fix(inccommand): do not change reg_prev_sub when previewing

3 files changed, 84 insertions, 5 deletions

diff --git a/src/nvim/ex_cmds.c b/src/nvim/ex_cmds.c
index 95390b1a70..66e9738f98 100644
--- a/src/nvim/ex_cmds.c
+++ b/src/nvim/ex_cmds.c
@@ -3629,8 +3629,14 @@ static buf_T *do_sub(exarg_T *eap, proftime_T timeout, bool do_buf_event, handle
   // We do it here once to avoid it to be replaced over and over again.
   // But don't do it when it starts with "\=", then it's an expression.
   assert(sub != NULL);
+
+  bool sub_needs_free = false;
   if (!(sub[0] == '\\' && sub[1] == '=')) {
+    char_u *source = sub;
     sub = regtilde(sub, p_magic);
+    // When previewing, the new pattern allocated by regtilde() needs to be freed
+    // in this function because it will not be used or freed by regtilde() later.
+    sub_needs_free = preview && sub != source;
   }
 
   // Check for a match on each line.
@@ -4425,6 +4431,10 @@ skip:
 
   kv_destroy(preview_lines.subresults);
 
+  if (sub_needs_free) {
+    xfree(sub);
+  }
+
   return preview_buf;
 #undef ADJUST_SUB_FIRSTLNUM
 #undef PUSH_PREVIEW_LINES
diff --git a/src/nvim/regexp.c b/src/nvim/regexp.c
index 45e580dbee..9bea54ad2c 100644
--- a/src/nvim/regexp.c
+++ b/src/nvim/regexp.c
@@ -6538,11 +6538,16 @@ char_u *regtilde(char_u *source, int magic)
     }
   }
 
-  xfree(reg_prev_sub);
-  if (newsub != source)         /* newsub was allocated, just keep it */
-    reg_prev_sub = newsub;
-  else                          /* no ~ found, need to save newsub  */
-    reg_prev_sub = vim_strsave(newsub);
+  // Only change reg_prev_sub when not previewing.
+  if (!(State & CMDPREVIEW)) {
+    xfree(reg_prev_sub);
+    if (newsub != source) {             // newsub was allocated, just keep it
+      reg_prev_sub = newsub;
+    } else {                            // no ~ found, need to save newsub
+      reg_prev_sub = vim_strsave(newsub);
+    }
+  }
+
   return newsub;
 }
 
diff --git a/test/functional/ui/inccommand_spec.lua b/test/functional/ui/inccommand_spec.lua
index b6e2f2311f..10700d9508 100644
--- a/test/functional/ui/inccommand_spec.lua
+++ b/test/functional/ui/inccommand_spec.lua
@@ -293,6 +293,70 @@ describe(":substitute, 'inccommand' preserves", function()
     end)
   end
 
+  for _, case in ipairs({'', 'split', 'nosplit'}) do
+    it('previous substitute string ~ (inccommand='..case..') #12109', function()
+      local screen = Screen.new(30,10)
+      common_setup(screen, case, default_text)
+
+      feed(':%s/Inc/SUB<CR>')
+      expect([[
+        SUB substitution on
+        two lines
+        ]])
+
+      feed(':%s/line/')
+      poke_eventloop()
+      feed('~')
+      poke_eventloop()
+      feed('<CR>')
+      expect([[
+        SUB substitution on
+        two SUBs
+        ]])
+
+      feed(':%s/sti/')
+      poke_eventloop()
+      feed('~')
+      poke_eventloop()
+      feed('B')
+      poke_eventloop()
+      feed('<CR>')
+      expect([[
+        SUB subSUBBtution on
+        two SUBs
+        ]])
+
+      feed(':%s/ion/NEW<CR>')
+      expect([[
+        SUB subSUBBtutNEW on
+        two SUBs
+        ]])
+
+      feed(':%s/two/')
+      poke_eventloop()
+      feed('N')
+      poke_eventloop()
+      feed('~')
+      poke_eventloop()
+      feed('<CR>')
+      expect([[
+        SUB subSUBBtutNEW on
+        NNEW SUBs
+        ]])
+
+      feed(':%s/bS/')
+      poke_eventloop()
+      feed('~')
+      poke_eventloop()
+      feed('W')
+      poke_eventloop()
+      feed('<CR>')
+      expect([[
+        SUB suNNEWWUBBtutNEW on
+        NNEW SUBs
+        ]])
+    end)
+  end
 end)
 
 describe(":substitute, 'inccommand' preserves undo", function()