '\0')
               p = skipwhite(skiptowhite(p));
             if (fword_ends && !can_compound(slang, p,
                     compflags + sp->ts_compsplit))
@@ -10027,7 +10027,7 @@ static void suggest_trie_walk(suginfo_T *su, langp_T *lp, char_u *fword, int sou
                     compflags, ((unsigned)flags >> 24)))) {
           try_compound = TRUE;
           compflags[sp->ts_complen] = ((unsigned)flags >> 24);
-          compflags[sp->ts_complen + 1] = NUL;
+          compflags[sp->ts_complen + 1] = '\0';
         }
 
         // For NOBREAK we never try splitting, it won't make any word
@@ -10044,7 +10044,7 @@ static void suggest_trie_walk(suginfo_T *su, langp_T *lp, char_u *fword, int sou
           try_compound = FALSE;
           sp->ts_flags |= TSF_DIDSPLIT;
           --sp->ts_curi;                    // do the same NUL again
-          compflags[sp->ts_complen] = NUL;
+          compflags[sp->ts_complen] = '\0';
         } else
           sp->ts_flags &= ~TSF_DIDSPLIT;
 
@@ -10058,7 +10058,7 @@ static void suggest_trie_walk(suginfo_T *su, langp_T *lp, char_u *fword, int sou
                 && (flags & WF_NEEDCOMP))
               break;
             p = preword;
-            while (*skiptowhite(p) != NUL)
+            while (*skiptowhite(p) != '\0')
               p = skipwhite(skiptowhite(p));
             if (sp->ts_complen > sp->ts_compsplit
                 && !can_compound(slang, p,
@@ -10108,7 +10108,7 @@ static void suggest_trie_walk(suginfo_T *su, langp_T *lp, char_u *fword, int sou
                       + sp->ts_fidx,
                       curwin))
                  || fword_ends)
-                && fword[sp->ts_fidx] != NUL
+                && fword[sp->ts_fidx] != '\0'
                 && goodword_ends) {
               int l;
 
@@ -10121,7 +10121,7 @@ static void suggest_trie_walk(suginfo_T *su, langp_T *lp, char_u *fword, int sou
                 memmove(preword + sp->ts_prewordlen,
                     fword + sp->ts_fidx, l);
                 sp->ts_prewordlen += l;
-                preword[sp->ts_prewordlen] = NUL;
+                preword[sp->ts_prewordlen] = '\0';
               } else
                 sp->ts_score -= SCORE_SPLIT - SCORE_SUBST;
               sp->ts_fidx += l;
@@ -10175,7 +10175,7 @@ static void suggest_trie_walk(suginfo_T *su, langp_T *lp, char_u *fword, int sou
     case STATE_ENDNUL:
       // Past the NUL bytes in the node.
       su->su_badflags = sp->ts_save_badflags;
-      if (fword[sp->ts_fidx] == NUL
+      if (fword[sp->ts_fidx] == '\0'
           && sp->ts_tcharlen == 0
           ) {
         // The badword ends, can't use STATE_PLAIN.
@@ -10339,7 +10339,7 @@ static void suggest_trie_walk(suginfo_T *su, langp_T *lp, char_u *fword, int sou
         newscore = 2 * SCORE_DEL / 3;
       else
         newscore = SCORE_DEL;
-      if (fword[sp->ts_fidx] != NUL
+      if (fword[sp->ts_fidx] != '\0'
           && TRY_DEEPER(su, stack, depth, newscore)) {
         go_deeper(stack, depth, newscore);
 #ifdef DEBUG_TRIEWALK
@@ -10390,7 +10390,7 @@ static void suggest_trie_walk(suginfo_T *su, langp_T *lp, char_u *fword, int sou
           sp->ts_state = STATE_SWAP;
           break;
         }
-        if (byts[n + sp->ts_curi] != NUL) {
+        if (byts[n + sp->ts_curi] != '\0') {
           // Found a byte to insert.
           sp->ts_state = STATE_INS;
           break;
@@ -10465,7 +10465,7 @@ static void suggest_trie_walk(suginfo_T *su, langp_T *lp, char_u *fword, int sou
       // STATE_UNSWAP.
       p = fword + sp->ts_fidx;
       c = *p;
-      if (c == NUL) {
+      if (c == '\0') {
         // End of word, can't swap or replace.
         sp->ts_state = STATE_FINAL;
         break;
@@ -10481,15 +10481,15 @@ static void suggest_trie_walk(suginfo_T *su, langp_T *lp, char_u *fword, int sou
       if (has_mbyte) {
         n = mb_cptr2len(p);
         c = mb_ptr2char(p);
-        if (p[n] == NUL)
-          c2 = NUL;
+        if (p[n] == '\0')
+          c2 = '\0';
         else if (!soundfold && !spell_iswordp(p + n, curwin))
           c2 = c;           // don't swap non-word char
         else
           c2 = mb_ptr2char(p + n);
       } else {
-        if (p[1] == NUL)
-          c2 = NUL;
+        if (p[1] == '\0')
+          c2 = '\0';
         else if (!soundfold && !spell_iswordp(p + 1, curwin))
           c2 = c;           // don't swap non-word char
         else
@@ -10497,7 +10497,7 @@ static void suggest_trie_walk(suginfo_T *su, langp_T *lp, char_u *fword, int sou
       }
 
       // When the second character is NUL we can't swap.
-      if (c2 == NUL) {
+      if (c2 == '\0') {
         sp->ts_state = STATE_REP_INI;
         break;
       }
@@ -10508,7 +10508,7 @@ static void suggest_trie_walk(suginfo_T *su, langp_T *lp, char_u *fword, int sou
         sp->ts_state = STATE_SWAP3;
         break;
       }
-      if (c2 != NUL && TRY_DEEPER(su, stack, depth, SCORE_SWAP)) {
+      if (c2 != '\0' && TRY_DEEPER(su, stack, depth, SCORE_SWAP)) {
         go_deeper(stack, depth, SCORE_SWAP);
 #ifdef DEBUG_TRIEWALK
         sprintf(changename[depth], "%.*s-%s: swap %c and %c",
@@ -10575,7 +10575,7 @@ static void suggest_trie_walk(suginfo_T *su, langp_T *lp, char_u *fword, int sou
       // Also skip when c3 is NUL.
       // Also get here when the third character is not a word character.
       // Second character may any char: "a.b" -> "b.a"
-      if (c == c3 || c3 == NUL) {
+      if (c == c3 || c3 == '\0') {
         sp->ts_state = STATE_REP_INI;
         break;
       }
@@ -10896,7 +10896,7 @@ static void find_keepcap_word(slang_T *slang, char_u *fword, char_u *kword)
 
   if (byts == NULL) {
     // array is empty: "cannot happen"
-    *kword = NUL;
+    *kword = '\0';
     return;
   }
 
@@ -10913,11 +10913,11 @@ static void find_keepcap_word(slang_T *slang, char_u *fword, char_u *kword)
   uwordidx[0] = 0;
   kwordlen[0] = 0;
   while (depth >= 0) {
-    if (fword[fwordidx[depth]] == NUL) {
+    if (fword[fwordidx[depth]] == '\0') {
       // We are at the end of "fword".  If the tree allows a word to end
       // here we have found a match.
       if (byts[arridx[depth] + 1] == 0) {
-        kword[kwordlen[depth]] = NUL;
+        kword[kwordlen[depth]] = '\0';
         return;
       }
 
@@ -10992,7 +10992,7 @@ static void find_keepcap_word(slang_T *slang, char_u *fword, char_u *kword)
   }
 
   // Didn't find it: "cannot happen".
-  *kword = NUL;
+  *kword = '\0';
 }
 
 // Compute the sound-a-like score for suggestions in su->su_ga and add them to
@@ -11170,8 +11170,8 @@ stp_sal_score (
     // removing the space.  Don't do it when the good word also contains a
     // space.
     if (vim_iswhite(su->su_badptr[su->su_badlen])
-        && *skiptowhite(stp->st_word) == NUL)
-      for (p = fword; *(p = skiptowhite(p)) != NUL; )
+        && *skiptowhite(stp->st_word) == '\0')
+      for (p = fword; *(p = skiptowhite(p)) != '\0'; )
         STRMOVE(p, p + 1);
 
     spell_soundfold(slang, fword, TRUE, badsound2);
@@ -11337,7 +11337,7 @@ add_sound_suggest (
   // Go over the list of good words that produce this soundfold word
   nrline = ml_get_buf(slang->sl_sugbuf, (linenr_T)(sfwordnr + 1), FALSE);
   orgnr = 0;
-  while (*nrline != NUL) {
+  while (*nrline != '\0') {
     // The wordnr was stored in a minimal nr of bytes as an offset to the
     // previous wordnr.
     orgnr += bytes2offset(&nrline);
@@ -11350,14 +11350,14 @@ add_sound_suggest (
     wordcount = 0;
     for (wlen = 0; wlen < MAXWLEN - 3; ++wlen) {
       i = 1;
-      if (wordcount == orgnr && byts[n + 1] == NUL)
+      if (wordcount == orgnr && byts[n + 1] == '\0')
         break;          // found end of word
 
-      if (byts[n + 1] == NUL)
+      if (byts[n + 1] == '\0')
         ++wordcount;
 
       // skip over the NUL bytes
-      for (; byts[n + i] == NUL; ++i)
+      for (; byts[n + i] == '\0'; ++i)
         if (i > byts[n]) {              // safety check
           STRCPY(theword + wlen, "BAD");
           wlen += 3;
@@ -11376,10 +11376,10 @@ add_sound_suggest (
       n = idxs[n + i];
     }
 badword:
-    theword[wlen] = NUL;
+    theword[wlen] = '\0';
 
     // Go over the possible flags and regions.
-    for (; i <= byts[n] && byts[n + i] == NUL; ++i) {
+    for (; i <= byts[n] && byts[n + i] == '\0'; ++i) {
       char_u cword[MAXWLEN];
       char_u      *p;
       int flags = (int)idxs[n + i];
@@ -11481,12 +11481,12 @@ static int soundfold_find(slang_T *slang, char_u *word)
     // If the first possible byte is a zero the word could end here.
     // If the word ends we found the word.  If not skip the NUL bytes.
     c = ptr[wlen];
-    if (byts[arridx] == NUL) {
-      if (c == NUL)
+    if (byts[arridx] == '\0') {
+      if (c == '\0')
         break;
 
       // Skip over the zeros, there can be several.
-      while (len > 0 && byts[arridx] == NUL) {
+      while (len > 0 && byts[arridx] == '\0') {
         ++arridx;
         --len;
       }
@@ -11496,7 +11496,7 @@ static int soundfold_find(slang_T *slang, char_u *word)
     }
 
     // If the word ends we didn't find it.
-    if (c == NUL)
+    if (c == '\0')
       return -1;
 
     // Perform a binary search in the list of accepted bytes.
@@ -11548,7 +11548,7 @@ static void set_map_str(slang_T *lp, char_u *map)
   int c;
   int i;
 
-  if (*map == NUL) {
+  if (*map == '\0') {
     lp->sl_has_map = FALSE;
     return;
   }
@@ -11562,7 +11562,7 @@ static void set_map_str(slang_T *lp, char_u *map)
   // The similar characters are stored separated with slashes:
   // "aaa/bbb/ccc/".  Fill sl_map_array[c] with the character before c and
   // before the same slash.  For characters above 255 sl_map_hash is used.
-  for (p = map; *p != NUL; ) {
+  for (p = map; *p != '\0'; ) {
     c = mb_cptr2char_adv(&p);
     if (c == '/')
       headc = 0;
@@ -11582,9 +11582,9 @@ static void set_map_str(slang_T *lp, char_u *map)
 
         b = alloc((unsigned)(cl + headcl + 2));
         mb_char2bytes(c, b);
-        b[cl] = NUL;
+        b[cl] = '\0';
         mb_char2bytes(headc, b + cl + 1);
-        b[cl + 1 + headcl] = NUL;
+        b[cl + 1 + headcl] = '\0';
         hash = hash_hash(b);
         hi = hash_lookup(&lp->sl_map_hash, b, hash);
         if (HASHITEM_EMPTY(hi))
@@ -11896,7 +11896,7 @@ char_u *eval_soundfold(char_u *word)
   char_u sound[MAXWLEN];
   int lpi;
 
-  if (curwin->w_p_spell && *curwin->w_s->b_p_spl != NUL)
+  if (curwin->w_p_spell && *curwin->w_s->b_p_spl != '\0')
     // Use the sound-folding of the first language that supports it.
     for (lpi = 0; lpi < curwin->w_s->b_langp.ga_len; ++lpi) {
       lp = LANGP_ENTRY(curwin->w_s->b_langp, lpi);
@@ -11965,7 +11965,7 @@ static void spell_soundfold_sofo(slang_T *slang, char_u *inword, char_u *res)
 
     // The sl_sal_first[] table contains the translation for chars up to
     // 255, sl_sal the rest.
-    for (s = inword; *s != NUL; ) {
+    for (s = inword; *s != '\0'; ) {
       c = mb_cptr2char_adv(&s);
       if (enc_utf8 ? utf_class(c) == 0 : vim_iswhite(c))
         c = ' ';
@@ -11974,11 +11974,11 @@ static void spell_soundfold_sofo(slang_T *slang, char_u *inword, char_u *res)
       else {
         ip = ((int **)slang->sl_sal.ga_data)[c & 0xff];
         if (ip == NULL)                 // empty list, can't match
-          c = NUL;
+          c = '\0';
         else
           for (;; ) {                   // find "c" in the list
             if (*ip == 0) {             // not found
-              c = NUL;
+              c = '\0';
               break;
             }
             if (*ip == c) {             // match!
@@ -11989,7 +11989,7 @@ static void spell_soundfold_sofo(slang_T *slang, char_u *inword, char_u *res)
           }
       }
 
-      if (c != NUL && c != prevc) {
+      if (c != '\0' && c != prevc) {
         ri += mb_char2bytes(c, res + ri);
         if (ri + MB_MAXBYTES > MAXWLEN)
           break;
@@ -11998,17 +11998,17 @@ static void spell_soundfold_sofo(slang_T *slang, char_u *inword, char_u *res)
     }
   } else {
     // The sl_sal_first[] table contains the translation.
-    for (s = inword; (c = *s) != NUL; ++s) {
+    for (s = inword; (c = *s) != '\0'; ++s) {
       if (vim_iswhite(c))
         c = ' ';
       else
         c = slang->sl_sal_first[c];
-      if (c != NUL && (ri == 0 || res[ri - 1] != c))
+      if (c != '\0' && (ri == 0 || res[ri - 1] != c))
         res[ri++] = c;
     }
   }
 
-  res[ri] = NUL;
+  res[ri] = '\0';
 }
 
 static void spell_soundfold_sal(slang_T *slang, char_u *inword, char_u *res)
@@ -12033,7 +12033,7 @@ static void spell_soundfold_sal(slang_T *slang, char_u *inword, char_u *res)
   // But keep white space.  We need a copy, the word may be changed here.
   if (slang->sl_rem_accents) {
     t = word;
-    while (*s != NUL) {
+    while (*s != '\0') {
       if (vim_iswhite(*s)) {
         *t++ = ' ';
         s = skipwhite(s);
@@ -12043,7 +12043,7 @@ static void spell_soundfold_sal(slang_T *slang, char_u *inword, char_u *res)
         ++s;
       }
     }
-    *t = NUL;
+    *t = '\0';
   } else
     vim_strncpy(word, s, MAXWLEN - 1);
 
@@ -12052,7 +12052,7 @@ static void spell_soundfold_sal(slang_T *slang, char_u *inword, char_u *res)
   // This comes from Aspell phonet.cpp.  Converted from C++ to C.
   // Changed to keep spaces.
   i = reslen = z = 0;
-  while ((c = word[i]) != NUL) {
+  while ((c = word[i]) != '\0') {
     // Start with the first rule that has the character in the word.
     n = slang->sl_sal_first[c];
     z0 = 0;
@@ -12077,9 +12077,9 @@ static void spell_soundfold_sal(slang_T *slang, char_u *inword, char_u *res)
 
         if ((pf = smp[n].sm_oneof) != NULL) {
           // Check for match with one of the chars in "sm_oneof".
-          while (*pf != NUL && *pf != word[i + k])
+          while (*pf != '\0' && *pf != word[i + k])
             ++pf;
-          if (*pf == NUL)
+          if (*pf == '\0')
             continue;
           ++k;
         }
@@ -12102,7 +12102,7 @@ static void spell_soundfold_sal(slang_T *slang, char_u *inword, char_u *res)
         if (*s == '^' && *(s + 1) == '^')
           s++;
 
-        if (*s == NUL
+        if (*s == '\0'
             || (*s == '^'
                 && (i == 0 || !(word[i - 1] == ' '
                                 || spell_iswordp(word + i - 1, curwin)))
@@ -12117,7 +12117,7 @@ static void spell_soundfold_sal(slang_T *slang, char_u *inword, char_u *res)
           n0 = slang->sl_sal_first[c0];
 
           if (slang->sl_followup && k > 1 && n0 >= 0
-              && p0 != '-' && word[i + k] != NUL) {
+              && p0 != '-' && word[i + k] != '\0') {
             // test follow-up rule for "word[i + k]"
             for (; (s = smp[n0].sm_lead)[0] == c0; ++n0) {
               // Quickly skip entries that don't match the word.
@@ -12139,9 +12139,9 @@ static void spell_soundfold_sal(slang_T *slang, char_u *inword, char_u *res)
               if ((pf = smp[n0].sm_oneof) != NULL) {
                 // Check for match with one of the chars in
                 // "sm_oneof".
-                while (*pf != NUL && *pf != word[i + k0])
+                while (*pf != '\0' && *pf != word[i + k0])
                   ++pf;
-                if (*pf == NUL)
+                if (*pf == '\0')
                   continue;
                 ++k0;
               }
@@ -12160,7 +12160,7 @@ static void spell_soundfold_sal(slang_T *slang, char_u *inword, char_u *res)
                 s++;
               }
 
-              if (*s == NUL
+              if (*s == '\0'
                   // *s == '^' cuts
                   || (*s == '$'
                       && !spell_iswordp(word + i + k0,
@@ -12189,13 +12189,13 @@ static void spell_soundfold_sal(slang_T *slang, char_u *inword, char_u *res)
           p0 = (vim_strchr(pf, '<') != NULL) ? 1 : 0;
           if (p0 == 1 && z == 0) {
             // rule with '<' is used
-            if (reslen > 0 && *s != NUL && (res[reslen - 1] == c
+            if (reslen > 0 && *s != '\0' && (res[reslen - 1] == c
                                             || res[reslen - 1] == *s))
               reslen--;
             z0 = 1;
             z = 1;
             k0 = 0;
-            while (*s != NUL && word[i + k0] != NUL) {
+            while (*s != '\0' && word[i + k0] != '\0') {
               word[i + k0] = *s;
               k0++;
               s++;
@@ -12209,7 +12209,7 @@ static void spell_soundfold_sal(slang_T *slang, char_u *inword, char_u *res)
             // no '<' rule used
             i += k - 1;
             z = 0;
-            while (*s != NUL && s[1] != NUL && reslen < MAXWLEN) {
+            while (*s != '\0' && s[1] != '\0' && reslen < MAXWLEN) {
               if (reslen == 0 || res[reslen - 1] != *s)
                 res[reslen++] = *s;
               s++;
@@ -12217,7 +12217,7 @@ static void spell_soundfold_sal(slang_T *slang, char_u *inword, char_u *res)
             // new "actual letter"
             c = *s;
             if (strstr((char *)pf, "^^") != NULL) {
-              if (c != NUL)
+              if (c != '\0')
                 res[reslen++] = c;
               STRMOVE(word, word + i + 1);
               i = 0;
@@ -12233,7 +12233,7 @@ static void spell_soundfold_sal(slang_T *slang, char_u *inword, char_u *res)
     }
 
     if (z0 == 0) {
-      if (k && !p0 && reslen < MAXWLEN && c != NUL
+      if (k && !p0 && reslen < MAXWLEN && c != '\0'
           && (!slang->sl_collapse || reslen == 0
               || res[reslen - 1] != c))
         // condense only double letters
@@ -12245,7 +12245,7 @@ static void spell_soundfold_sal(slang_T *slang, char_u *inword, char_u *res)
     }
   }
 
-  res[reslen] = NUL;
+  res[reslen] = '\0';
 }
 
 // Turn "inword" into its sound-a-like equivalent in "res[MAXWLEN]".
@@ -12278,7 +12278,7 @@ static void spell_soundfold_wsal(slang_T *slang, char_u *inword, char_u *res)
   // Remove accents, if wanted.  We actually remove all non-word characters.
   // But keep white space.
   wordlen = 0;
-  for (s = inword; *s != NUL; ) {
+  for (s = inword; *s != '\0'; ) {
     t = s;
     c = mb_cptr2char_adv(&s);
     if (slang->sl_rem_accents) {
@@ -12295,13 +12295,13 @@ static void spell_soundfold_wsal(slang_T *slang, char_u *inword, char_u *res)
     }
     word[wordlen++] = c;
   }
-  word[wordlen] = NUL;
+  word[wordlen] = '\0';
 
   // This algorithm comes from Aspell phonet.cpp.
   // Converted from C++ to C.  Added support for multi-byte chars.
   // Changed to keep spaces.
   i = reslen = z = 0;
-  while ((c = word[i]) != NUL) {
+  while ((c = word[i]) != '\0') {
     // Start with the first rule that has the character in the word.
     n = slang->sl_sal_first[c & 0xff];
     z0 = 0;
@@ -12311,7 +12311,7 @@ static void spell_soundfold_wsal(slang_T *slang, char_u *inword, char_u *res)
       // If c is 0x300 need extra check for the end of the array, as
       // (c & 0xff) is NUL.
       for (; ((ws = smp[n].sm_lead_w)[0] & 0xff) == (c & 0xff)
-           && ws[0] != NUL; ++n) {
+           && ws[0] != '\0'; ++n) {
         // Quickly skip entries that don't match the word.  Most
         // entries are less then three chars, optimize for that.
         if (c != ws[0])
@@ -12331,9 +12331,9 @@ static void spell_soundfold_wsal(slang_T *slang, char_u *inword, char_u *res)
 
         if ((pf = smp[n].sm_oneof_w) != NULL) {
           // Check for match with one of the chars in "sm_oneof".
-          while (*pf != NUL && *pf != word[i + k])
+          while (*pf != '\0' && *pf != word[i + k])
             ++pf;
-          if (*pf == NUL)
+          if (*pf == '\0')
             continue;
           ++k;
         }
@@ -12356,7 +12356,7 @@ static void spell_soundfold_wsal(slang_T *slang, char_u *inword, char_u *res)
         if (*s == '^' && *(s + 1) == '^')
           s++;
 
-        if (*s == NUL
+        if (*s == '\0'
             || (*s == '^'
                 && (i == 0 || !(word[i - 1] == ' '
                                 || spell_iswordp_w(word + i - 1, curwin)))
@@ -12371,7 +12371,7 @@ static void spell_soundfold_wsal(slang_T *slang, char_u *inword, char_u *res)
           n0 = slang->sl_sal_first[c0 & 0xff];
 
           if (slang->sl_followup && k > 1 && n0 >= 0
-              && p0 != '-' && word[i + k] != NUL) {
+              && p0 != '-' && word[i + k] != '\0') {
             // Test follow-up rule for "word[i + k]"; loop over
             // all entries with the same index byte.
             for (; ((ws = smp[n0].sm_lead_w)[0] & 0xff)
@@ -12397,9 +12397,9 @@ static void spell_soundfold_wsal(slang_T *slang, char_u *inword, char_u *res)
               if ((pf = smp[n0].sm_oneof_w) != NULL) {
                 // Check for match with one of the chars in
                 // "sm_oneof".
-                while (*pf != NUL && *pf != word[i + k0])
+                while (*pf != '\0' && *pf != word[i + k0])
                   ++pf;
-                if (*pf == NUL)
+                if (*pf == '\0')
                   continue;
                 ++k0;
               }
@@ -12418,7 +12418,7 @@ static void spell_soundfold_wsal(slang_T *slang, char_u *inword, char_u *res)
                 s++;
               }
 
-              if (*s == NUL
+              if (*s == '\0'
                   // *s == '^' cuts
                   || (*s == '$'
                       && !spell_iswordp_w(word + i + k0,
@@ -12446,7 +12446,7 @@ static void spell_soundfold_wsal(slang_T *slang, char_u *inword, char_u *res)
           p0 = (vim_strchr(s, '<') != NULL) ? 1 : 0;
           if (p0 == 1 && z == 0) {
             // rule with '<' is used
-            if (reslen > 0 && ws != NULL && *ws != NUL
+            if (reslen > 0 && ws != NULL && *ws != '\0'
                 && (wres[reslen - 1] == c
                     || wres[reslen - 1] == *ws))
               reslen--;
@@ -12454,7 +12454,7 @@ static void spell_soundfold_wsal(slang_T *slang, char_u *inword, char_u *res)
             z = 1;
             k0 = 0;
             if (ws != NULL)
-              while (*ws != NUL && word[i + k0] != NUL) {
+              while (*ws != '\0' && word[i + k0] != '\0') {
                 word[i + k0] = *ws;
                 k0++;
                 ws++;
@@ -12470,7 +12470,7 @@ static void spell_soundfold_wsal(slang_T *slang, char_u *inword, char_u *res)
             i += k - 1;
             z = 0;
             if (ws != NULL)
-              while (*ws != NUL && ws[1] != NUL
+              while (*ws != '\0' && ws[1] != '\0'
                      && reslen < MAXWLEN) {
                 if (reslen == 0 || wres[reslen - 1] != *ws)
                   wres[reslen++] = *ws;
@@ -12478,11 +12478,11 @@ static void spell_soundfold_wsal(slang_T *slang, char_u *inword, char_u *res)
               }
             // new "actual letter"
             if (ws == NULL)
-              c = NUL;
+              c = '\0';
             else
               c = *ws;
             if (strstr((char *)s, "^^") != NULL) {
-              if (c != NUL)
+              if (c != '\0')
                 wres[reslen++] = c;
               memmove(word, word + i + 1,
                   sizeof(int) * (wordlen - (i + 1) + 1));
@@ -12499,7 +12499,7 @@ static void spell_soundfold_wsal(slang_T *slang, char_u *inword, char_u *res)
     }
 
     if (z0 == 0) {
-      if (k && !p0 && reslen < MAXWLEN && c != NUL
+      if (k && !p0 && reslen < MAXWLEN && c != '\0'
           && (!slang->sl_collapse || reslen == 0
               || wres[reslen - 1] != c))
         // condense only double letters
@@ -12518,7 +12518,7 @@ static void spell_soundfold_wsal(slang_T *slang, char_u *inword, char_u *res)
     if (l + MB_MAXBYTES > MAXWLEN)
       break;
   }
-  res[l] = NUL;
+  res[l] = '\0';
 }
 
 // Compute a score for two sound-a-like words.
@@ -12543,17 +12543,17 @@ soundalike_score (
   // Adding/inserting "*" at the start (word starts with vowel) shouldn't be
   // counted so much, vowels halfway the word aren't counted at all.
   if ((*badsound == '*' || *goodsound == '*') && *badsound != *goodsound) {
-    if ((badsound[0] == NUL && goodsound[1] == NUL)
-        || (goodsound[0] == NUL && badsound[1] == NUL))
+    if ((badsound[0] == '\0' && goodsound[1] == '\0')
+        || (goodsound[0] == '\0' && badsound[1] == '\0'))
       // changing word with vowel to word without a sound
       return SCORE_DEL;
-    if (badsound[0] == NUL || goodsound[0] == NUL)
+    if (badsound[0] == '\0' || goodsound[0] == '\0')
       // more than two changes
       return SCORE_MAXMAX;
 
     if (badsound[1] == goodsound[1]
-        || (badsound[1] != NUL
-            && goodsound[1] != NUL
+        || (badsound[1] != '\0'
+            && goodsound[1] != '\0'
             && badsound[2] == goodsound[2])) {
       // handle like a substitute
     } else {
@@ -12583,7 +12583,7 @@ soundalike_score (
   }
 
   // Skip over the identical part.
-  while (*pl == *ps && *pl != NUL) {
+  while (*pl == *ps && *pl != '\0') {
     ++pl;
     ++ps;
   }
@@ -12612,7 +12612,7 @@ soundalike_score (
     pl2 = pl + 1;
     ps2 = ps;
     while (*pl2 == *ps2) {
-      if (*pl2 == NUL)                  // reached the end
+      if (*pl2 == '\0')                  // reached the end
         return score + SCORE_DEL;
       ++pl2;
       ++ps2;
@@ -12658,7 +12658,7 @@ soundalike_score (
     // Lengths are equal, thus changes must result in same length: An
     // insert is only possible in combination with a delete.
     // 1: check if for identical strings
-    if (*pl == NUL)
+    if (*pl == '\0')
       return score;
 
     // 2: swap
@@ -12666,7 +12666,7 @@ soundalike_score (
       pl2 = pl + 2;                 // swap, skip two chars
       ps2 = ps + 2;
       while (*pl2 == *ps2) {
-        if (*pl2 == NUL)                // reached the end
+        if (*pl2 == '\0')                // reached the end
           return score + SCORE_SWAP;
         ++pl2;
         ++ps2;
@@ -12685,7 +12685,7 @@ soundalike_score (
     pl2 = pl + 1;
     ps2 = ps + 1;
     while (*pl2 == *ps2) {
-      if (*pl2 == NUL)                  // reached the end
+      if (*pl2 == '\0')                  // reached the end
         return score + SCORE_SUBST;
       ++pl2;
       ++ps2;
@@ -12749,10 +12749,10 @@ static int spell_edit_score(slang_T *slang, char_u *badword, char_u *goodword)
   if (has_mbyte) {
     // Get the characters from the multi-byte strings and put them in an
     // int array for easy access.
-    for (p = badword, badlen = 0; *p != NUL; )
+    for (p = badword, badlen = 0; *p != '\0'; )
       wbadword[badlen++] = mb_cptr2char_adv(&p);
     wbadword[badlen++] = 0;
-    for (p = goodword, goodlen = 0; *p != NUL; )
+    for (p = goodword, goodlen = 0; *p != '\0'; )
       wgoodword[goodlen++] = mb_cptr2char_adv(&p);
     wgoodword[goodlen++] = 0;
   } else {
@@ -12873,7 +12873,7 @@ static int spell_edit_score_limit(slang_T *slang, char_u *badword, char_u *goodw
       gc = goodword[gi];
       if (bc != gc)             // stop at a char that's different
         break;
-      if (bc == NUL) {          // both words end
+      if (bc == '\0') {          // both words end
         if (score < minscore)
           minscore = score;
         goto pop;               // do next alternative
@@ -12882,17 +12882,17 @@ static int spell_edit_score_limit(slang_T *slang, char_u *badword, char_u *goodw
       ++gi;
     }
 
-    if (gc == NUL) {      // goodword ends, delete badword chars
+    if (gc == '\0') {      // goodword ends, delete badword chars
       do {
         if ((score += SCORE_DEL) >= minscore)
           goto pop;                 // do next alternative
-      } while (badword[++bi] != NUL);
+      } while (badword[++bi] != '\0');
       minscore = score;
-    } else if (bc == NUL)   { // badword ends, insert badword chars
+    } else if (bc == '\0')   { // badword ends, insert badword chars
       do {
         if ((score += SCORE_INS) >= minscore)
           goto pop;                 // do next alternative
-      } while (goodword[++gi] != NUL);
+      } while (goodword[++gi] != '\0');
       minscore = score;
     } else {                  // both words continue
       // If not close to the limit, perform a change.  Only try changes
@@ -12909,7 +12909,7 @@ static int spell_edit_score_limit(slang_T *slang, char_u *badword, char_u *goodw
             bi2 = bi + 1 - round;
             gi2 = gi + round;
             while (goodword[gi2] == badword[bi2]) {
-              if (goodword[gi2] == NUL) {
+              if (goodword[gi2] == '\0') {
                 minscore = score_off;
                 break;
               }
@@ -13001,11 +13001,11 @@ static int spell_edit_score_limit_w(slang_T *slang, char_u *badword, char_u *goo
   // Get the characters from the multi-byte strings and put them in an
   // int array for easy access.
   bi = 0;
-  for (p = badword; *p != NUL; )
+  for (p = badword; *p != '\0'; )
     wbadword[bi++] = mb_cptr2char_adv(&p);
   wbadword[bi++] = 0;
   gi = 0;
-  for (p = goodword; *p != NUL; )
+  for (p = goodword; *p != '\0'; )
     wgoodword[gi++] = mb_cptr2char_adv(&p);
   wgoodword[gi++] = 0;
 
@@ -13030,7 +13030,7 @@ static int spell_edit_score_limit_w(slang_T *slang, char_u *badword, char_u *goo
 
       if (bc != gc)             // stop at a char that's different
         break;
-      if (bc == NUL) {          // both words end
+      if (bc == '\0') {          // both words end
         if (score < minscore)
           minscore = score;
         goto pop;               // do next alternative
@@ -13039,17 +13039,17 @@ static int spell_edit_score_limit_w(slang_T *slang, char_u *badword, char_u *goo
       ++gi;
     }
 
-    if (gc == NUL) {      // goodword ends, delete badword chars
+    if (gc == '\0') {      // goodword ends, delete badword chars
       do {
         if ((score += SCORE_DEL) >= minscore)
           goto pop;                 // do next alternative
-      } while (wbadword[++bi] != NUL);
+      } while (wbadword[++bi] != '\0');
       minscore = score;
-    } else if (bc == NUL)   { // badword ends, insert badword chars
+    } else if (bc == '\0')   { // badword ends, insert badword chars
       do {
         if ((score += SCORE_INS) >= minscore)
           goto pop;                 // do next alternative
-      } while (wgoodword[++gi] != NUL);
+      } while (wgoodword[++gi] != '\0');
       minscore = score;
     } else {                  // both words continue
       // If not close to the limit, perform a change.  Only try changes
@@ -13066,7 +13066,7 @@ static int spell_edit_score_limit_w(slang_T *slang, char_u *badword, char_u *goo
             bi2 = bi + 1 - round;
             gi2 = gi + round;
             while (wgoodword[gi2] == wbadword[bi2]) {
-              if (wgoodword[gi2] == NUL) {
+              if (wgoodword[gi2] == '\0') {
                 minscore = score_off;
                 break;
               }
@@ -13331,7 +13331,7 @@ spell_dump_compl (
                     || (flags & WF_REGION) == 0
                     || (((unsigned)flags >> 16)
                         & lp->lp_region) != 0)) {
-              word[depth] = NUL;
+              word[depth] = '\0';
               if (!do_region)
                 flags &= ~WF_REGION;
 
@@ -13551,7 +13551,7 @@ char_u *spell_to_word_end(char_u *start, win_T *win)
 {
   char_u  *p = start;
 
-  while (*p != NUL && spell_iswordp(p, win))
+  while (*p != '\0' && spell_iswordp(p, win))
     mb_ptr_adv(p);
   return p;
 }
diff --git a/src/syntax.c b/src/syntax.c
index 5105a8c161..917d7214c4 100644
--- a/src/syntax.c
+++ b/src/syntax.c
@@ -485,7 +485,7 @@ void syntax_start(win_T *wp, linenr_T lnum)
   int dist;
   static int changedtick = 0;           /* remember the last change ID */
 
-  current_sub_char = NUL;
+  current_sub_char = '\0';
 
   /*
    * After switching buffers, invalidate current_state.
@@ -741,7 +741,7 @@ static void syn_sync(win_T *wp, linenr_T start_lnum, synstate_T *last_valid)
      */
     for (; start_lnum > 1; --start_lnum) {
       line = ml_get(start_lnum - 1);
-      if (*line == NUL || *(line + STRLEN(line) - 1) != '\\')
+      if (*line == '\0' || *(line + STRLEN(line) - 1) != '\\')
         break;
     }
     current_lnum = start_lnum;
@@ -858,7 +858,7 @@ static void syn_sync(win_T *wp, linenr_T start_lnum, synstate_T *last_valid)
              * an item that ends here, need to do that now.  Be
              * careful not to go past the NUL. */
             prev_current_col = current_col;
-            if (syn_getcurline()[current_col] != NUL)
+            if (syn_getcurline()[current_col] != '\0')
               ++current_col;
             check_state_ends();
             current_col = prev_current_col;
@@ -1629,7 +1629,7 @@ syn_finish_line (
          * that ends here, need to do that now.  Be careful not to go
          * past the NUL. */
         prev_current_col = current_col;
-        if (syn_getcurline()[current_col] != NUL)
+        if (syn_getcurline()[current_col] != '\0')
           ++current_col;
         check_state_ends();
         current_col = prev_current_col;
@@ -1740,7 +1740,7 @@ syn_current_attr (
    * Do try matching with an empty line (could be the start of a region).
    */
   line = syn_getcurline();
-  if (line[current_col] == NUL && current_col != 0) {
+  if (line[current_col] == '\0' && current_col != 0) {
     /*
      * If we found a match after the last column, use it.
      */
@@ -1754,7 +1754,7 @@ syn_current_attr (
   }
 
   /* if the current or next character is NUL, we will finish the line now */
-  if (line[current_col] == NUL || line[current_col + 1] == NUL) {
+  if (line[current_col] == '\0' || line[current_col + 1] == '\0') {
     current_finished = TRUE;
     current_state_stored = FALSE;
   }
@@ -2084,7 +2084,7 @@ syn_current_attr (
         if (((current_next_flags & HL_SKIPWHITE)
              && vim_iswhite(line[current_col]))
             || ((current_next_flags & HL_SKIPEMPTY)
-                && *line == NUL))
+                && *line == '\0'))
           break;
       }
 
@@ -2185,7 +2185,7 @@ syn_current_attr (
     if (!syncing && !keep_state) {
       check_state_ends();
       if (current_state.ga_len > 0
-          && syn_getcurline()[current_col] != NUL) {
+          && syn_getcurline()[current_col] != '\0') {
         ++current_col;
         check_state_ends();
         --current_col;
@@ -2200,7 +2200,7 @@ syn_current_attr (
 
   /* nextgroup ends at end of line, unless "skipnl" or "skipempty" present */
   if (current_next_list != NULL
-      && syn_getcurline()[current_col + 1] == NUL
+      && syn_getcurline()[current_col + 1] == '\0'
       && !(current_next_flags & (HL_SKIPNL | HL_SKIPEMPTY)))
     current_next_list = NULL;
 
@@ -2364,7 +2364,7 @@ static void check_state_ends(void)
         current_next_list = cur_si->si_next_list;
         current_next_flags = cur_si->si_flags;
         if (!(current_next_flags & (HL_SKIPNL | HL_SKIPEMPTY))
-            && syn_getcurline()[current_col] == NUL)
+            && syn_getcurline()[current_col] == '\0')
           current_next_list = NULL;
 
         /* When the ended item has "extend", another item with
@@ -2401,7 +2401,7 @@ static void check_state_ends(void)
           check_keepend();
           if ((current_next_flags & HL_HAS_EOL)
               && keepend_level < 0
-              && syn_getcurline()[current_col] == NUL)
+              && syn_getcurline()[current_col] == '\0')
             break;
         }
       }
@@ -2744,12 +2744,12 @@ find_endpos (
         else
           /* Be careful not to jump over the NUL at the end-of-line */
           for (matchcol = regmatch.endpos[0].col;
-               line[matchcol] != NUL && matchcol < pos.col;
+               line[matchcol] != '\0' && matchcol < pos.col;
                ++matchcol)
             ;
 
         /* if the skip pattern includes end-of-line, break here */
-        if (line[matchcol] == NUL)
+        if (line[matchcol] == '\0')
           break;
 
         continue;                   /* start with first end pattern again */
@@ -2873,7 +2873,7 @@ syn_add_end_off (
     base = ml_get_buf(syn_buf, result->lnum, FALSE);
     p = base + col;
     if (off > 0) {
-      while (off-- > 0 && *p != NUL)
+      while (off-- > 0 && *p != '\0')
         mb_ptr_adv(p);
     } else if (off < 0)   {
       while (off++ < 0 && base < p)
@@ -2920,7 +2920,7 @@ syn_add_start_off (
     base = ml_get_buf(syn_buf, result->lnum, FALSE);
     p = base + col;
     if (off > 0) {
-      while (off-- && *p != NUL)
+      while (off-- && *p != '\0')
         mb_ptr_adv(p);
     } else if (off < 0)   {
       while (off++ && base < p)
@@ -3715,7 +3715,7 @@ static void put_pattern(char *s, int c, synpat_T *spp, int attr)
 
   /* output the pattern, in between a char that is not in the pattern */
   for (i = 0; vim_strchr(spp->sp_pattern, sepchars[i]) != NULL; )
-    if (sepchars[++i] == NUL) {
+    if (sepchars[++i] == '\0') {
       i = 0;            /* no good char found, just use the first one */
       break;
     }
@@ -3978,7 +3978,7 @@ get_group_name (
    * Check if there are enough arguments.  The first argument may be a
    * pattern, where '|' is allowed, so only check for NUL.
    */
-  if (ends_excmd(*arg) || *rest == NUL)
+  if (ends_excmd(*arg) || *rest == '\0')
     return NULL;
   return rest;
 }
@@ -4046,10 +4046,10 @@ get_syn_options (
 
     for (fidx = sizeof(flagtab) / sizeof(struct flag); --fidx >= 0; ) {
       p = flagtab[fidx].name;
-      for (i = 0, len = 0; p[i] != NUL; i += 2, ++len)
+      for (i = 0, len = 0; p[i] != '\0'; i += 2, ++len)
         if (arg[len] != p[i] && arg[len] != p[i + 1])
           break;
-      if (p[i] == NUL && (vim_iswhite(arg[len])
+      if (p[i] == '\0' && (vim_iswhite(arg[len])
                           || (flagtab[fidx].argtype > 0
                               ? arg[len] == '='
                               : ends_excmd(arg[len])))) {
@@ -4242,7 +4242,7 @@ static void syn_cmd_keyword(exarg_T *eap, int syncing)
   char_u      *kw;
   syn_opt_arg_T syn_opt_arg;
   int cnt;
-  int conceal_char = NUL;
+  int conceal_char = '\0';
 
   rest = get_group_name(arg, &group_name_end);
 
@@ -4270,12 +4270,12 @@ static void syn_cmd_keyword(exarg_T *eap, int syncing)
       if (rest == NULL || ends_excmd(*rest))
         break;
       /* Copy the keyword, removing backslashes, and add a NUL. */
-      while (*rest != NUL && !vim_iswhite(*rest)) {
-        if (*rest == '\\' && rest[1] != NUL)
+      while (*rest != '\0' && !vim_iswhite(*rest)) {
+        if (*rest == '\\' && rest[1] != '\0')
           ++rest;
         *p++ = *rest++;
       }
-      *p++ = NUL;
+      *p++ = '\0';
       ++cnt;
     }
 
@@ -4289,13 +4289,13 @@ static void syn_cmd_keyword(exarg_T *eap, int syncing)
       for (kw = keyword_copy; --cnt >= 0; kw += STRLEN(kw) + 1) {
         for (p = vim_strchr(kw, '[');; ) {
           if (p != NULL)
-            *p = NUL;
+            *p = '\0';
           add_keyword(kw, syn_id, syn_opt_arg.flags,
               syn_opt_arg.cont_in_list,
               syn_opt_arg.next_list, conceal_char);
           if (p == NULL)
             break;
-          if (p[1] == NUL) {
+          if (p[1] == '\0') {
             EMSG2(_("E789: Missing ']': %s"), kw);
             kw = p + 2;                       /* skip over the NUL */
             break;
@@ -4350,7 +4350,7 @@ syn_cmd_match (
   int idx;
   syn_opt_arg_T syn_opt_arg;
   int sync_idx = 0;
-  int conceal_char = NUL;
+  int conceal_char = '\0';
 
   /* Isolate the group name, check for validity */
   rest = get_group_name(arg, &group_name_end);
@@ -4469,7 +4469,7 @@ syn_cmd_region (
   int success = FALSE;
   int idx;
   syn_opt_arg_T syn_opt_arg;
-  int conceal_char = NUL;
+  int conceal_char = '\0';
 
   /* Isolate the group name, check for validity */
   rest = get_group_name(arg, &group_name_end);
@@ -4528,7 +4528,7 @@ syn_cmd_region (
       break;
     }
     rest = skipwhite(rest + 1);
-    if (*rest == NUL) {
+    if (*rest == '\0') {
       not_enough = TRUE;
       break;
     }
@@ -4980,7 +4980,7 @@ static char_u *get_syn_pattern(char_u *arg, synpat_T *ci)
   char_u      *cpo_save;
 
   /* need at least three chars */
-  if (arg == NULL || arg[1] == NUL || arg[2] == NUL)
+  if (arg == NULL || arg[1] == '\0' || arg[2] == '\0')
     return NULL;
 
   end = skip_regexp(arg + 1, *arg, TRUE, NULL);
@@ -5619,11 +5619,11 @@ void set_context_in_syntax_cmd(expand_T *xp, char_u *arg)
   include_default = 0;
 
   /* (part of) subcommand already typed */
-  if (*arg != NUL) {
+  if (*arg != '\0') {
     p = skiptowhite(arg);
-    if (*p != NUL) {                /* past first word */
+    if (*p != '\0') {                /* past first word */
       xp->xp_pattern = skipwhite(p);
-      if (*skiptowhite(xp->xp_pattern) != NUL)
+      if (*skiptowhite(xp->xp_pattern) != '\0')
         xp->xp_context = EXPAND_NOTHING;
       else if (STRNICMP(arg, "case", p - arg) == 0)
         expand_what = EXP_CASE;
@@ -6448,7 +6448,7 @@ do_highlight (
             || STRCMP(key, "GUI") == 0) {
         attr = 0;
         off = 0;
-        while (arg[off] != NUL) {
+        while (arg[off] != '\0') {
           for (i = sizeof(hl_attr_table) / sizeof(int); --i >= 0; ) {
             len = (int)STRLEN(hl_name_table[i]);
             if (STRNICMP(arg + off, hl_name_table[i], len) == 0) {
@@ -6609,11 +6609,11 @@ do_highlight (
                  * probably an xterm-like terminal.  Use the changed
                  * order for colors.
                  */
-                if (*T_CAF != NUL)
+                if (*T_CAF != '\0')
                   p = T_CAF;
                 else
                   p = T_CSF;
-                if (*p != NUL && *(p + STRLEN(p) - 1) == 'm')
+                if (*p != '\0' && *(p + STRLEN(p) - 1) == 'm')
                   switch (t_colors) {
                   case 16:
                     color = color_numbers_8[i];
@@ -6712,7 +6712,7 @@ do_highlight (
         if (STRNCMP(arg, "t_", 2) == 0) {
           off = 0;
           buf[0] = 0;
-          while (arg[off] != NUL) {
+          while (arg[off] != '\0') {
             /* Isolate one termcap name */
             for (len = 0; arg[off + len] &&
                  arg[off + len] != ','; ++len)
@@ -6752,7 +6752,7 @@ do_highlight (
             else                            /* copy as normal char */
               buf[off++] = *p++;
           }
-          buf[off] = NUL;
+          buf[off] = '\0';
         }
         if (error)
           break;
@@ -7168,10 +7168,10 @@ static int highlight_list_arg(int id, int didh, int type, int iarg, char_u *sarg
     else if (type == LIST_STRING)
       ts = sarg;
     else {   /* type == LIST_ATTR */
-      buf[0] = NUL;
+      buf[0] = '\0';
       for (i = 0; hl_attr_table[i] != 0; ++i) {
         if (iarg & hl_attr_table[i]) {
-          if (buf[0] != NUL)
+          if (buf[0] != '\0')
             vim_strcat(buf, (char_u *)",", 100);
           vim_strcat(buf, (char_u *)hl_name_table[i], 100);
           iarg &= ~hl_attr_table[i];                /* don't want "inverse" */
@@ -7183,7 +7183,7 @@ static int highlight_list_arg(int id, int didh, int type, int iarg, char_u *sarg
         (int)(vim_strsize(ts) + STRLEN(name) + 1), id);
     didh = TRUE;
     if (!got_int) {
-      if (*name != NUL) {
+      if (*name != '\0') {
         MSG_PUTS_ATTR(name, hl_attr(HLF_D));
         MSG_PUTS_ATTR("=", hl_attr(HLF_D));
       }
@@ -7475,7 +7475,7 @@ static int syn_add_group(char_u *name)
   char_u      *p;
 
   /* Check that the name is ASCII letters, digits and underscore. */
-  for (p = name; *p != NUL; ++p) {
+  for (p = name; *p != '\0'; ++p) {
     if (!vim_isprintc(*p)) {
       EMSG(_("E669: Unprintable character in group name"));
       vim_free(name);
@@ -7622,7 +7622,7 @@ int highlight_changed(void)
         if (hl_flags[hlf] == *p)
           break;
       ++p;
-      if (hlf == (int)HLF_COUNT || *p == NUL)
+      if (hlf == (int)HLF_COUNT || *p == '\0')
         return FAIL;
 
       /*
@@ -7654,7 +7654,7 @@ int highlight_changed(void)
         case 'c':   attr |= HL_UNDERCURL;
           break;
         case ':':   ++p;                            /* highlight group name */
-          if (attr || *p == NUL)                         /* no combinations */
+          if (attr || *p == '\0')                         /* no combinations */
             return FAIL;
           end = vim_strchr(p, ',');
           if (end == NULL)
@@ -7758,16 +7758,16 @@ void set_context_in_highlight_cmd(expand_T *xp, char_u *arg)
   include_default = 1;
 
   /* (part of) subcommand already typed */
-  if (*arg != NUL) {
+  if (*arg != '\0') {
     p = skiptowhite(arg);
-    if (*p != NUL) {                    /* past "default" or group name */
+    if (*p != '\0') {                    /* past "default" or group name */
       include_default = 0;
       if (STRNCMP("default", arg, p - arg) == 0) {
         arg = skipwhite(p);
         xp->xp_pattern = arg;
         p = skiptowhite(arg);
       }
-      if (*p != NUL) {                          /* past group name */
+      if (*p != '\0') {                          /* past group name */
         include_link = 0;
         if (arg[1] == 'i' && arg[0] == 'N')
           highlight_list();
@@ -7775,12 +7775,12 @@ void set_context_in_highlight_cmd(expand_T *xp, char_u *arg)
             || STRNCMP("clear", arg, p - arg) == 0) {
           xp->xp_pattern = skipwhite(p);
           p = skiptowhite(xp->xp_pattern);
-          if (*p != NUL) {                      /* past first group name */
+          if (*p != '\0') {                      /* past first group name */
             xp->xp_pattern = skipwhite(p);
             p = skiptowhite(xp->xp_pattern);
           }
         }
-        if (*p != NUL)                          /* past group name(s) */
+        if (*p != '\0')                          /* past group name(s) */
           xp->xp_context = EXPAND_NOTHING;
       }
     }
diff --git a/src/tag.c b/src/tag.c
index 47f7e1b97f..f775640961 100644
--- a/src/tag.c
+++ b/src/tag.c
@@ -211,7 +211,7 @@ do_tag (
   /*
    * Don't add a tag to the tagstack if 'tagstack' has been reset.
    */
-  if ((!p_tgst && *tag != NUL)) {
+  if ((!p_tgst && *tag != '\0')) {
     use_tagstack = FALSE;
     new_tag = TRUE;
   } else {
@@ -221,7 +221,7 @@ do_tag (
       use_tagstack = TRUE;
 
     /* new pattern, add to the tag stack */
-    if (*tag != NUL
+    if (*tag != '\0'
         && (type == DT_TAG || type == DT_SELECT || type == DT_JUMP
             || type == DT_LTAG
             || type == DT_CSCOPE
@@ -451,7 +451,7 @@ do_tag (
        * If a count is supplied to the ":tag <name>" command, then
        * jump to count'th matching tag.
        */
-      if (type == DT_TAG && *tag != NUL && count > 0)
+      if (type == DT_TAG && *tag != '\0' && count > 0)
         cur_match = count - 1;
 
       if (type == DT_SELECT || type == DT_JUMP
@@ -699,7 +699,7 @@ do_tag (
           if (len > 128)
             len = 128;
           vim_strncpy(tag_name, tagp.tagname, len);
-          tag_name[len] = NUL;
+          tag_name[len] = '\0';
 
           /* Save the tag file name */
           p = tag_full_fname(&tagp);
@@ -749,7 +749,7 @@ do_tag (
               cmd_end--;
 
             len = 0;
-            cmd[0] = NUL;
+            cmd[0] = '\0';
 
             /*
              * If "^" is present in the tag search pattern, then
@@ -784,7 +784,7 @@ do_tag (
               len++;
             }
 
-            cmd[len] = NUL;
+            cmd[len] = '\0';
           }
 
           if ((dict = dict_alloc()) == NULL)
@@ -1012,7 +1012,7 @@ static int tag_strnicmp(char_u *s1, char_u *s2, size_t len)
     i = (int)TOUPPER_ASC(*s1) - (int)TOUPPER_ASC(*s2);
     if (i != 0)
       return i;                         /* this character different */
-    if (*s1 == NUL)
+    if (*s1 == '\0')
       break;                            /* strings match until NUL */
     ++s1;
     ++s2;
@@ -1051,7 +1051,7 @@ static void prepare_pats(pat_T *pats, int has_re)
     if (pats->head == pats->pat)
       pats->headlen = 0;
     else
-      for (pats->headlen = 0; pats->head[pats->headlen] != NUL;
+      for (pats->headlen = 0; pats->head[pats->headlen] != '\0';
            ++pats->headlen)
         if (vim_strchr((char_u *)(p_magic ? ".[~*\\$" : "\\$"),
                 pats->head[pats->headlen]) != NULL)
@@ -1118,7 +1118,7 @@ find_tags (
   char_u      *p;
   char_u      *s;
   int i;
-  int tag_file_sorted = NUL;            /* !_TAG_FILE_SORTED value */
+  int tag_file_sorted = '\0';            /* !_TAG_FILE_SORTED value */
   struct tag_search_info        /* Binary search file offsets */
   {
     off_t low_offset;           /* offset for first char of first line that
@@ -1287,14 +1287,14 @@ find_tags (
             help_pri = 0;
           else {
             help_pri = 1;
-            for (s = p_hlg; *s != NUL; ++s) {
+            for (s = p_hlg; *s != '\0'; ++s) {
               if (STRNICMP(s, help_lang, 2) == 0)
                 break;
               ++help_pri;
               if ((s = vim_strchr(s, ',')) == NULL)
                 break;
             }
-            if (s == NULL || *s == NUL) {
+            if (s == NULL || *s == '\0') {
               /* Language not in 'helplang': use last, prefer English,
                * unless found already. */
               ++help_pri;
@@ -1467,7 +1467,7 @@ line_read_in:
                * encoding to 'encoding'. */
               for (p = lbuf + 20; *p > ' ' && *p < 127; ++p)
                 ;
-              *p = NUL;
+              *p = '\0';
               convert_setup(&vimconv, lbuf + 20, p_enc);
             }
 
@@ -1488,7 +1488,7 @@ line_read_in:
            */
           if (linear || use_cscope)
             state = TS_LINEAR;
-          else if (tag_file_sorted == NUL)
+          else if (tag_file_sorted == '\0')
             state = TS_BINARY;
           else if (tag_file_sorted == '1')
             state = TS_BINARY;
@@ -1542,7 +1542,7 @@ parse_line:
           tagp.tagname = lbuf;
 #ifdef FEAT_TAG_ANYWHITE
           tagp.tagname_end = skiptowhite(lbuf);
-          if (*tagp.tagname_end == NUL)
+          if (*tagp.tagname_end == '\0')
 #else
           tagp.tagname_end = vim_strchr(lbuf, TAB);
           if (tagp.tagname_end == NULL)
@@ -1707,7 +1707,7 @@ parse_line:
 #ifdef FEAT_TAG_ANYWHITE
           tagp.fname_end = skiptowhite(tagp.fname);
           tagp.command = skipwhite(tagp.fname_end);
-          if (*tagp.command == NUL)
+          if (*tagp.command == '\0')
 #else
           tagp.fname_end = vim_strchr(tagp.fname, TAB);
           tagp.command = tagp.fname_end + 1;
@@ -1752,7 +1752,7 @@ parse_line:
           int cc;
 
           cc = *tagp.tagname_end;
-          *tagp.tagname_end = NUL;
+          *tagp.tagname_end = '\0';
           match = vim_regexec(&orgpat.regmatch, tagp.tagname, (colnr_T)0);
           if (match) {
             matchoff = (int)(orgpat.regmatch.startp[0] - tagp.tagname);
@@ -1820,7 +1820,7 @@ parse_line:
                * Append the help-heuristic number after the
                * tagname, for sorting it later.
                */
-              *tagp.tagname_end = NUL;
+              *tagp.tagname_end = '\0';
               len = (int)(tagp.tagname_end - tagp.tagname);
               mfp = (struct match_found *)
                     alloc((int)sizeof(struct match_found) + len
@@ -1940,7 +1940,7 @@ parse_line:
       if (vimconv.vc_type != CONV_NONE)
         convert_setup(&vimconv, NULL, NULL);
 
-      tag_file_sorted = NUL;
+      tag_file_sorted = '\0';
       if (sort_error) {
         EMSG2(_("E432: Tags file not sorted: %s"), tag_fname);
         sort_error = FALSE;
@@ -2085,7 +2085,7 @@ get_tagfname (
     if (tnp->tn_hf_idx >= tag_fnames.ga_len) {
       /* Not found in 'runtimepath', use 'helpfile', if it exists and
        * wasn't used yet, replacing "help.txt" with "tags". */
-      if (tnp->tn_hf_idx > tag_fnames.ga_len || *p_hf == NUL)
+      if (tnp->tn_hf_idx > tag_fnames.ga_len || *p_hf == '\0')
         return FAIL;
       ++tnp->tn_hf_idx;
       STRCPY(buf, p_hf);
@@ -2099,7 +2099,7 @@ get_tagfname (
   if (first) {
     /* Init.  We make a copy of 'tags', because autocommands may change
      * the value without notifying us. */
-    tnp->tn_tags = vim_strsave((*curbuf->b_p_tags != NUL)
+    tnp->tn_tags = vim_strsave((*curbuf->b_p_tags != '\0')
         ? curbuf->b_p_tags : p_tags);
     if (tnp->tn_tags == NULL)
       return FAIL;
@@ -2123,7 +2123,7 @@ get_tagfname (
       char_u  *filename = NULL;
 
       /* Stop when used all parts of 'tags'. */
-      if (*tnp->tn_np == NUL) {
+      if (*tnp->tn_np == '\0') {
         vim_findfile_cleanup(tnp->tn_search_ctx);
         tnp->tn_search_ctx = NULL;
         return FAIL;
@@ -2132,7 +2132,7 @@ get_tagfname (
       /*
        * Copy next file name into buf.
        */
-      buf[0] = NUL;
+      buf[0] = '\0';
       (void)copy_option_part(&tnp->tn_np, buf, MAXPATHL - 1, " ,");
 
       r_ptr = vim_findfile_stopdir(buf);
@@ -2140,7 +2140,7 @@ get_tagfname (
        * filepath with a NUL */
       filename = path_tail(buf);
       STRMOVE(filename + 1, filename);
-      *filename++ = NUL;
+      *filename++ = '\0';
 
       tnp->tn_search_ctx = vim_findfile_init(buf, filename,
           r_ptr, 100,
@@ -2199,7 +2199,7 @@ parse_tag_line (
 #ifdef FEAT_TAG_ANYWHITE
   p = skipwhite(p);
 #else
-  if (*p != NUL)
+  if (*p != '\0')
     ++p;
 #endif
   tagp->fname = p;
@@ -2216,10 +2216,10 @@ parse_tag_line (
 #ifdef FEAT_TAG_ANYWHITE
   p = skipwhite(p);
 #else
-  if (*p != NUL)
+  if (*p != '\0')
     ++p;
 #endif
-  if (*p == NUL)
+  if (*p == '\0')
     return FAIL;
   tagp->command = p;
 
@@ -2346,7 +2346,7 @@ static char_u *tag_full_fname(tagptrs_T *tagp)
 
   {
     c = *tagp->fname_end;
-    *tagp->fname_end = NUL;
+    *tagp->fname_end = '\0';
   }
   fullname = expand_tag_fname(tagp->fname, tagp->tag_fname, FALSE);
 
@@ -2396,7 +2396,7 @@ jumpto_tag (
 
   /* truncate the file name, so it can be used as a string */
   csave = *tagp.fname_end;
-  *tagp.fname_end = NUL;
+  *tagp.fname_end = '\0';
   fname = tagp.fname;
 
   /* copy the command to pbuf[], remove trailing CR/NL */
@@ -2404,7 +2404,7 @@ jumpto_tag (
   for (pbuf_end = pbuf; *str && *str != '\n' && *str != '\r'; ) {
     *pbuf_end++ = *str++;
   }
-  *pbuf_end = NUL;
+  *pbuf_end = '\0';
 
   {
     /*
@@ -2413,7 +2413,7 @@ jumpto_tag (
     str = pbuf;
     if (find_extra(&str) == OK) {
       pbuf_end = str;
-      *pbuf_end = NUL;
+      *pbuf_end = '\0';
     }
   }
 
@@ -2548,7 +2548,7 @@ jumpto_tag (
           found = 2;
           (void)test_for_static(&tagp);
           cc = *tagp.tagname_end;
-          *tagp.tagname_end = NUL;
+          *tagp.tagname_end = '\0';
           sprintf((char *)pbuf, "^%s\\s\\*(", tagp.tagname);
           if (!do_search(NULL, '/', pbuf, (long)1,
                   search_options, NULL)) {
@@ -2710,7 +2710,7 @@ static int test_for_current(char_u *fname, char_u *fname_end, char_u *tag_fname,
   if (buf_ffname != NULL) {     /* if the buffer has a name */
     {
       c = *fname_end;
-      *fname_end = NUL;
+      *fname_end = '\0';
     }
     fullname = expand_tag_fname(fname, tag_fname, TRUE);
     if (fullname != NULL) {
@@ -2843,7 +2843,7 @@ add_tag_field (
       len = MAXPATHL - 1;
     vim_strncpy(buf, start, len);
   }
-  buf[len] = NUL;
+  buf[len] = '\0';
   retval = dict_add_nr_str(dict, field_name, 0L, buf);
   vim_free(buf);
   return retval;
@@ -2892,7 +2892,7 @@ int get_tags(list_T *list, char_u *pat)
 
       if (tp.command_end != NULL) {
         for (p = tp.command_end + 3;
-             *p != NUL && *p != '\n' && *p != '\r'; ++p) {
+             *p != '\0' && *p != '\n' && *p != '\r'; ++p) {
           if (p == tp.tagkind || (p + 5 == tp.tagkind
                                   && STRNCMP(p, "kind:", 5) == 0))
             /* skip "kind:<kind>" and "<kind>" */
@@ -2907,22 +2907,22 @@ int get_tags(list_T *list, char_u *pat)
             /* Add extra field as a dict entry.  Fields are
              * separated by Tabs. */
             n = p;
-            while (*p != NUL && *p >= ' ' && *p < 127 && *p != ':')
+            while (*p != '\0' && *p >= ' ' && *p < 127 && *p != ':')
               ++p;
             len = (int)(p - n);
             if (*p == ':' && len > 0) {
               s = ++p;
-              while (*p != NUL && *p >= ' ')
+              while (*p != '\0' && *p >= ' ')
                 ++p;
-              n[len] = NUL;
+              n[len] = '\0';
               if (add_tag_field(dict, (char *)n, s, p) == FAIL)
                 ret = FAIL;
               n[len] = ':';
             } else
               /* Skip field without colon. */
-              while (*p != NUL && *p >= ' ')
+              while (*p != '\0' && *p >= ' ')
                 ++p;
-            if (*p == NUL)
+            if (*p == '\0')
               break;
           }
         }
diff --git a/src/term.c b/src/term.c
index 0483af8401..ea0d8d79ae 100644
--- a/src/term.c
+++ b/src/term.c
@@ -1232,7 +1232,7 @@ static void set_color_count(int nr)
   if (t_colors > 1)
     sprintf((char *)nr_colors, "%d", t_colors);
   else
-    *nr_colors = NUL;
+    *nr_colors = '\0';
   set_string_option_direct((char_u *)"t_Co", -1, nr_colors, OPT_FREE, 0);
 }
 
@@ -1520,9 +1520,9 @@ int set_termname(char_u *term)
   {
     bs_p = find_termcode((char_u *)"kb");
     del_p = find_termcode((char_u *)"kD");
-    if (bs_p == NULL || *bs_p == NUL)
+    if (bs_p == NULL || *bs_p == '\0')
       add_termcode((char_u *)"kb", (bs_p = (char_u *)CTRL_H_STR), FALSE);
-    if ((del_p == NULL || *del_p == NUL) &&
+    if ((del_p == NULL || *del_p == '\0') &&
         (bs_p == NULL || *bs_p != DEL))
       add_termcode((char_u *)"kD", (char_u *)DEL_STR, FALSE);
   }
@@ -1771,7 +1771,7 @@ getlinecol (
 {
   char_u tbuf[TBUFSZ];
 
-  if (T_NAME != NULL && *T_NAME != NUL &&
+  if (T_NAME != NULL && *T_NAME != '\0' &&
       tgetent_error(tbuf, T_NAME) == NULL) {
     if (*cp == 0)
       *cp = tgetnum("co");
@@ -1812,7 +1812,7 @@ int add_termcap_entry(char_u *name, int force)
     return OK;
 
   term = T_NAME;
-  if (term == NULL || *term == NUL)         /* 'term' not defined yet */
+  if (term == NULL || *term == '\0')         /* 'term' not defined yet */
     return FAIL;
 
   if (term_is_builtin(term)) {              /* name starts with "builtin_" */
@@ -1861,7 +1861,7 @@ int add_termcap_entry(char_u *name, int force)
     error_msg = tgetent_error(tbuf, term);
     if (error_msg == NULL) {
       string = TGETSTR((char *)name, &tp);
-      if (string != NULL && *string != NUL) {
+      if (string != NULL && *string != '\0') {
         add_termcode(name, string, FALSE);
         return OK;
       }
@@ -1992,13 +1992,13 @@ void termcapinit(char_u *name)
 {
   char_u      *term;
 
-  if (name != NULL && *name == NUL)
+  if (name != NULL && *name == '\0')
     name = NULL;            /* empty name is equal to no name */
   term = name;
 
   if (term == NULL)
     term = (char_u *)os_getenv("TERM");
-  if (term == NULL || *term == NUL)
+  if (term == NULL || *term == '\0')
     term = DEFAULT_TERM;
   set_string_option_direct((char_u *)"term", -1, term, OPT_FREE, 0);
 
@@ -2200,7 +2200,7 @@ static void term_color(char_u *s, int n)
   /* Also accept CSI instead of <Esc>[ */
   if (n >= 8 && t_colors >= 16
       && ((s[0] == ESC && s[1] == '[') || (s[0] == CSI && (i = 1) == 1))
-      && s[i] != NUL
+      && s[i] != '\0'
       && (STRCMP(s + i + 1, "%p1%dm") == 0
           || STRCMP(s + i + 1, "%dm") == 0)
       && (s[i] == '3' || s[i] == '4')) {
@@ -2246,13 +2246,13 @@ void ttest(int pairs)
   /*
    * MUST have "cm": cursor motion.
    */
-  if (*T_CM == NUL)
+  if (*T_CM == '\0')
     EMSG(_("E437: terminal capability \"cm\" required"));
 
   /*
    * if "cs" defined, use a scroll region, it's faster.
    */
-  if (*T_CS != NUL)
+  if (*T_CS != '\0')
     scroll_region = TRUE;
   else
     scroll_region = FALSE;
@@ -2262,62 +2262,62 @@ void ttest(int pairs)
      * optional pairs
      */
     /* TP goes to normal mode for TI (invert) and TB (bold) */
-    if (*T_ME == NUL)
+    if (*T_ME == '\0')
       T_ME = T_MR = T_MD = T_MB = empty_option;
-    if (*T_SO == NUL || *T_SE == NUL)
+    if (*T_SO == '\0' || *T_SE == '\0')
       T_SO = T_SE = empty_option;
-    if (*T_US == NUL || *T_UE == NUL)
+    if (*T_US == '\0' || *T_UE == '\0')
       T_US = T_UE = empty_option;
-    if (*T_CZH == NUL || *T_CZR == NUL)
+    if (*T_CZH == '\0' || *T_CZR == '\0')
       T_CZH = T_CZR = empty_option;
 
     /* T_VE is needed even though T_VI is not defined */
-    if (*T_VE == NUL)
+    if (*T_VE == '\0')
       T_VI = empty_option;
 
     /* if 'mr' or 'me' is not defined use 'so' and 'se' */
-    if (*T_ME == NUL) {
+    if (*T_ME == '\0') {
       T_ME = T_SE;
       T_MR = T_SO;
       T_MD = T_SO;
     }
 
     /* if 'so' or 'se' is not defined use 'mr' and 'me' */
-    if (*T_SO == NUL) {
+    if (*T_SO == '\0') {
       T_SE = T_ME;
-      if (*T_MR == NUL)
+      if (*T_MR == '\0')
         T_SO = T_MD;
       else
         T_SO = T_MR;
     }
 
     /* if 'ZH' or 'ZR' is not defined use 'mr' and 'me' */
-    if (*T_CZH == NUL) {
+    if (*T_CZH == '\0') {
       T_CZR = T_ME;
-      if (*T_MR == NUL)
+      if (*T_MR == '\0')
         T_CZH = T_MD;
       else
         T_CZH = T_MR;
     }
 
     /* "Sb" and "Sf" come in pairs */
-    if (*T_CSB == NUL || *T_CSF == NUL) {
+    if (*T_CSB == '\0' || *T_CSF == '\0') {
       T_CSB = empty_option;
       T_CSF = empty_option;
     }
 
     /* "AB" and "AF" come in pairs */
-    if (*T_CAB == NUL || *T_CAF == NUL) {
+    if (*T_CAB == '\0' || *T_CAF == '\0') {
       T_CAB = empty_option;
       T_CAF = empty_option;
     }
 
     /* if 'Sb' and 'AB' are not defined, reset "Co" */
-    if (*T_CSB == NUL && *T_CAB == NUL)
+    if (*T_CSB == '\0' && *T_CAB == '\0')
       free_one_termoption(T_CCO);
 
     /* Set 'weirdinvert' according to value of 't_xs' */
-    p_wiv = (*T_XS != NUL);
+    p_wiv = (*T_XS != '\0');
   }
   need_gather = TRUE;
 
@@ -2387,13 +2387,13 @@ static int get_bytes_from_buf(char_u *buf, char_u *bytes, int num_bytes)
   char_u c;
 
   for (i = 0; i < num_bytes; i++) {
-    if ((c = buf[len++]) == NUL)
+    if ((c = buf[len++]) == '\0')
       return -1;
     if (c == K_SPECIAL) {
-      if (buf[len] == NUL || buf[len + 1] == NUL)           /* cannot happen? */
+      if (buf[len] == '\0' || buf[len + 1] == '\0')           /* cannot happen? */
         return -1;
       if (buf[len++] == (int)KS_ZERO)
-        c = NUL;
+        c = '\0';
       /* else it should be KS_SPECIAL; when followed by KE_FILLER c is
        * K_SPECIAL, or followed by KE_CSI and c must be CSI. */
       if (buf[len++] == (int)KE_CSI)
@@ -2691,7 +2691,7 @@ void may_req_termresponse(void)
       && isatty(1)
       && isatty(read_cmd_fd)
 # endif
-      && *T_CRV != NUL) {
+      && *T_CRV != '\0') {
     LOG_TR("Sending CRV");
     out_str(T_CRV);
     crv_status = CRV_SENT;
@@ -2721,7 +2721,7 @@ void may_req_ambiguous_char_width(void)
       && isatty(1)
       && isatty(read_cmd_fd)
 #  endif
-      && *T_U7 != NUL
+      && *T_U7 != '\0'
       && !option_was_set((char_u *)"ambiwidth")) {
     char_u buf[16];
 
@@ -2770,7 +2770,7 @@ static void log_tr(char *msg)                 {
  */
 int swapping_screen(void)
 {
-  return full_screen && *T_TI != NUL;
+  return full_screen && *T_TI != '\0';
 }
 
 /*
@@ -2782,7 +2782,7 @@ void setmouse(void)
 
 
   /* be quick when mouse is off */
-  if (*p_mouse == NUL || has_mouse_termcode == 0)
+  if (*p_mouse == '\0' || has_mouse_termcode == 0)
     return;
 
   /* don't switch mouse on when not in raw mode (Ex mode) */
@@ -2849,7 +2849,7 @@ int mouse_model_popup(void)
  */
 void scroll_start(void)
 {
-  if (*T_VS != NUL) {
+  if (*T_VS != '\0') {
     out_str(T_VS);
     out_str(T_VE);
     screen_start();                     /* don't know where cursor is now */
@@ -2888,7 +2888,7 @@ void term_cursor_shape(void)
 {
   static int showing_insert_mode = MAYBE;
 
-  if (!full_screen || *T_CSI == NUL || *T_CEI == NUL)
+  if (!full_screen || *T_CSI == '\0' || *T_CEI == '\0')
     return;
 
   if (State & INSERT) {
@@ -2912,7 +2912,7 @@ void scroll_region_set(win_T *wp, int off)
 {
   OUT_STR(tgoto((char *)T_CS, W_WINROW(wp) + wp->w_height - 1,
           W_WINROW(wp) + off));
-  if (*T_CSV != NUL && wp->w_width != Columns)
+  if (*T_CSV != '\0' && wp->w_width != Columns)
     OUT_STR(tgoto((char *)T_CSV, W_WINCOL(wp) + wp->w_width - 1,
             W_WINCOL(wp)));
   screen_start();                   /* don't know where cursor is now */
@@ -2924,7 +2924,7 @@ void scroll_region_set(win_T *wp, int off)
 void scroll_region_reset(void)
 {
   OUT_STR(tgoto((char *)T_CS, (int)Rows - 1, 0));
-  if (*T_CSV != NUL)
+  if (*T_CSV != '\0')
     OUT_STR(tgoto((char *)T_CSV, (int)Columns - 1, 0));
   screen_start();                   /* don't know where cursor is now */
 }
@@ -2956,7 +2956,7 @@ void clear_termcodes(void)
 #ifdef HAVE_TGETENT
   BC = (char *)empty_option;
   UP = (char *)empty_option;
-  PC = NUL;                     /* set pad character to NUL */
+  PC = '\0';                     /* set pad character to NUL */
   ospeed = 0;
 #endif
 
@@ -2978,7 +2978,7 @@ void add_termcode(char_u *name, char_u *string, int flags)
   char_u          *s;
   int len;
 
-  if (string == NULL || *string == NUL) {
+  if (string == NULL || *string == '\0') {
     del_termcode(name);
     return;
   }
@@ -3271,7 +3271,7 @@ int check_termcode(int max_offset, char_u *buf, int bufsize, int *buflen)
     i = *tp;
     for (p = termleader; *p && *p != i; ++p)
       ;
-    if (*p == NUL)
+    if (*p == '\0')
       continue;
 
     /*
@@ -3281,8 +3281,8 @@ int check_termcode(int max_offset, char_u *buf, int bufsize, int *buflen)
     if (*tp == ESC && !p_ek && (State & INSERT))
       continue;
 
-    key_name[0] = NUL;          /* no key name found yet */
-    key_name[1] = NUL;          /* no key name found yet */
+    key_name[0] = '\0';          /* no key name found yet */
+    key_name[1] = '\0';          /* no key name found yet */
     modifiers = 0;              /* no modifiers yet */
 
     {
@@ -3377,7 +3377,7 @@ int check_termcode(int max_offset, char_u *buf, int bufsize, int *buflen)
       }
     }
 
-    if (key_name[0] == NUL
+    if (key_name[0] == '\0'
         /* URXVT mouse uses <ESC>[#;#;#M, but we are matching <ESC>[ */
         || key_name[0] == KS_URXVT_MOUSE
         || u7_status == U7_SENT
@@ -3396,7 +3396,7 @@ int check_termcode(int max_offset, char_u *buf, int bufsize, int *buflen)
        *   ambiguous-width character state.
        */
       p = tp[0] == CSI ? tp + 1 : tp + 2;
-      if ((*T_CRV != NUL || *T_U7 != NUL)
+      if ((*T_CRV != '\0' || *T_U7 != '\0')
           && ((tp[0] == ESC && tp[1] == '[' && len >= 3)
               || (tp[0] == CSI && len >= 2))
           && (VIM_ISDIGIT(*p) || *p == '>' || *p == '?')) {
@@ -3459,7 +3459,7 @@ int check_termcode(int max_offset, char_u *buf, int bufsize, int *buflen)
           slen = i + 1;
         } else
         /* eat it when at least one digit and ending in 'c' */
-        if (*T_CRV != NUL && i > 2 + (tp[0] != CSI) && tp[i] == 'c') {
+        if (*T_CRV != '\0' && i > 2 + (tp[0] != CSI) && tp[i] == 'c') {
           LOG_TR("Received CRV");
           crv_status = CRV_GOT;
           did_cursorhold = TRUE;
@@ -3533,7 +3533,7 @@ int check_termcode(int max_offset, char_u *buf, int bufsize, int *buflen)
       }
     }
 
-    if (key_name[0] == NUL)
+    if (key_name[0] == '\0')
       continue;             /* No match at this position, try next one */
 
     /* We only get here when we have a complete termcode match */
@@ -3592,8 +3592,8 @@ int check_termcode(int max_offset, char_u *buf, int bufsize, int *buflen)
           j = termcodes[idx].len;
           if (STRNCMP(tp, tp + slen, (size_t)j) == 0
               && tp[slen + j] == mouse_code
-              && tp[slen + j + 1] != NUL
-              && tp[slen + j + 2] != NUL
+              && tp[slen + j + 1] != '\0'
+              && tp[slen + j + 2] != '\0'
               )
             slen += j;
           else
@@ -4103,7 +4103,7 @@ int check_termcode(int max_offset, char_u *buf, int bufsize, int *buflen)
       string[new_slen++] = key_name[0];
       string[new_slen++] = key_name[1];
     }
-    string[new_slen] = NUL;
+    string[new_slen] = '\0';
     extra = new_slen - slen;
     if (buf == NULL) {
       if (extra < 0)
@@ -4208,7 +4208,7 @@ replace_termcodes (
   /*
    * Copy each byte from *from to result[dlen]
    */
-  while (*src != NUL) {
+  while (*src != '\0') {
     /*
      * If 'cpoptions' does not contain '<', check for special key codes,
      * like "<C-S-LeftMouse>"
@@ -4278,11 +4278,11 @@ replace_termcodes (
       }
       if (len != 0) {
         /* Allow up to 8 * 6 characters for "mapleader". */
-        if (p == NULL || *p == NUL || STRLEN(p) > 8 * 6)
+        if (p == NULL || *p == '\0' || STRLEN(p) > 8 * 6)
           s = (char_u *)"\\";
         else
           s = p;
-        while (*s != NUL)
+        while (*s != '\0')
           result[dlen++] = *s++;
         src += len;
         continue;
@@ -4298,7 +4298,7 @@ replace_termcodes (
     key = *src;
     if (key == Ctrl_V || (do_backslash && key == '\\')) {
       ++src;                                    /* skip CTRL-V or backslash */
-      if (*src == NUL) {
+      if (*src == '\0') {
         if (from_part)
           result[dlen++] = key;
         break;
@@ -4321,7 +4321,7 @@ replace_termcodes (
       ++src;
     }
   }
-  result[dlen] = NUL;
+  result[dlen] = '\0';
 
   /*
    * Copy the new string to allocated memory.
@@ -4362,12 +4362,12 @@ static void gather_termleader(void)
   if (check_for_codes)
     termleader[len++] = DCS;        /* the termcode response starts with DCS
                                        in 8-bit mode */
-  termleader[len] = NUL;
+  termleader[len] = '\0';
 
   for (i = 0; i < tc_len; ++i)
     if (vim_strchr(termleader, termcodes[i].code[0]) == NULL) {
       termleader[len++] = termcodes[i].code[0];
-      termleader[len] = NUL;
+      termleader[len] = '\0';
     }
 
   need_gather = FALSE;
@@ -4476,12 +4476,12 @@ int show_one_termcode(char_u *name, char_u *code, int printit)
   if (p[1] != 't')
     STRCPY(IObuff + 5, p);
   else
-    IObuff[5] = NUL;
+    IObuff[5] = '\0';
   len = (int)STRLEN(IObuff);
   do
     IObuff[len++] = ' ';
   while (len < 17);
-  IObuff[len] = NUL;
+  IObuff[len] = '\0';
   if (code == NULL)
     len += 4;
   else
@@ -4564,7 +4564,7 @@ static void got_code_from_term(char_u *code, int len)
     /* Get the name from the response and find it in the table. */
     name[0] = hexhex2nr(code + 3);
     name[1] = hexhex2nr(code + 5);
-    name[2] = NUL;
+    name[2] = '\0';
     for (i = 0; key_names[i] != NULL; ++i) {
       if (STRCMP(key_names[i], name) == 0) {
         xt_index_in = i;
@@ -4582,7 +4582,7 @@ static void got_code_from_term(char_u *code, int len)
     if (key_names[i] != NULL) {
       for (i = 8; (c = hexhex2nr(code + i)) >= 0; i += 2)
         str[j++] = c;
-      str[j] = NUL;
+      str[j] = '\0';
       if (name[0] == 'C' && name[1] == 'o') {
         /* Color count is not a key code. */
         i = atoi((char *)str);
@@ -4641,7 +4641,7 @@ static void check_for_codes_from_term(void)
   ++allow_keys;
   for (;; ) {
     c = vpeekc();
-    if (c == NUL)           /* nothing available */
+    if (c == '\0')           /* nothing available */
       break;
 
     /* If a response is recognized it's replaced with K_IGNORE, must read
@@ -4695,7 +4695,7 @@ translate_mapping (
 
   for (; *str; ++str) {
     c = *str;
-    if (c == K_SPECIAL && str[1] != NUL && str[2] != NUL) {
+    if (c == K_SPECIAL && str[1] != '\0' && str[2] != '\0') {
       modifiers = 0;
       if (str[1] == KS_MODIFIER) {
         str++;
@@ -4716,14 +4716,14 @@ translate_mapping (
           continue;           /* for (str) */
         }
       }
-      if (c == K_SPECIAL && str[1] != NUL && str[2] != NUL) {
+      if (c == K_SPECIAL && str[1] != '\0' && str[2] != '\0') {
         if (expmap && cpo_special) {
           ga_clear(&ga);
           return NULL;
         }
         c = TO_SPECIAL(str[1], str[2]);
         if (c == K_ZERO)                /* display <Nul> as ^@ */
-          c = NUL;
+          c = '\0';
         str += 2;
       }
       if (IS_SPECIAL(c) || modifiers) {         /* special key */
@@ -4741,7 +4741,7 @@ translate_mapping (
     if (c)
       ga_append(&ga, c);
   }
-  ga_append(&ga, NUL);
+  ga_append(&ga, '\0');
   return (char_u *)(ga.ga_data);
 }
 
diff --git a/src/ui.c b/src/ui.c
index 6f4f6b200d..2cbc43a2a8 100644
--- a/src/ui.c
+++ b/src/ui.c
@@ -1075,7 +1075,7 @@ int vcol2col(win_T *wp, linenr_T lnum, int vcol)
   char_u      *start;
 
   start = ptr = ml_get_buf(wp->w_buffer, lnum, FALSE);
-  while (count < vcol && *ptr != NUL) {
+  while (count < vcol && *ptr != '\0') {
     count += win_lbr_chartabsize(wp, ptr, count, NULL);
     mb_ptr_adv(ptr);
   }
diff --git a/src/undo.c b/src/undo.c
index 243d865513..39162aea29 100644
--- a/src/undo.c
+++ b/src/undo.c
@@ -694,7 +694,7 @@ char_u *u_get_undo_file_name(char_u *buf_ffname, int reading)
   /* Loop over 'undodir'.  When reading find the first file that exists.
    * When not reading use the first directory that exists or ".". */
   dirp = p_udir;
-  while (*dirp != NUL) {
+  while (*dirp != '\0') {
     dir_len = copy_option_part(&dirp, dir_name, IOSIZE, ",");
     if (dir_len == 1 && dir_name[0] == '.') {
       /* Use same directory as the ffname,
@@ -707,13 +707,13 @@ char_u *u_get_undo_file_name(char_u *buf_ffname, int reading)
       *p = '.';
       STRCAT(p, ".un~");
     } else {
-      dir_name[dir_len] = NUL;
+      dir_name[dir_len] = '\0';
       if (os_isdir(dir_name)) {
         if (munged_name == NULL) {
           munged_name = vim_strsave(ffname);
           if (munged_name == NULL)
             return NULL;
-          for (p = munged_name; *p != NUL; mb_ptr_adv(p))
+          for (p = munged_name; *p != '\0'; mb_ptr_adv(p))
             if (vim_ispathsep(*p))
               *p = '%';
         }
@@ -763,7 +763,7 @@ static size_t fwrite_crypt(buf_T *buf, char_u *ptr, size_t len, FILE *fp)
   char_u small_buf[100];
   size_t i;
 
-  if (*buf->b_p_key == NUL)
+  if (*buf->b_p_key == '\0')
     return fwrite(ptr, len, (size_t)1, fp);
   if (len < 100)
     copy = small_buf;      /* no malloc()/free() for short strings */
@@ -786,7 +786,7 @@ static char_u *read_string_decrypt(buf_T *buf, FILE *fd, int len)
   char_u  *ptr;
 
   ptr = read_string(fd, len);
-  if (ptr != NULL && *buf->b_p_key != NUL)
+  if (ptr != NULL && *buf->b_p_key != '\0')
     crypt_decode(ptr, len);
   return ptr;
 }
@@ -801,7 +801,7 @@ static int serialize_header(FILE *fp, buf_T *buf, char_u *hash)
 
   /* If the buffer is encrypted then all text bytes following will be
    * encrypted.  Numbers and other info is not crypted. */
-  if (*buf->b_p_key != NUL) {
+  if (*buf->b_p_key != '\0') {
     char_u *header;
     int header_len;
 
@@ -1253,7 +1253,7 @@ void u_write_undo(char_u *name, int forceit, buf_T *buf, char_u *hash)
    */
   if (serialize_header(fp, buf, hash) == FAIL)
     goto write_error;
-  if (*buf->b_p_key != NUL)
+  if (*buf->b_p_key != '\0')
     do_crypt = TRUE;
 
   /*
@@ -1404,7 +1404,7 @@ void u_read_undo(char_u *name, char_u *hash, char_u *orig_name)
   }
   version = get2c(fp);
   if (version == UF_VERSION_CRYPT) {
-    if (*curbuf->b_p_key == NUL) {
+    if (*curbuf->b_p_key == '\0') {
       EMSG2(_("E832: Non-encrypted file has encrypted undo file: %s"),
           file_name);
       goto error;
@@ -2317,7 +2317,7 @@ u_undo_end (
     uhp = curbuf->b_u_newhead;
 
   if (uhp == NULL)
-    *msgbuf = NUL;
+    *msgbuf = '\0';
   else
     u_add_time(msgbuf, sizeof(msgbuf), uhp->uh_time);
 
diff --git a/src/version.c b/src/version.c
index 6e41f41c97..f504264134 100644
--- a/src/version.c
+++ b/src/version.c
@@ -517,7 +517,7 @@ int has_patch(int n)
 void ex_version(exarg_T *eap)
 {
   // Ignore a ":version 9.99" command.
-  if (*eap->arg == NUL) {
+  if (*eap->arg == '\0') {
     msg_putchar('\n');
     list_version();
   }
@@ -639,15 +639,15 @@ void list_version(void)
 
 #ifdef HAVE_PATHDEF
 
-  if ((*compiled_user != NUL) || (*compiled_sys != NUL)) {
+  if ((*compiled_user != '\0') || (*compiled_sys != '\0')) {
     MSG_PUTS(_("\nCompiled "));
 
-    if (*compiled_user != NUL) {
+    if (*compiled_user != '\0') {
       MSG_PUTS(_("by "));
       MSG_PUTS(compiled_user);
     }
 
-    if (*compiled_sys != NUL) {
+    if (*compiled_sys != '\0') {
       MSG_PUTS("@");
       MSG_PUTS(compiled_sys);
     }
@@ -692,13 +692,13 @@ void list_version(void)
 #endif  // ifdef USR_EXRC_FILE2
 #ifdef HAVE_PATHDEF
 
-  if (*default_vim_dir != NUL) {
+  if (*default_vim_dir != '\0') {
     version_msg(_("  fall-back for $VIM: \""));
     version_msg((char *)default_vim_dir);
     version_msg("\"\n");
   }
 
-  if (*default_vimruntime_dir != NUL) {
+  if (*default_vimruntime_dir != '\0') {
     version_msg(_(" f-b for $VIMRUNTIME: \""));
     version_msg((char *)default_vimruntime_dir);
     version_msg("\"\n");
@@ -832,7 +832,7 @@ void intro_message(int colon)
         }
       }
 
-      if (*p != NUL) {
+      if (*p != '\0') {
         do_intro_line(row, (char_u *)_(p), i == 2, 0);
       }
       row++;
@@ -890,10 +890,10 @@ static void do_intro_line(int row, char_u *mesg, int add_version, int attr)
   }
 
   // Split up in parts to highlight <> items differently.
-  for (p = mesg; *p != NUL; p += l) {
+  for (p = mesg; *p != '\0'; p += l) {
     clen = 0;
 
-    for (l = 0; p[l] != NUL
+    for (l = 0; p[l] != '\0'
          && (l == 0 || (p[l] != '<' && p[l - 1] != '>')); ++l) {
       if (has_mbyte) {
         clen += ptr2cells(p + l);
diff --git a/src/window.c b/src/window.c
index 305bf6b7c9..9ed9e4de59 100644
--- a/src/window.c
+++ b/src/window.c
@@ -185,7 +185,7 @@ newwindow:
       /* window height */
       vim_snprintf((char *)cbuf, sizeof(cbuf) - 5, "%" PRId64, (int64_t)Prenum);
     else
-      cbuf[0] = NUL;
+      cbuf[0] = '\0';
     if (nchar == 'v' || nchar == Ctrl_V)
       STRCAT(cbuf, "v");
     STRCAT(cbuf, "new");
@@ -418,7 +418,7 @@ newwindow:
 
     /* Execute the command right here, required when
      * "wincmd ]" was used in a function. */
-    do_nv_ident(Ctrl_RSB, NUL);
+    do_nv_ident(Ctrl_RSB, '\0');
     break;
 
   /* edit file name under cursor in a new window */
@@ -487,7 +487,7 @@ wingotofile:
 #endif
     ++ no_mapping;
     ++allow_keys;               /* no mapping for xchar, but allow key codes */
-    if (xchar == NUL)
+    if (xchar == '\0')
       xchar = plain_vgetc();
     LANGMAP_ADJUST(xchar, TRUE);
     --no_mapping;
@@ -4791,9 +4791,9 @@ file_name_in_line (
    * search forward for what could be the start of a file name
    */
   ptr = line + col;
-  while (*ptr != NUL && !vim_isfilec(*ptr))
+  while (*ptr != '\0' && !vim_isfilec(*ptr))
     mb_ptr_adv(ptr);
-  if (*ptr == NUL) {            /* nothing found */
+  if (*ptr == '\0') {            /* nothing found */
     if (options & FNAME_MESS)
       EMSG(_("E446: No file name under cursor"));
     return NULL;
@@ -4839,7 +4839,7 @@ file_name_in_line (
     /* Get the number after the file name and a separator character */
     p = ptr + len;
     p = skipwhite(p);
-    if (*p != NUL) {
+    if (*p != '\0') {
       if (!isdigit(*p))
         ++p;                        /* skip the separator */
       p = skipwhite(p);
@@ -5224,7 +5224,7 @@ int match_add(win_T *wp, char_u *grp, char_u *pat, int prio, int id)
   int hlg_id;
   regprog_T   *regprog;
 
-  if (*grp == NUL || *pat == NUL)
+  if (*grp == '\0' || *pat == '\0')
     return -1;
   if (id < -1 || id == 0) {
     EMSGN("E799: Invalid ID: %" PRId64