Merge #5561 'inccommand'

Initial work by:
  Robin Elrharbi-Fleury (Robinhola)
  Audrey Rayé (Adrey06)
  Philémon Hullot (DesbyP)
  Aymeric Collange (aym7)
  Clément Guyomard (Clement0)

Major revisions by:
  KillTheMule
  Björn Linse <bjorn.linse@gmail.com>
  Justin M. Keyes <justinkz@gmail.com>

19 files changed, 716 insertions, 384 deletions

diff --git a/src/nvim/buffer.c b/src/nvim/buffer.c
index a66fdc1304..75caf2223b 100644
--- a/src/nvim/buffer.c
+++ b/src/nvim/buffer.c
@@ -268,6 +268,9 @@ open_buffer (
 bool buf_valid(buf_T *buf)
   FUNC_ATTR_PURE FUNC_ATTR_WARN_UNUSED_RESULT
 {
+  if (buf == NULL) {
+    return false;
+  }
   FOR_ALL_BUFFERS(bp) {
     if (bp == buf) {
       return true;
@@ -479,6 +482,18 @@ void buf_clear_file(buf_T *buf)
   buf->b_ml.ml_flags = ML_EMPTY;                /* empty buffer */
 }
 
+/// Clears the current buffer contents.
+void buf_clear(void)
+{
+  linenr_T line_count = curbuf->b_ml.ml_line_count;
+  while (!(curbuf->b_ml.ml_flags & ML_EMPTY)) {
+    ml_delete((linenr_T)1, false);
+  }
+  deleted_lines_mark(1, line_count);  // prepare for display
+  ml_close(curbuf, true);             // free memline_T
+  buf_clear_file(curbuf);
+}
+
 /// buf_freeall() - free all things allocated for a buffer that are related to
 /// the file.  Careful: get here with "curwin" NULL when exiting.
 ///
@@ -671,14 +686,15 @@ void handle_swap_exists(buf_T *old_curbuf)
      * aborting() returns FALSE when closing a buffer. */
     enter_cleanup(&cs);
 
-    /* User selected Quit at ATTENTION prompt.  Go back to previous
-     * buffer.  If that buffer is gone or the same as the current one,
-     * open a new, empty buffer. */
-    swap_exists_action = SEA_NONE;      /* don't want it again */
-    swap_exists_did_quit = TRUE;
-    close_buffer(curwin, curbuf, DOBUF_UNLOAD, FALSE);
-    if (!buf_valid(old_curbuf) || old_curbuf == curbuf)
+    // User selected Quit at ATTENTION prompt.  Go back to previous
+    // buffer.  If that buffer is gone or the same as the current one,
+    // open a new, empty buffer.
+    swap_exists_action = SEA_NONE;      // don't want it again
+    swap_exists_did_quit = true;
+    close_buffer(curwin, curbuf, DOBUF_UNLOAD, false);
+    if (!buf_valid(old_curbuf) || old_curbuf == curbuf) {
       old_curbuf = buflist_new(NULL, NULL, 1L, BLN_CURBUF | BLN_LISTED);
+    }
     if (old_curbuf != NULL) {
       enter_buffer(old_curbuf);
       if (old_tw != curbuf->b_p_tw)
@@ -1319,28 +1335,29 @@ void do_autochdir(void)
   }
 }
 
-/*
- * functions for dealing with the buffer list
- */
+//
+// functions for dealing with the buffer list
+//
 
-/*
- * Add a file name to the buffer list.  Return a pointer to the buffer.
- * If the same file name already exists return a pointer to that buffer.
- * If it does not exist, or if fname == NULL, a new entry is created.
- * If (flags & BLN_CURBUF) is TRUE, may use current buffer.
- * If (flags & BLN_LISTED) is TRUE, add new buffer to buffer list.
- * If (flags & BLN_DUMMY) is TRUE, don't count it as a real buffer.
- * This is the ONLY way to create a new buffer.
- */
-static int top_file_num = 1;            /* highest file number */
-
-buf_T *
-buflist_new (
-    char_u *ffname,            /* full path of fname or relative */
-    char_u *sfname,            /* short fname or NULL */
-    linenr_T lnum,                  /* preferred cursor line */
-    int flags                      /* BLN_ defines */
-)
+static int top_file_num = 1;            ///< highest file number
+
+/// Add a file name to the buffer list.
+/// If the same file name already exists return a pointer to that buffer.
+/// If it does not exist, or if fname == NULL, a new entry is created.
+/// If (flags & BLN_CURBUF) is TRUE, may use current buffer.
+/// If (flags & BLN_LISTED) is TRUE, add new buffer to buffer list.
+/// If (flags & BLN_DUMMY) is TRUE, don't count it as a real buffer.
+/// If (flags & BLN_NEW) is TRUE, don't use an existing buffer.
+/// This is the ONLY way to create a new buffer.
+///
+/// @param ffname full path of fname or relative
+/// @param sfname short fname or NULL
+/// @param lnum   preferred cursor line
+/// @param flags  BLN_ defines
+/// @param bufnr
+///
+/// @return pointer to the buffer
+buf_T * buflist_new(char_u *ffname, char_u *sfname, linenr_T lnum, int flags)
 {
   buf_T       *buf;
 
@@ -2374,10 +2391,11 @@ buf_T *setaltfname(char_u *ffname, char_u *sfname, linenr_T lnum)
 {
   buf_T       *buf;
 
-  /* Create a buffer.  'buflisted' is not set if it's a new buffer */
+  // Create a buffer.  'buflisted' is not set if it's a new buffer
   buf = buflist_new(ffname, sfname, lnum, 0);
-  if (buf != NULL && !cmdmod.keepalt)
+  if (buf != NULL && !cmdmod.keepalt) {
     curwin->w_alt_fnum = buf->b_fnum;
+  }
   return buf;
 }
 
@@ -2412,8 +2430,9 @@ int buflist_add(char_u *fname, int flags)
   buf_T       *buf;
 
   buf = buflist_new(fname, NULL, (linenr_T)0, flags);
-  if (buf != NULL)
+  if (buf != NULL) {
     return buf->b_fnum;
+  }
   return 0;
 }
 
@@ -5253,3 +5272,15 @@ wipe_buffer (
     unblock_autocmds();
   }
 }
+
+/// Creates or switches to a special-purpose buffer.
+///
+/// @param bufnr     Buffer to switch to, or 0 to create a new buffer.
+void buf_open_special(handle_T bufnr, char *bufname, char *buftype)
+{
+  (void)do_ecmd((int)bufnr, NULL, NULL, NULL, ECMD_ONE, ECMD_HIDE, NULL);
+  (void)setfname(curbuf, (char_u *)bufname, NULL, true);
+  set_option_value((char_u *)"bt", 0L, (char_u *)buftype, OPT_LOCAL);
+  set_option_value((char_u *)"swf", 0L, NULL, OPT_LOCAL);
+  RESET_BINDING(curwin);
+}
diff --git a/src/nvim/ex_cmds.c b/src/nvim/ex_cmds.c
index 4674460a16..13a0282b76 100644
--- a/src/nvim/ex_cmds.c
+++ b/src/nvim/ex_cmds.c
@@ -8,7 +8,11 @@
 #include <string.h>
 #include <stdlib.h>
 #include <inttypes.h>
+#include <math.h>
 
+#include "nvim/api/private/defs.h"
+#include "nvim/api/buffer.h"
+#include "nvim/log.h"
 #include "nvim/vim.h"
 #include "nvim/ascii.h"
 #include "nvim/ex_cmds.h"
@@ -83,6 +87,15 @@ typedef struct {
   SubIgnoreType do_ic;  ///< ignore case flag
 } subflags_T;
 
+/// Lines matched during :substitute.
+typedef struct {
+  linenr_T lnum;
+  long nmatch;
+  char_u *line;
+  kvec_t(colnr_T) cols;  ///< columns of in-line matches
+} MatchedLine;
+typedef kvec_t(MatchedLine) MatchedLineVec;
+
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "ex_cmds.c.generated.h"
 #endif
@@ -1524,8 +1537,9 @@ int rename_buffer(char_u *new_fname)
   curbuf->b_flags |= BF_NOTEDITED;
   if (xfname != NULL && *xfname != NUL) {
     buf = buflist_new(fname, xfname, curwin->w_cursor.lnum, 0);
-    if (buf != NULL && !cmdmod.keepalt)
+    if (buf != NULL && !cmdmod.keepalt) {
       curwin->w_alt_fnum = buf->b_fnum;
+    }
   }
   xfree(fname);
   xfree(sfname);
@@ -2019,37 +2033,34 @@ theend:
   return retval;
 }
 
-/*
- * start editing a new file
- *
- *     fnum: file number; if zero use ffname/sfname
- *   ffname: the file name
- *		- full path if sfname used,
- *		- any file name if sfname is NULL
- *		- empty string to re-edit with the same file name (but may be
- *		    in a different directory)
- *		- NULL to start an empty buffer
- *   sfname: the short file name (or NULL)
- *	eap: contains the command to be executed after loading the file and
- *	     forced 'ff' and 'fenc'
- *  newlnum: if > 0: put cursor on this line number (if possible)
- *	     if ECMD_LASTL: use last position in loaded file
- *	     if ECMD_LAST: use last position in all files
- *	     if ECMD_ONE: use first line
- *    flags:
- *	   ECMD_HIDE: if TRUE don't free the current buffer
- *     ECMD_SET_HELP: set b_help flag of (new) buffer before opening file
- *	 ECMD_OLDBUF: use existing buffer if it exists
- *	ECMD_FORCEIT: ! used for Ex command
- *	 ECMD_ADDBUF: don't edit, just add to buffer list
- *   oldwin: Should be "curwin" when editing a new buffer in the current
- *	     window, NULL when splitting the window first.  When not NULL info
- *	     of the previous buffer for "oldwin" is stored.
- *
- * return FAIL for failure, OK otherwise
- */
-int 
-do_ecmd (
+/// start editing a new file
+///
+/// @param fnum     file number; if zero use ffname/sfname
+/// @param ffname   the file name
+///                 - full path if sfname used,
+///                 - any file name if sfname is NULL
+///                 - empty string to re-edit with the same file name (but may
+///                   be in a different directory)
+///                 - NULL to start an empty buffer
+/// @param sfname   the short file name (or NULL)
+/// @param eap      contains the command to be executed after loading the file
+///                 and forced 'ff' and 'fenc'
+/// @param newlnum  if > 0: put cursor on this line number (if possible)
+///                 ECMD_LASTL: use last position in loaded file
+///                 ECMD_LAST: use last position in all files
+///                 ECMD_ONE: use first line
+/// @param flags    ECMD_HIDE: if TRUE don't free the current buffer
+///                 ECMD_SET_HELP: set b_help flag of (new) buffer before
+///                 opening file
+///                 ECMD_OLDBUF: use existing buffer if it exists
+///                 ECMD_FORCEIT: ! used for Ex command
+///                 ECMD_ADDBUF: don't edit, just add to buffer list
+/// @param oldwin   Should be "curwin" when editing a new buffer in the current
+///                 window, NULL when splitting the window first.  When not NULL
+///                 info of the previous buffer for "oldwin" is stored.
+///
+/// @return FAIL for failure, OK otherwise
+int do_ecmd(
     int fnum,
     char_u *ffname,
     char_u *sfname,
@@ -2174,9 +2185,10 @@ do_ecmd (
         buflist_altfpos(oldwin);
     }
 
-    if (fnum)
+    if (fnum) {
       buf = buflist_findnr(fnum);
-    else {
+    } else {
+      ILOG("here");
       if (flags & ECMD_ADDBUF) {
         linenr_T tlnum = 1L;
 
@@ -2189,7 +2201,7 @@ do_ecmd (
         goto theend;
       }
       buf = buflist_new(ffname, sfname, 0L,
-          BLN_CURBUF | ((flags & ECMD_SET_HELP) ? 0 : BLN_LISTED));
+                        BLN_CURBUF | (flags & ECMD_SET_HELP ? 0 : BLN_LISTED));
       // Autocmds may change curwin and curbuf.
       if (oldwin != NULL) {
         oldwin = curwin;
@@ -2978,10 +2990,12 @@ static bool sub_joining_lines(exarg_T *eap, char_u *pat,
       ex_may_print(eap);
     }
 
-    if (!cmdmod.keeppatterns) {
-      save_re_pat(RE_SUBST, pat, p_magic);
+    if (!eap->is_live) {
+      if (!cmdmod.keeppatterns) {
+        save_re_pat(RE_SUBST, pat, p_magic);
+      }
+      add_to_history(HIST_SEARCH, pat, true, NUL);
     }
-    add_to_history(HIST_SEARCH, pat, TRUE, NUL);
 
     return true;
   }
@@ -3087,16 +3101,17 @@ static char_u *sub_parse_flags(char_u *cmd, subflags_T *subflags,
   return cmd;
 }
 
-/* do_sub()
- *
- * Perform a substitution from line eap->line1 to line eap->line2 using the
- * command pointed to by eap->arg which should be of the form:
- *
- * /pattern/substitution/{flags}
- *
- * The usual escapes are supported as described in the regexp docs.
- */
-void do_sub(exarg_T *eap)
+/// do_sub()
+///
+/// Perform a substitution from line eap->line1 to line eap->line2 using the
+/// command pointed to by eap->arg which should be of the form:
+///
+/// /pattern/substitution/{flags}
+///
+/// The usual escapes are supported as described in the regexp docs.
+///
+/// @return buffer used for 'inccommand' preview
+buf_T *do_sub(exarg_T *eap)
 {
   long i = 0;
   regmmatch_T regmatch;
@@ -3112,20 +3127,22 @@ void do_sub(exarg_T *eap)
   };
   char_u *pat = NULL, *sub = NULL;  // init for GCC
   int delimiter;
+  bool has_second_delim = false;
   int sublen;
   int got_quit = false;
   int got_match = false;
   int which_pat;
   char_u *cmd = eap->arg;
   linenr_T first_line = 0;  // first changed line
-  linenr_T last_line= 0;    // below last changed line AFTER the
-                            // change
+  linenr_T last_line= 0;    // below last changed line AFTER the change
   linenr_T old_line_count = curbuf->b_ml.ml_line_count;
-  char_u      *sub_firstline;  // allocated copy of first sub line
-  bool endcolumn = false;        // cursor in last column when done
+  char_u *sub_firstline;    // allocated copy of first sub line
+  bool endcolumn = false;   // cursor in last column when done
+  MatchedLineVec matched_lines = KV_INITIAL_VALUE;
   pos_T old_cursor = curwin->w_cursor;
   int start_nsubs;
   int save_ma = 0;
+  int save_b_changed = curbuf->b_changed;
 
   if (!global_busy) {
     sub_nsubs = 0;
@@ -3144,7 +3161,7 @@ void do_sub(exarg_T *eap)
     /* don't accept alphanumeric for separator */
     if (isalpha(*cmd)) {
       EMSG(_("E146: Regular expressions can't be delimited by letters"));
-      return;
+      return NULL;
     }
     /*
      * undocumented vi feature:
@@ -3155,21 +3172,26 @@ void do_sub(exarg_T *eap)
       ++cmd;
       if (vim_strchr((char_u *)"/?&", *cmd) == NULL) {
         EMSG(_(e_backslash));
-        return;
+        return NULL;
       }
-      if (*cmd != '&')
-        which_pat = RE_SEARCH;              /* use last '/' pattern */
-      pat = (char_u *)"";                   /* empty search pattern */
-      delimiter = *cmd++;                   /* remember delimiter character */
-    } else {          /* find the end of the regexp */
-      if (p_altkeymap && curwin->w_p_rl)
+      if (*cmd != '&') {
+        which_pat = RE_SEARCH;              // use last '/' pattern
+      }
+      pat = (char_u *)"";                   // empty search pattern
+      delimiter = *cmd++;                   // remember delimiter character
+      has_second_delim = true;
+    } else {          // find the end of the regexp
+      if (p_altkeymap && curwin->w_p_rl) {
         lrF_sub(cmd);
-      which_pat = RE_LAST;                  /* use last used regexp */
-      delimiter = *cmd++;                   /* remember delimiter character */
-      pat = cmd;                            /* remember start of search pat */
+      }
+      which_pat = RE_LAST;                  // use last used regexp
+      delimiter = *cmd++;                   // remember delimiter character
+      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 */
+      if (cmd[0] == delimiter) {            // end delimiter found
+        *cmd++ = NUL;                       // replace it with a NUL
+        has_second_delim = true;
+      }
     }
 
     /*
@@ -3188,7 +3210,7 @@ void do_sub(exarg_T *eap)
       mb_ptr_adv(cmd);
     }
 
-    if (!eap->skip) {
+    if (!eap->skip && !eap->is_live) {
       sub_set_replacement((SubReplacementString) {
         .sub = xstrdup((char *) sub),
         .timestamp = os_time(),
@@ -3198,7 +3220,7 @@ void do_sub(exarg_T *eap)
   } else if (!eap->skip) {    /* use previous pattern and substitution */
     if (old_sub.sub == NULL) {      /* there is no previous command */
       EMSG(_(e_nopresub));
-      return;
+      return NULL;
     }
     pat = NULL;                 /* search_regcomp() will use previous pattern */
     sub = (char_u *) old_sub.sub;
@@ -3209,7 +3231,7 @@ void do_sub(exarg_T *eap)
   }
 
   if (sub_joining_lines(eap, pat, sub, cmd)) {
-    return;
+    return NULL;
   }
 
   cmd = sub_parse_flags(cmd, &subflags, &which_pat);
@@ -3223,7 +3245,7 @@ void do_sub(exarg_T *eap)
     i = getdigits_long(&cmd);
     if (i <= 0 && !eap->skip && subflags.do_error) {
       EMSG(_(e_zerocount));
-      return;
+      return NULL;
     }
     eap->line1 = eap->line2;
     eap->line2 += i - 1;
@@ -3239,24 +3261,27 @@ void do_sub(exarg_T *eap)
     eap->nextcmd = check_nextcmd(cmd);
     if (eap->nextcmd == NULL) {
       EMSG(_(e_trailing));
-      return;
+      return NULL;
     }
   }
 
-  if (eap->skip)            /* not executing commands, only parsing */
-    return;
+  if (eap->skip) {          // not executing commands, only parsing
+    return NULL;
+  }
 
   if (!subflags.do_count && !MODIFIABLE(curbuf)) {
     // Substitution is not allowed in non-'modifiable' buffer
     EMSG(_(e_modifiable));
-    return;
+    return NULL;
   }
 
-  if (search_regcomp(pat, RE_SUBST, which_pat, SEARCH_HIS, &regmatch) == FAIL) {
+  int search_options = eap->is_live ? 0 : SEARCH_HIS;
+  if (search_regcomp(pat, RE_SUBST, which_pat, search_options,
+                     &regmatch) == FAIL) {
     if (subflags.do_error) {
       EMSG(_(e_invcmd));
     }
-    return;
+    return NULL;
   }
 
   // the 'i' or 'I' flag overrules 'ignorecase' and 'smartcase'
@@ -3343,6 +3368,8 @@ void do_sub(exarg_T *eap)
       sub_firstlnum = lnum;
       copycol = 0;
       matchcol = 0;
+      // the current match
+      MatchedLine matched_line = { 0, 0, NULL, KV_INITIAL_VALUE };
 
       /* At first match, remember current cursor position. */
       if (!got_match) {
@@ -3379,6 +3406,12 @@ void do_sub(exarg_T *eap)
         curwin->w_cursor.lnum = lnum;
         do_again = FALSE;
 
+        if (eap->is_live) {
+          // Increment the in-line match count and store the column.
+          matched_line.nmatch++;
+          kv_push(matched_line.cols, regmatch.startpos[0].col);
+        }
+
         /*
          * 1. Match empty string does not count, except for first
          * match.  This reproduces the strange vi behaviour.
@@ -3426,7 +3459,7 @@ void do_sub(exarg_T *eap)
             goto skip;
         }
 
-        if (subflags.do_ask) {
+        if (subflags.do_ask && !eap->is_live) {
           int typed = 0;
 
           /* change State to CONFIRM, so that the mouse works
@@ -3597,134 +3630,131 @@ void do_sub(exarg_T *eap)
          * use "\=col("."). */
         curwin->w_cursor.col = regmatch.startpos[0].col;
 
-        /*
-         * 3. substitute the string.
-         */
-        if (subflags.do_count) {
-          // prevent accidentally changing the buffer by a function
-          save_ma = curbuf->b_p_ma;
-          curbuf->b_p_ma = false;
-          sandbox++;
-        }
-        // Save flags for recursion.  They can change for e.g.
-        // :s/^/\=execute("s#^##gn")
-        subflags_T subflags_save = subflags;
-        // get length of substitution part
-        sublen = vim_regsub_multi(&regmatch,
-                                  sub_firstlnum - regmatch.startpos[0].lnum,
-                                  sub, sub_firstline, false, p_magic, true);
-        // Don't keep flags set by a recursive call
-        subflags = subflags_save;
-        if (subflags.do_count) {
-          curbuf->b_p_ma = save_ma;
-          if (sandbox > 0) {
-            sandbox--;
+        // 3. Substitute the string. During 'inccommand' only do this if there
+        //    is a replace pattern.
+        if (!eap->is_live || has_second_delim) {
+          if (subflags.do_count) {
+            // prevent accidentally changing the buffer by a function
+            save_ma = curbuf->b_p_ma;
+            curbuf->b_p_ma = false;
+            sandbox++;
+          }
+          // Save flags for recursion.  They can change for e.g.
+          // :s/^/\=execute("s#^##gn")
+          subflags_T subflags_save = subflags;
+          // get length of substitution part
+          sublen = vim_regsub_multi(&regmatch,
+                                    sub_firstlnum - regmatch.startpos[0].lnum,
+                                    sub, sub_firstline, false, p_magic, true);
+          // Don't keep flags set by a recursive call
+          subflags = subflags_save;
+          if (subflags.do_count) {
+            curbuf->b_p_ma = save_ma;
+            if (sandbox > 0) {
+              sandbox--;
+            }
+            goto skip;
           }
-          goto skip;
-        }
-
-        /* When the match included the "$" of the last line it may
-         * go beyond the last line of the buffer. */
-        if (nmatch > curbuf->b_ml.ml_line_count - sub_firstlnum + 1) {
-          nmatch = curbuf->b_ml.ml_line_count - sub_firstlnum + 1;
-          skip_match = TRUE;
-        }
 
-        /* Need room for:
-         * - result so far in new_start (not for first sub in line)
-         * - original text up to match
-         * - length of substituted part
-         * - original text after match
-         */
-        if (nmatch == 1)
-          p1 = sub_firstline;
-        else {
-          p1 = ml_get(sub_firstlnum + nmatch - 1);
-          nmatch_tl += nmatch - 1;
-        }
-        size_t copy_len = regmatch.startpos[0].col - copycol;
-        new_end = sub_grow_buf(&new_start,
-                               copy_len + (STRLEN(p1) - regmatch.endpos[0].col)
-                               + sublen + 1);
+          // When the match included the "$" of the last line it may
+          // go beyond the last line of the buffer.
+          if (nmatch > curbuf->b_ml.ml_line_count - sub_firstlnum + 1) {
+            nmatch = curbuf->b_ml.ml_line_count - sub_firstlnum + 1;
+            skip_match = true;
+          }
 
-        /*
-         * copy the text up to the part that matched
-         */
-        memmove(new_end, sub_firstline + copycol, (size_t)copy_len);
-        new_end += copy_len;
-
-        (void)vim_regsub_multi(&regmatch,
-            sub_firstlnum - regmatch.startpos[0].lnum,
-            sub, new_end, TRUE, p_magic, TRUE);
-        sub_nsubs++;
-        did_sub = TRUE;
-
-        /* Move the cursor to the start of the line, to avoid that it
-         * is beyond the end of the line after the substitution. */
-        curwin->w_cursor.col = 0;
-
-        /* For a multi-line match, make a copy of the last matched
-         * line and continue in that one. */
-        if (nmatch > 1) {
-          sub_firstlnum += nmatch - 1;
-          xfree(sub_firstline);
-          sub_firstline = vim_strsave(ml_get(sub_firstlnum));
-          // When going beyond the last line, stop substituting.
-          if (sub_firstlnum <= line2) {
-            do_again = true;
+          // Need room for:
+          // - result so far in new_start (not for first sub in line)
+          // - original text up to match
+          // - length of substituted part
+          // - original text after match
+          if (nmatch == 1) {
+            p1 = sub_firstline;
           } else {
-            subflags.do_all = false;
+            p1 = ml_get(sub_firstlnum + nmatch - 1);
+            nmatch_tl += nmatch - 1;
           }
-        }
+          size_t copy_len = regmatch.startpos[0].col - copycol;
+          new_end = sub_grow_buf(&new_start,
+                                 (STRLEN(p1) - regmatch.endpos[0].col)
+                                 + copy_len + sublen + 1);
+
+          // copy the text up to the part that matched
+          memmove(new_end, sub_firstline + copycol, (size_t)copy_len);
+          new_end += copy_len;
+
+          (void)vim_regsub_multi(&regmatch,
+                                 sub_firstlnum - regmatch.startpos[0].lnum,
+                                 sub, new_end, true, p_magic, true);
+          sub_nsubs++;
+          did_sub = true;
 
-        /* Remember next character to be copied. */
-        copycol = regmatch.endpos[0].col;
+          // Move the cursor to the start of the line, to avoid that it
+          // is beyond the end of the line after the substitution.
+          curwin->w_cursor.col = 0;
 
-        if (skip_match) {
-          /* Already hit end of the buffer, sub_firstlnum is one
-           * less than what it ought to be. */
-          xfree(sub_firstline);
-          sub_firstline = vim_strsave((char_u *)"");
-          copycol = 0;
-        }
+          // For a multi-line match, make a copy of the last matched
+          // line and continue in that one.
+          if (nmatch > 1) {
+            sub_firstlnum += nmatch - 1;
+            xfree(sub_firstline);
+            sub_firstline = vim_strsave(ml_get(sub_firstlnum));
+            // When going beyond the last line, stop substituting.
+            if (sub_firstlnum <= line2) {
+              do_again = true;
+            } else {
+              subflags.do_all = false;
+            }
+          }
 
-        /*
-         * Now the trick is to replace CTRL-M chars with a real line
-         * break.  This would make it impossible to insert a CTRL-M in
-         * the text.  The line break can be avoided by preceding the
-         * CTRL-M with a backslash.  To be able to insert a backslash,
-         * they must be doubled in the string and are halved here.
-         * That is Vi compatible.
-         */
-        for (p1 = new_end; *p1; ++p1) {
-          if (p1[0] == '\\' && p1[1] != NUL)            /* 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 */
-              ml_append(lnum - 1, new_start,
-                  (colnr_T)(p1 - new_start + 1), FALSE);
-              mark_adjust(lnum + 1, (linenr_T)MAXLNUM, 1L, 0L);
-              if (subflags.do_ask) {
-                appended_lines(lnum - 1, 1L);
-              } else {
-                if (first_line == 0) {
-                  first_line = lnum;
+          // Remember next character to be copied.
+          copycol = regmatch.endpos[0].col;
+
+          if (skip_match) {
+            // Already hit end of the buffer, sub_firstlnum is one
+            // less than what it ought to be.
+            xfree(sub_firstline);
+            sub_firstline = vim_strsave((char_u *)"");
+            copycol = 0;
+          }
+
+          // Now the trick is to replace CTRL-M chars with a real line
+          // break.  This would make it impossible to insert a CTRL-M in
+          // the text.  The line break can be avoided by preceding the
+          // CTRL-M with a backslash.  To be able to insert a backslash,
+          // they must be doubled in the string and are halved here.
+          // That is Vi compatible.
+          for (p1 = new_end; *p1; p1++) {
+            if (p1[0] == '\\' && p1[1] != NUL) {            // 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
+                ml_append(lnum - 1, new_start,
+                          (colnr_T)(p1 - new_start + 1), false);
+                mark_adjust(lnum + 1, (linenr_T)MAXLNUM, 1L, 0L);
+                if (subflags.do_ask) {
+                  appended_lines(lnum - 1, 1L);
+                } else {
+                  if (first_line == 0) {
+                    first_line = lnum;
+                  }
+                  last_line = lnum + 1;
                 }
-                last_line = lnum + 1;
+                // All line numbers increase.
+                sub_firstlnum++;
+                lnum++;
+                line2++;
+                // move the cursor to the new line, like Vi
+                curwin->w_cursor.lnum++;
+                // copy the rest
+                STRMOVE(new_start, p1 + 1);
+                p1 = new_start - 1;
               }
-              /* All line numbers increase. */
-              ++sub_firstlnum;
-              ++lnum;
-              ++line2;
-              /* move the cursor to the new line, like Vi */
-              ++curwin->w_cursor.lnum;
-              /* copy the rest */
-              STRMOVE(new_start, p1 + 1);
-              p1 = new_start - 1;
+            } else if (has_mbyte) {
+              p1 += (*mb_ptr2len)(p1) - 1;
             }
-          } else if (has_mbyte)
-            p1 += (*mb_ptr2len)(p1) - 1;
+          }
         }
 
         // 4. If subflags.do_all is set, find next match.
@@ -3845,6 +3875,12 @@ skip:
       xfree(new_start);              /* for when substitute was cancelled */
       xfree(sub_firstline);          /* free the copy of the original line */
       sub_firstline = NULL;
+
+      if (eap->is_live) {
+        matched_line.lnum = lnum;
+        matched_line.line = vim_strsave(ml_get(lnum));
+        kv_push(matched_lines, matched_line);
+      }
     }
 
     line_breakcheck();
@@ -3880,7 +3916,7 @@ skip:
           beginline(BL_WHITE | BL_FIX);
         }
       }
-      if (!do_sub_msg(subflags.do_count) && subflags.do_ask) {
+      if (!eap->is_live && !do_sub_msg(subflags.do_count) && subflags.do_ask) {
         MSG("");
       }
     } else {
@@ -3912,6 +3948,26 @@ skip:
   // Restore the flag values, they can be used for ":&&".
   subflags.do_all = save_do_all;
   subflags.do_ask = save_do_ask;
+
+  // Show 'inccommand' preview if there are matched lines.
+  buf_T *preview_buf = NULL;
+  if (eap->is_live && matched_lines.size != 0 && pat != NULL && *p_icm != NUL) {
+    curbuf->b_changed = save_b_changed;  // preserve 'modified' during preview
+    preview_buf = show_sub(old_cursor, pat, sub, eap->line1, eap->line2,
+                           &matched_lines);
+
+  } else if (*p_icm != NUL && eap->is_live) {
+    curwin->w_cursor = old_cursor;  // don't move the cursor
+  }
+
+  for (MatchedLine m; kv_size(matched_lines);) {
+    m = kv_pop(matched_lines);
+    xfree(m.line);
+    kv_destroy(m.cols);
+  }
+  kv_destroy(matched_lines);
+
+  return preview_buf;
 }  // NOLINT(readability/fn_size)
 
 /*
@@ -5793,50 +5849,39 @@ static enum
     EXP_SIGN_NAMES	/* expand with name of placed signs */
 } expand_what;
 
-/*
- * Function given to ExpandGeneric() to obtain the sign command
- * expansion.
- */
+/// Function given to ExpandGeneric() to obtain the sign command
+/// expansion.
 char_u * get_sign_name(expand_T *xp, int idx)
 {
-    sign_T	*sp;
-    int		current_idx;
-
-    switch (expand_what)
-    {
+  switch (expand_what)
+  {
     case EXP_SUBCMD:
-	return (char_u *)cmds[idx];
-    case EXP_DEFINE:
-	{
-	    char *define_arg[] =
-	    {
-		"icon=", "linehl=", "text=", "texthl=", NULL
-	    };
-	    return (char_u *)define_arg[idx];
-	}
-    case EXP_PLACE:
-	{
-	    char *place_arg[] =
-	    {
-		"line=", "name=", "file=", "buffer=", NULL
-	    };
-	    return (char_u *)place_arg[idx];
-	}
-    case EXP_UNPLACE:
-	{
-	    char *unplace_arg[] = { "file=", "buffer=", NULL };
-	    return (char_u *)unplace_arg[idx];
-	}
-    case EXP_SIGN_NAMES:
-	/* Complete with name of signs already defined */
-	current_idx = 0;
-	for (sp = first_sign; sp != NULL; sp = sp->sn_next)
-	    if (current_idx++ == idx)
-		return sp->sn_name;
-	return NULL;
+      return (char_u *)cmds[idx];
+    case EXP_DEFINE: {
+        char *define_arg[] = { "icon=", "linehl=", "text=", "texthl=", NULL };
+        return (char_u *)define_arg[idx];
+      }
+    case EXP_PLACE: {
+        char *place_arg[] = { "line=", "name=", "file=", "buffer=", NULL };
+        return (char_u *)place_arg[idx];
+      }
+    case EXP_UNPLACE: {
+        char *unplace_arg[] = { "file=", "buffer=", NULL };
+        return (char_u *)unplace_arg[idx];
+      }
+    case EXP_SIGN_NAMES: {
+        // Complete with name of signs already defined
+        int current_idx = 0;
+        for (sign_T *sp = first_sign; sp != NULL; sp = sp->sn_next) {
+          if (current_idx++ == idx) {
+            return sp->sn_name;
+          }
+        }
+      }
+      return NULL;
     default:
-	return NULL;
-    }
+      return NULL;
+  }
 }
 
 /*
@@ -5956,3 +6001,174 @@ void set_context_in_sign_cmd(expand_T *xp, char_u *arg)
     }
   }
 }
+
+/// Shows the effects of the :substitute command being typed ('inccommand').
+/// If inccommand=split, shows a preview window and later restores the layout.
+static buf_T *show_sub(pos_T old_cusr, char_u *pat, char_u *sub, linenr_T line1,
+                       linenr_T line2, MatchedLineVec *matched_lines)
+  FUNC_ATTR_NONNULL_ALL
+{
+  static handle_T bufnr = 0;  // special buffer, re-used on each visit
+
+  garray_T save_winsizes;
+  win_T *save_curwin = curwin;
+  cmdmod_T save_cmdmod = cmdmod;
+  char_u *save_shm_p = vim_strsave(p_shm);
+  size_t sub_size = mb_string2cells(sub);
+  size_t pat_size = mb_string2cells(pat);
+
+  // We keep a special-purpose buffer around, but don't assume it exists.
+  buf_T *preview_buf = bufnr ? buflist_findnr(bufnr) : 0;
+  win_size_save(&save_winsizes);  // Save current window sizes.
+  cmdmod.tab = 0;                 // disable :tab modifier
+  cmdmod.noswapfile = true;       // disable swap for preview buffer
+  // disable file info message
+  set_option_value((char_u *)"shm", 0L, (char_u *)"F", 0);
+
+  bool outside_curline = (line1 != curwin->w_cursor.lnum
+                          || line2 != curwin->w_cursor.lnum);
+  bool split = outside_curline && (*p_icm != 'n') && (sub_size || pat_size);
+  if (preview_buf == curbuf) {  // Preview buffer cannot preview itself!
+    split = false;
+    preview_buf = NULL;
+  }
+
+  // Place cursor on the first match after the cursor. (If all matches are
+  // above, then do_sub already placed cursor on the last match.)
+  colnr_T cur_col = -1;
+  MatchedLine curmatch;
+  for (size_t j = 0; j < matched_lines->size && cur_col == -1; j++) {
+    curmatch = matched_lines->items[j];
+    if (curmatch.lnum == old_cusr.lnum) {
+      // On cursor line; iterate in-line matches to find one after cursor.
+      for (size_t i = 0; i < curmatch.cols.size; i++) {
+        if (curmatch.cols.items[i] >= old_cusr.col) {
+          cur_col = curmatch.cols.items[i];
+          curwin->w_cursor.lnum = curmatch.lnum;
+          curwin->w_cursor.col = cur_col;
+          break;
+        }
+      }
+    } else if (curmatch.lnum > old_cusr.lnum) {
+      // After cursor; put cursor on first match there.
+      cur_col = curmatch.cols.items[0];
+      curwin->w_cursor.lnum = curmatch.lnum;
+      curwin->w_cursor.col = cur_col;
+    }
+  }
+
+  if (split && win_split((int)p_cwh, WSP_BOT) != FAIL) {
+    buf_open_special(preview_buf ? bufnr : 0, "[Preview]", "incsub");
+    buf_clear();
+    preview_buf = curbuf;
+    set_option_value((char_u *)"bufhidden", 0L, (char_u *)"hide", OPT_LOCAL);
+    bufnr = preview_buf->handle;
+    curbuf->b_p_bl = false;
+    curbuf->b_p_ma = true;
+    curbuf->b_p_ul = -1;
+    curbuf->b_p_tw = 0;         // Reset 'textwidth' (was set by ftplugin)
+    curwin->w_p_cul = false;
+    curwin->w_p_spell = false;
+    curwin->w_p_fen = false;
+
+    // Width of the "| lnum|..." column which displays the line numbers.
+    linenr_T highest_num_line = kv_last(*matched_lines).lnum;
+    int col_width = log10(highest_num_line) + 1 + 3;
+
+    char *str = NULL;
+    size_t old_line_size = 0;
+    size_t line_size;
+    int src_id_highlight = 0;
+    int hl_id = syn_check_group((char_u *)"Substitute", 13);
+
+    // Dump the lines into the preview buffer.
+    for (size_t line = 0; line < matched_lines->size; line++) {
+      MatchedLine mat = matched_lines->items[line];
+      line_size = mb_string2cells(mat.line) + col_width + 1;
+
+      // Reallocate if str not long enough
+      if (line_size > old_line_size) {
+        str = xrealloc(str, line_size * sizeof(char));
+        old_line_size = line_size;
+      }
+
+      // put " | lnum|line" into str and append it to the preview buffer
+      snprintf(str, line_size, "|%*ld| %s", col_width - 3, mat.lnum, mat.line);
+      ml_append(line, (char_u *)str, (colnr_T)line_size, false);
+
+      // highlight the replaced part
+      if (sub_size > 0) {
+        for (size_t i = 0; i < mat.cols.size; i++) {
+          colnr_T col_start = mat.cols.items[i] + col_width
+            + i * (sub_size - pat_size) + 1;
+          colnr_T col_end = col_start - 1 + sub_size;
+          src_id_highlight = bufhl_add_hl(curbuf, src_id_highlight, hl_id,
+                                          line + 1, col_start, col_end);
+        }
+      }
+    }
+    xfree(str);
+  }
+
+  redraw_later(SOME_VALID);
+
+  win_enter(save_curwin, false);  // Return to original window
+  win_size_restore(&save_winsizes);
+  ga_clear(&save_winsizes);
+
+  set_option_value((char_u *)"shm", 0L, save_shm_p, 0);
+  xfree(save_shm_p);
+
+  // Update screen now. Must do this _before_ close_windows().
+  int save_rd = RedrawingDisabled;
+  RedrawingDisabled = 0;
+  update_screen(NOT_VALID);
+  RedrawingDisabled = save_rd;
+
+  cmdmod = save_cmdmod;
+
+  return preview_buf;
+}
+
+/// :substitute command
+///
+/// If 'inccommand' is empty this just calls do_sub().
+/// If 'inccommand' is set, shows a "live" preview then removes the changes
+/// from undo history.
+void ex_substitute(exarg_T *eap)
+{
+  if (*p_icm == NUL || !eap->is_live) {  // 'inccommand' is disabled
+    (void)do_sub(eap);
+    return;
+  }
+
+  char_u *save_eap = eap->arg;
+  save_search_patterns();
+  int save_changedtick = curbuf->b_changedtick;
+  time_t save_b_u_time_cur = curbuf->b_u_time_cur;
+  u_header_T *save_b_u_newhead = curbuf->b_u_newhead;
+  long save_b_p_ul = curbuf->b_p_ul;
+  curbuf->b_p_ul = LONG_MAX;  // make sure we can undo all changes
+  block_autocmds();   // disable events before show_sub() opens window/buffer
+  emsg_off++;         // No error messages for live commands
+
+  buf_T *preview_buf = do_sub(eap);
+
+  if (save_changedtick != curbuf->b_changedtick) {
+    if (!u_undo_and_forget(1)) { abort(); }
+    // Restore newhead. It is meaningless when curhead is valid, but we must
+    // restore it so that undotree() is identical before/after the preview.
+    curbuf->b_u_newhead = save_b_u_newhead;
+    curbuf->b_u_time_cur = save_b_u_time_cur;
+    curbuf->b_changedtick = save_changedtick;
+  }
+  if (buf_valid(preview_buf)) {
+    // XXX: Must do this *after* u_undo_and_forget(), why?
+    close_windows(preview_buf, false);
+  }
+  curbuf->b_p_ul = save_b_p_ul;
+  eap->arg = save_eap;
+  restore_search_patterns();
+  emsg_off--;
+  unblock_autocmds();
+}
diff --git a/src/nvim/ex_cmds.h b/src/nvim/ex_cmds.h
index 721145efd8..ccb2202edb 100644
--- a/src/nvim/ex_cmds.h
+++ b/src/nvim/ex_cmds.h
@@ -5,14 +5,17 @@
 
 #include "nvim/os/time.h"
 #include "nvim/eval_defs.h"
+#include "nvim/pos.h"
+
+// flags for do_ecmd()
+#define ECMD_HIDE            0x01    // don't free the current buffer
+#define ECMD_SET_HELP        0x02    // set b_help flag of (new) buffer before
+                                     // opening file
+#define ECMD_OLDBUF          0x04    // use existing buffer if it exists
+#define ECMD_FORCEIT         0x08    // ! used in Ex command
+#define ECMD_ADDBUF          0x10    // don't edit, just add to buffer list
+#define ECMD_RESERVED_BUFNR  0x20    // bufnr argument is reserved bufnr
 
-/* flags for do_ecmd() */
-#define ECMD_HIDE       0x01    /* don't free the current buffer */
-#define ECMD_SET_HELP   0x02    /* set b_help flag of (new) buffer before
-                                   opening file */
-#define ECMD_OLDBUF     0x04    /* use existing buffer if it exists */
-#define ECMD_FORCEIT    0x08    /* ! used in Ex command */
-#define ECMD_ADDBUF     0x10    /* don't edit, just add to buffer list */
 
 /* for lnum argument in do_ecmd() */
 #define ECMD_LASTL      (linenr_T)0     /* use last position in loaded file */
diff --git a/src/nvim/ex_cmds.lua b/src/nvim/ex_cmds.lua
index 3f5d9b3244..95ede4bdc5 100644
--- a/src/nvim/ex_cmds.lua
+++ b/src/nvim/ex_cmds.lua
@@ -2194,7 +2194,7 @@ return {
     command='substitute',
     flags=bit.bor(RANGE, WHOLEFOLD, EXTRA, CMDWIN),
     addr_type=ADDR_LINES,
-    func='do_sub',
+    func='ex_substitute',
   },
   {
     command='sNext',
@@ -3181,7 +3181,7 @@ return {
     enum='CMD_and',
     flags=bit.bor(RANGE, WHOLEFOLD, EXTRA, CMDWIN, MODIFY),
     addr_type=ADDR_LINES,
-    func='do_sub',
+    func='ex_substitute',
   },
   {
     command='<',
@@ -3222,6 +3222,6 @@ return {
     enum='CMD_tilde',
     flags=bit.bor(RANGE, WHOLEFOLD, EXTRA, CMDWIN, MODIFY),
     addr_type=ADDR_LINES,
-    func='do_sub',
+    func='ex_substitute',
   },
 }
diff --git a/src/nvim/ex_cmds_defs.h b/src/nvim/ex_cmds_defs.h
index 8148eb5cee..8a2ebe2cd4 100644
--- a/src/nvim/ex_cmds_defs.h
+++ b/src/nvim/ex_cmds_defs.h
@@ -124,6 +124,7 @@ struct exarg {
   LineGetter getline;           ///< Function used to get the next line
   void   *cookie;               ///< argument for getline()
   struct condstack *cstack;     ///< condition stack for ":if" etc.
+  bool is_live;                 ///< 'inccommand' live preview
 };
 
 #define FORCE_BIN 1             // ":edit ++bin file"
diff --git a/src/nvim/ex_docmd.c b/src/nvim/ex_docmd.c
index 5e418bf099..30347cbe85 100644
--- a/src/nvim/ex_docmd.c
+++ b/src/nvim/ex_docmd.c
@@ -271,7 +271,7 @@ do_exmode (
 int do_cmdline_cmd(char *cmd)
 {
   return do_cmdline((char_u *)cmd, NULL, NULL,
-      DOCMD_VERBOSE|DOCMD_NOWAIT|DOCMD_KEYTYPED);
+                    DOCMD_NOWAIT|DOCMD_KEYTYPED);
 }
 
 /*
@@ -597,11 +597,11 @@ int do_cmdline(char_u *cmdline, LineGetter fgetline,
      *    do_one_cmd() will return NULL if there is no trailing '|'.
      *    "cmdline_copy" can change, e.g. for '%' and '#' expansion.
      */
-    ++recursive;
-    next_cmdline = do_one_cmd(&cmdline_copy, flags & DOCMD_VERBOSE,
-        &cstack,
-        cmd_getline, cmd_cookie);
-    --recursive;
+    recursive++;
+    next_cmdline = do_one_cmd(&cmdline_copy, flags,
+                              &cstack,
+                              cmd_getline, cmd_cookie);
+    recursive--;
 
     if (cmd_cookie == (void *)&cmd_loop_cookie)
       /* Use "current_line" from "cmd_loop_cookie", it may have been
@@ -1225,7 +1225,7 @@ static void get_wincmd_addr_type(char_u *arg, exarg_T *eap)
  * This function may be called recursively!
  */
 static char_u * do_one_cmd(char_u **cmdlinep,
-                           int sourcing,
+                           int flags,
                            struct condstack *cstack,
                            LineGetter fgetline,
                            void *cookie /* argument for fgetline() */
@@ -1248,7 +1248,8 @@ static char_u * do_one_cmd(char_u **cmdlinep,
   memset(&ea, 0, sizeof(ea));
   ea.line1 = 1;
   ea.line2 = 1;
-  ++ex_nesting_level;
+  ea.is_live = flags & DOCMD_LIVE;
+  ex_nesting_level++;
 
   /* When the last file has not been edited :q has to be typed twice. */
   if (quitmore
@@ -1726,8 +1727,9 @@ static char_u * do_one_cmd(char_u **cmdlinep,
   if (ea.cmdidx == CMD_SIZE) {
     if (!ea.skip) {
       STRCPY(IObuff, _("E492: Not an editor command"));
-      if (!sourcing)
+      if (!(flags & DOCMD_VERBOSE)) {
         append_command(*cmdlinep);
+      }
       errormsg = IObuff;
       did_emsg_syntax = TRUE;
     }
@@ -1809,7 +1811,7 @@ static char_u * do_one_cmd(char_u **cmdlinep,
      */
     if (!global_busy && ea.line1 > ea.line2) {
       if (msg_silent == 0) {
-        if (sourcing || exmode_active) {
+        if ((flags & DOCMD_VERBOSE) || exmode_active) {
           errormsg = (char_u *)_("E493: Backwards range given");
           goto doend;
         }
@@ -2221,7 +2223,7 @@ doend:
     curwin->w_cursor.lnum = 1;
 
   if (errormsg != NULL && *errormsg != NUL && !did_emsg) {
-    if (sourcing) {
+    if (flags & DOCMD_VERBOSE) {
       if (errormsg != IObuff) {
         STRCPY(IObuff, errormsg);
         errormsg = IObuff;
@@ -9647,3 +9649,25 @@ static void ex_terminal(exarg_T *eap)
     xfree(name);
   }
 }
+
+/// Check whether commandline starts with a live command
+///
+/// @param[in] cmd Commandline to check. May start with a range.
+///
+/// @return True if first command is a live command
+bool cmd_is_live(char_u *cmd)
+{
+  if (cmd == NULL) {
+    return false;
+  }
+
+  exarg_T ea;
+  // parse the command line
+  ea.cmd = skip_range(cmd, NULL);
+  if (*ea.cmd == '*') {
+    ea.cmd = skipwhite(ea.cmd + 1);
+  }
+  find_command(&ea, NULL);
+
+  return (ea.cmdidx == CMD_substitute);
+}
diff --git a/src/nvim/ex_docmd.h b/src/nvim/ex_docmd.h
index bafad20169..84f1a13a15 100644
--- a/src/nvim/ex_docmd.h
+++ b/src/nvim/ex_docmd.h
@@ -3,13 +3,14 @@
 
 #include "nvim/ex_cmds_defs.h"
 
-/* flags for do_cmdline() */
-#define DOCMD_VERBOSE   0x01    /* included command in error message */
-#define DOCMD_NOWAIT    0x02    /* don't call wait_return() and friends */
-#define DOCMD_REPEAT    0x04    /* repeat exec. until getline() returns NULL */
-#define DOCMD_KEYTYPED  0x08    /* don't reset KeyTyped */
-#define DOCMD_EXCRESET  0x10    /* reset exception environment (for debugging)*/
-#define DOCMD_KEEPLINE  0x20    /* keep typed line for repeating with "." */
+// flags for do_cmdline()
+#define DOCMD_VERBOSE   0x01      // included command in error message
+#define DOCMD_NOWAIT    0x02      // don't call wait_return() and friends
+#define DOCMD_REPEAT    0x04      // repeat exec. until getline() returns NULL
+#define DOCMD_KEYTYPED  0x08      // don't reset KeyTyped
+#define DOCMD_EXCRESET  0x10      // reset exception environment (for debugging
+#define DOCMD_KEEPLINE  0x20      // keep typed line for repeating with "."
+#define DOCMD_LIVE      0x40      // show updates as-you-type ("live" command)
 
 /* defines for eval_vars() */
 #define VALID_PATH              1
diff --git a/src/nvim/ex_getln.c b/src/nvim/ex_getln.c
index e525c949bd..17693ecfc8 100644
--- a/src/nvim/ex_getln.c
+++ b/src/nvim/ex_getln.c
@@ -1591,6 +1591,15 @@ static int command_line_changed(CommandLineState *s)
     msg_starthere();
     redrawcmdline();
     s->did_incsearch = true;
+  } else if (s->firstc == ':'
+             && KeyTyped            // only if interactive
+             && *p_icm != NUL       // 'inccommand' is set
+             && curbuf->b_p_ma      // buffer is modifiable
+             && cmdline_star == 0   // not typing a password
+             && cmd_is_live(ccline.cmdbuff)) {
+    // process a "live" command ('inccommand')
+    do_cmdline(ccline.cmdbuff, NULL, NULL, DOCMD_KEEPLINE|DOCMD_LIVE);
+    redrawcmdline();
   }
 
   if (cmdmsg_rl || (p_arshape && !p_tbidi && enc_utf8)) {
@@ -5137,16 +5146,12 @@ static int ex_window(void)
   }
   cmdwin_type = get_cmdline_type();
 
-  /* Create the command-line buffer empty. */
-  (void)do_ecmd(0, NULL, NULL, NULL, ECMD_ONE, ECMD_HIDE, NULL);
-  (void)setfname(curbuf, (char_u *)"[Command Line]", NULL, TRUE);
-  set_option_value((char_u *)"bt", 0L, (char_u *)"nofile", OPT_LOCAL);
-  set_option_value((char_u *)"swf", 0L, NULL, OPT_LOCAL);
-  curbuf->b_p_ma = TRUE;
-  curwin->w_p_fen = FALSE;
+  // Create empty command-line buffer.
+  buf_open_special(0, "[Command Line]", "nofile");
   curwin->w_p_rl = cmdmsg_rl;
-  cmdmsg_rl = FALSE;
-  RESET_BINDING(curwin);
+  cmdmsg_rl = false;
+  curbuf->b_p_ma = true;
+  curwin->w_p_fen = false;
 
   /* Do execute autocommands for setting the filetype (load syntax). */
   unblock_autocmds();
diff --git a/src/nvim/fileio.c b/src/nvim/fileio.c
index 0eb475d425..934d81dcd0 100644
--- a/src/nvim/fileio.c
+++ b/src/nvim/fileio.c
@@ -5062,10 +5062,10 @@ void buf_reload(buf_T *buf, int orig_mode)
    * the old contents.  Can't use memory only, the file might be
    * too big.  Use a hidden buffer to move the buffer contents to.
    */
-  if (bufempty() || saved == FAIL)
+  if (bufempty() || saved == FAIL) {
     savebuf = NULL;
-  else {
-    /* Allocate a buffer without putting it in the buffer list. */
+  } else {
+    // Allocate a buffer without putting it in the buffer list.
     savebuf = buflist_new(NULL, NULL, (linenr_T)1, BLN_DUMMY);
     if (savebuf != NULL && buf == curbuf) {
       /* Open the memline. */
@@ -6342,27 +6342,24 @@ aucmd_prepbuf (
   aco->new_curbuf = curbuf;
 }
 
-/*
- * Cleanup after executing autocommands for a (hidden) buffer.
- * Restore the window as it was (if possible).
- */
-void 
-aucmd_restbuf (
-    aco_save_T *aco               /* structure holding saved values */
-)
+/// Cleanup after executing autocommands for a (hidden) buffer.
+/// Restore the window as it was (if possible).
+///
+/// @param aco  structure holding saved values
+void aucmd_restbuf(aco_save_T *aco)
 {
   int dummy;
 
   if (aco->use_aucmd_win) {
-    --curbuf->b_nwindows;
-    /* Find "aucmd_win", it can't be closed, but it may be in another tab
-     * page. Do not trigger autocommands here. */
+    curbuf->b_nwindows--;
+    // Find "aucmd_win", it can't be closed, but it may be in another tab page.
+    // Do not trigger autocommands here.
     block_autocmds();
     if (curwin != aucmd_win) {
       FOR_ALL_TAB_WINDOWS(tp, wp) {
         if (wp == aucmd_win) {
           if (tp != curtab) {
-            goto_tabpage_tp(tp, TRUE, TRUE);
+            goto_tabpage_tp(tp, true, true);
           }
           win_goto(aucmd_win);
           goto win_found;
@@ -6371,49 +6368,50 @@ aucmd_restbuf (
     }
 win_found:
 
-    /* Remove the window and frame from the tree of frames. */
+    // Remove the window and frame from the tree of frames.
     (void)winframe_remove(curwin, &dummy, NULL);
     win_remove(curwin, NULL);
-    aucmd_win_used = FALSE;
-    last_status(FALSE);             /* may need to remove last status line */
-    restore_snapshot(SNAP_AUCMD_IDX, FALSE);
-    (void)win_comp_pos();       /* recompute window positions */
+    aucmd_win_used = false;
+    last_status(false);         // may need to remove last status line
+    restore_snapshot(SNAP_AUCMD_IDX, false);
+    (void)win_comp_pos();       // recompute window positions
     unblock_autocmds();
 
-    if (win_valid(aco->save_curwin))
+    if (win_valid(aco->save_curwin)) {
       curwin = aco->save_curwin;
-    else
-      /* Hmm, original window disappeared.  Just use the first one. */
+    } else {
+      // Hmm, original window disappeared.  Just use the first one.
       curwin = firstwin;
-    vars_clear(&aucmd_win->w_vars->dv_hashtab);      /* free all w: variables */
-    hash_init(&aucmd_win->w_vars->dv_hashtab);       /* re-use the hashtab */
+    }
+    vars_clear(&aucmd_win->w_vars->dv_hashtab);      // free all w: variables
+    hash_init(&aucmd_win->w_vars->dv_hashtab);       // re-use the hashtab
     curbuf = curwin->w_buffer;
 
     xfree(globaldir);
     globaldir = aco->globaldir;
 
-    /* the buffer contents may have changed */
+    // the buffer contents may have changed
     check_cursor();
     if (curwin->w_topline > curbuf->b_ml.ml_line_count) {
       curwin->w_topline = curbuf->b_ml.ml_line_count;
       curwin->w_topfill = 0;
     }
   } else {
-    /* restore curwin */
+    // restore curwin
     if (win_valid(aco->save_curwin)) {
-      /* Restore the buffer which was previously edited by curwin, if
-       * it was changed, we are still the same window and the buffer is
-       * valid. */
+      // Restore the buffer which was previously edited by curwin, if it was
+      // changed, we are still the same window and the buffer is valid.
       if (curwin == aco->new_curwin
           && curbuf != aco->new_curbuf
           && buf_valid(aco->new_curbuf)
           && aco->new_curbuf->b_ml.ml_mfp != NULL) {
-        if (curwin->w_s == &curbuf->b_s)
+        if (curwin->w_s == &curbuf->b_s) {
           curwin->w_s = &aco->new_curbuf->b_s;
-        --curbuf->b_nwindows;
+        }
+        curbuf->b_nwindows--;
         curbuf = aco->new_curbuf;
         curwin->w_buffer = curbuf;
-        ++curbuf->b_nwindows;
+        curbuf->b_nwindows++;
       }
 
       curwin = aco->save_curwin;
diff --git a/src/nvim/globals.h b/src/nvim/globals.h
index e42382ad00..301a2c1663 100644
--- a/src/nvim/globals.h
+++ b/src/nvim/globals.h
@@ -600,9 +600,9 @@ EXTERN int redraw_tabline INIT(= FALSE);           /* need to redraw tabline */
  * All buffers are linked in a list. 'firstbuf' points to the first entry,
  * 'lastbuf' to the last entry and 'curbuf' to the currently active buffer.
  */
-EXTERN buf_T    *firstbuf INIT(= NULL); /* first buffer */
-EXTERN buf_T    *lastbuf INIT(= NULL);  /* last buffer */
-EXTERN buf_T    *curbuf INIT(= NULL);   /* currently active buffer */
+EXTERN buf_T    *firstbuf INIT(= NULL);  // first buffer
+EXTERN buf_T    *lastbuf INIT(= NULL);   // last buffer
+EXTERN buf_T    *curbuf INIT(= NULL);    // currently active buffer
 
 // Iterates over all buffers in the buffer list.
 # define FOR_ALL_BUFFERS(buf) for (buf_T *buf = firstbuf; buf != NULL; buf = buf->b_next)
diff --git a/src/nvim/mark.c b/src/nvim/mark.c
index 2a65cf396b..6453c41415 100644
--- a/src/nvim/mark.c
+++ b/src/nvim/mark.c
@@ -473,7 +473,7 @@ static void fname2fnum(xfmark_T *fm)
     os_dirname(IObuff, IOSIZE);
     p = path_shorten_fname(NameBuff, IObuff);
 
-    /* buflist_new() will call fmarks_check_names() */
+    // buflist_new() will call fmarks_check_names()
     (void)buflist_new(NameBuff, p, (linenr_T)1, 0);
   }
 }
diff --git a/src/nvim/memline.c b/src/nvim/memline.c
index 5505335769..a9a53ebca7 100644
--- a/src/nvim/memline.c
+++ b/src/nvim/memline.c
@@ -2340,14 +2340,13 @@ int ml_replace(linenr_T lnum, char_u *line, int copy)
   return OK;
 }
 
-/*
- * Delete line 'lnum' in the current buffer.
- *
- * Check: The caller of this function should probably also call
- * deleted_lines() after this.
- *
- * return FAIL for failure, OK otherwise
- */
+/// Delete line `lnum` in the current buffer.
+///
+/// @note The caller of this function should probably also call
+/// deleted_lines() after this.
+///
+/// @param message  Show "--No lines in buffer--" message.
+/// @return FAIL for failure, OK otherwise
 int ml_delete(linenr_T lnum, int message)
 {
   ml_flush_line(curbuf);
diff --git a/src/nvim/option.c b/src/nvim/option.c
index 311982982b..3bd1ce217e 100644
--- a/src/nvim/option.c
+++ b/src/nvim/option.c
@@ -288,6 +288,7 @@ static char *(p_fdm_values[]) =       { "manual", "expr", "marker", "indent",
 static char *(p_fcl_values[]) =       { "all", NULL };
 static char *(p_cot_values[]) =       { "menu", "menuone", "longest", "preview",
                                         "noinsert", "noselect", NULL };
+static char *(p_icm_values[]) =       { "nosplit", "split", NULL };
 
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "option.c.generated.h"
@@ -3100,9 +3101,13 @@ did_set_string_option (
   else if (gvarp == &p_cino) {
     /* TODO: recognize errors */
     parse_cino(curbuf);
-  }
-  /* Options that are a list of flags. */
-  else {
+  // inccommand
+  } else if (varp == &p_icm) {
+      if (check_opt_strings(p_icm, p_icm_values, false) != OK) {
+        errmsg = e_invarg;
+      }
+  // Options that are a list of flags.
+  } else {
     p = NULL;
     if (varp == &p_ww)
       p = (char_u *)WW_ALL;
diff --git a/src/nvim/option_defs.h b/src/nvim/option_defs.h
index 8af6f2194f..6e89a093c8 100644
--- a/src/nvim/option_defs.h
+++ b/src/nvim/option_defs.h
@@ -468,6 +468,7 @@ EXTERN int p_icon;              // 'icon'
 EXTERN char_u   *p_iconstring;  // 'iconstring'
 EXTERN int p_ic;                // 'ignorecase'
 EXTERN int p_is;                // 'incsearch'
+EXTERN char_u   *p_icm;         // 'inccommand'
 EXTERN int p_im;                // 'insertmode'
 EXTERN char_u   *p_isf;         // 'isfname'
 EXTERN char_u   *p_isi;         // 'isident'
diff --git a/src/nvim/options.lua b/src/nvim/options.lua
index 583c63614a..14707aaa6c 100644
--- a/src/nvim/options.lua
+++ b/src/nvim/options.lua
@@ -1188,6 +1188,14 @@ return {
       }
     },
     {
+      full_name='inccommand', abbreviation='icm',
+      type='string', scope={'global'},
+      vi_def=true,
+      redraw={'everything'},
+      varname='p_icm',
+      defaults={if_true={vi=""}}
+    },
+    {
       full_name='include', abbreviation='inc',
       type='string', scope={'global', 'buffer'},
       vi_def=true,
diff --git a/src/nvim/quickfix.c b/src/nvim/quickfix.c
index a7aff15121..f23037613b 100644
--- a/src/nvim/quickfix.c
+++ b/src/nvim/quickfix.c
@@ -3326,10 +3326,11 @@ load_dummy_buffer (
   int failed = TRUE;
   aco_save_T aco;
 
-  /* Allocate a buffer without putting it in the buffer list. */
+  // Allocate a buffer without putting it in the buffer list.
   newbuf = buflist_new(NULL, NULL, (linenr_T)1, BLN_DUMMY);
-  if (newbuf == NULL)
+  if (newbuf == NULL) {
     return NULL;
+  }
 
   /* Init the options. */
   buf_copy_options(newbuf, BCO_ENTER | BCO_NOHELP);
diff --git a/src/nvim/syntax.c b/src/nvim/syntax.c
index b49ae9da21..e57965ac2c 100644
--- a/src/nvim/syntax.c
+++ b/src/nvim/syntax.c
@@ -5902,6 +5902,7 @@ static char *highlight_init_both[] =
   "WildMenu     ctermbg=Yellow ctermfg=Black guibg=Yellow guifg=Black",
   "default link EndOfBuffer NonText",
   "default link QuickFixLine Search",
+  "default link Substitute Search",
   NULL
 };
 
@@ -7159,16 +7160,13 @@ int syn_namen2id(char_u *linep, int len)
  */
 int syn_check_group(char_u *pp, int len)
 {
-  int id;
-  char_u  *name;
-
-  name = vim_strnsave(pp, len);
-
-  id = syn_name2id(name);
-  if (id == 0)                          /* doesn't exist yet */
+  char_u  *name = vim_strnsave(pp, len);
+  int id = syn_name2id(name);
+  if (id == 0) {  // doesn't exist yet
     id = syn_add_group(name);
-  else
+  } else {
     xfree(name);
+  }
   return id;
 }
 
diff --git a/src/nvim/undo.c b/src/nvim/undo.c
index d80aaf443a..2f4317980a 100644
--- a/src/nvim/undo.c
+++ b/src/nvim/undo.c
@@ -1669,7 +1669,7 @@ void u_undo(int count)
     undo_undoes = TRUE;
   else
     undo_undoes = !undo_undoes;
-  u_doit(count);
+  u_doit(count, false);
 }
 
 /*
@@ -1678,15 +1678,56 @@ void u_undo(int count)
  */
 void u_redo(int count)
 {
-  if (vim_strchr(p_cpo, CPO_UNDO) == NULL)
-    undo_undoes = FALSE;
-  u_doit(count);
+  if (vim_strchr(p_cpo, CPO_UNDO) == NULL) {
+    undo_undoes = false;
+  }
+
+  u_doit(count, false);
 }
 
-/*
- * Undo or redo, depending on 'undo_undoes', 'count' times.
- */
-static void u_doit(int startcount)
+/// undo, and remove the undo branch from the undo tree.
+bool u_undo_and_forget(int count)
+{
+  if (curbuf->b_u_synced == false) {
+    u_sync(true);
+    count = 1;
+  }
+  undo_undoes = true;
+  u_doit(count, true);
+
+  if (curbuf->b_u_curhead == NULL) {
+    // nothing was undone.
+    return false;
+  }
+
+  // Delete the current redo header
+  // set the redo header to the next alternative branch (if any)
+  // otherwise we will be in the leaf state
+  u_header_T *to_forget = curbuf->b_u_curhead;
+  curbuf->b_u_newhead = to_forget->uh_next.ptr;
+  curbuf->b_u_curhead = to_forget->uh_alt_next.ptr;
+  if (curbuf->b_u_curhead) {
+    to_forget->uh_alt_next.ptr = NULL;
+    curbuf->b_u_curhead->uh_alt_prev.ptr = to_forget->uh_alt_prev.ptr;
+    curbuf->b_u_seq_cur = curbuf->b_u_curhead->uh_seq-1;
+  } else if (curbuf->b_u_newhead) {
+    curbuf->b_u_seq_cur = curbuf->b_u_newhead->uh_seq;
+  }
+  if (to_forget->uh_alt_prev.ptr) {
+    to_forget->uh_alt_prev.ptr->uh_alt_next.ptr = curbuf->b_u_curhead;
+  }
+  if (curbuf->b_u_newhead) {
+    curbuf->b_u_newhead->uh_prev.ptr = curbuf->b_u_curhead;
+  }
+  if (curbuf->b_u_seq_last == to_forget->uh_seq) {
+    curbuf->b_u_seq_last--;
+  }
+  u_freebranch(curbuf, to_forget, NULL);
+  return true;
+}
+
+/// Undo or redo, depending on `undo_undoes`, `count` times.
+static void u_doit(int startcount, bool quiet)
 {
   int count = startcount;
 
@@ -1722,7 +1763,7 @@ static void u_doit(int startcount)
         break;
       }
 
-      u_undoredo(TRUE);
+      u_undoredo(true);
     } else {
       if (curbuf->b_u_curhead == NULL || get_undolevel() <= 0) {
         beep_flush();           /* nothing to redo */
@@ -1742,7 +1783,7 @@ static void u_doit(int startcount)
       curbuf->b_u_curhead = curbuf->b_u_curhead->uh_prev.ptr;
     }
   }
-  u_undo_end(undo_undoes, FALSE);
+  u_undo_end(undo_undoes, false, quiet);
 }
 
 /*
@@ -2055,7 +2096,7 @@ void undo_time(long step, int sec, int file, int absolute)
       }
     }
   }
-  u_undo_end(did_undo, absolute);
+  u_undo_end(did_undo, absolute, false);
 }
 
 /*
@@ -2299,16 +2340,13 @@ static void u_undoredo(int undo)
 #endif
 }
 
-/*
- * If we deleted or added lines, report the number of less/more lines.
- * Otherwise, report the number of changes (this may be incorrect
- * in some cases, but it's better than nothing).
- */
-static void 
-u_undo_end (
-    int did_undo,                   /* just did an undo */
-    int absolute                   /* used ":undo N" */
-)
+/// If we deleted or added lines, report the number of less/more lines.
+/// Otherwise, report the number of changes (this may be incorrect
+/// in some cases, but it's better than nothing).
+static void u_undo_end(
+    int did_undo,     ///< just did an undo
+    int absolute,     ///< used ":undo N"
+    bool quiet)
 {
   char        *msgstr;
   u_header_T  *uhp;
@@ -2317,9 +2355,11 @@ u_undo_end (
   if ((fdo_flags & FDO_UNDO) && KeyTyped)
     foldOpenCursor();
 
-  if (global_busy           /* no messages now, wait until global is finished */
-      || !messaging())        /* 'lazyredraw' set, don't do messages now */
+  if (quiet
+      || global_busy        // no messages until global is finished
+      || !messaging()) {    // 'lazyredraw' set, don't do messages now
     return;
+  }
 
   if (curbuf->b_ml.ml_flags & ML_EMPTY)
     --u_newcount;
diff --git a/src/nvim/window.c b/src/nvim/window.c
index 9c6a2e26a6..8512556c0a 100644
--- a/src/nvim/window.c
+++ b/src/nvim/window.c
@@ -2904,8 +2904,9 @@ static int win_alloc_firstwin(win_T *oldwin)
     /* Very first window, need to create an empty buffer for it and
      * initialize from scratch. */
     curbuf = buflist_new(NULL, NULL, 1L, BLN_LISTED);
-    if (curbuf == NULL)
+    if (curbuf == NULL) {
       return FAIL;
+    }
     curwin->w_buffer = curbuf;
     curwin->w_s = &(curbuf->b_s);
     curbuf->b_nwindows = 1;     /* there is one window */