diff --git a/src/cmdexpand.c b/src/cmdexpand.c
index b7597ea31e..13d540e77d 100644
--- a/src/cmdexpand.c
+++ b/src/cmdexpand.c
@@ -4686,6 +4686,82 @@ copy_substring_from_pos(pos_T *start, pos_T *end, char_u **match,
     return OK;
 }
 
+/*
+ * Returns TRUE if the given string `str` matches the regex pattern `pat`.
+ * Honors the 'ignorecase' (p_ic) and 'smartcase' (p_scs) settings to determine
+ * case sensitivity.
+ */
+    static int
+is_regex_match(char_u *pat, char_u *str)
+{
+    regmatch_T	regmatch;
+    int		result;
+
+    regmatch.regprog = vim_regcomp(pat, RE_MAGIC + RE_STRING);
+    if (regmatch.regprog == NULL)
+	return FALSE;
+    regmatch.rm_ic = p_ic;
+    if (p_ic && p_scs)
+	regmatch.rm_ic = !pat_has_uppercase(pat);
+
+    result = vim_regexec_nl(&regmatch, str, (colnr_T)0);
+
+    vim_regfree(regmatch.regprog);
+    return result;
+}
+
+/*
+ * Constructs a new match string by appending text from the buffer (starting at
+ * end_match_pos) to the given pattern `pat`. The result is a concatenation of
+ * `pat` and the word following end_match_pos.
+ * If 'lowercase' is TRUE, the appended text is converted to lowercase before
+ * being combined. Returns the newly allocated match string, or NULL on failure.
+ */
+    static char_u *
+concat_pattern_with_buffer_match(
+	char_u *pat,
+	int pat_len,
+	pos_T *end_match_pos,
+	int lowercase UNUSED)
+{
+    char_u  *line = ml_get(end_match_pos->lnum);
+    char_u  *word_end = find_word_end(line + end_match_pos->col);
+    int	    match_len = (int)(word_end - (line + end_match_pos->col));
+    char_u  *match = alloc(match_len + pat_len + 1);  // +1 for NUL
+
+    if (match == NULL)
+	return NULL;
+    mch_memmove(match, pat, pat_len);
+    if (match_len > 0)
+    {
+#if defined(FEAT_EVAL) || defined(FEAT_SPELL) || defined(PROTO)
+	if (lowercase)
+	{
+	    char_u  *mword = vim_strnsave(line + end_match_pos->col,
+		    match_len);
+	    if (mword == NULL)
+		goto cleanup;
+	    char_u  *lower = strlow_save(mword);
+	    vim_free(mword);
+	    if (lower == NULL)
+		goto cleanup;
+	    mch_memmove(match + pat_len, lower, match_len);
+	    vim_free(lower);
+	}
+	else
+#endif
+	    mch_memmove(match + pat_len, line + end_match_pos->col, match_len);
+    }
+    match[pat_len + match_len] = NUL;
+    return match;
+
+#if defined(FEAT_EVAL) || defined(FEAT_SPELL) || defined(PROTO)
+cleanup:
+    vim_free(match);
+    return NULL;
+#endif
+}
+
 /*
  * Search for strings matching "pat" in the specified range and return them.
  * Returns OK on success, FAIL otherwise.
@@ -4701,12 +4777,11 @@ expand_pattern_in_buf(
     garray_T	ga;
     int		found_new_match;
     int		looped_around = FALSE;
-    int		pat_len, match_len;
+    int		pat_len;
     int		has_range = FALSE;
     int		compl_started = FALSE;
     int		search_flags;
-    char_u	*match, *line, *word_end;
-    regmatch_T	regmatch;
+    char_u	*match, *full_match;
 
 #ifdef FEAT_SEARCH_EXTRA
     has_range = search_first_line != 0;
@@ -4731,11 +4806,6 @@ expand_pattern_in_buf(
     search_flags = SEARCH_OPT | SEARCH_NOOF | SEARCH_PEEK | SEARCH_NFMSG
 	| (has_range ? SEARCH_START : 0);
 
-    regmatch.regprog = vim_regcomp(pat, RE_MAGIC + RE_STRING);
-    if (regmatch.regprog == NULL)
-	return FAIL;
-    regmatch.rm_ic = p_ic;
-
     ga_init2(&ga, sizeof(char_u *), 10); // Use growable array of char_u*
 
     for (;;)
@@ -4796,30 +4866,30 @@ expand_pattern_in_buf(
 	}
 
 	// Extract the matching text prepended to completed word
-	if (!copy_substring_from_pos(&cur_match_pos, &end_match_pos, &match,
+	if (!copy_substring_from_pos(&cur_match_pos, &end_match_pos, &full_match,
 		    &word_end_pos))
 	    break;
 
-	// Verify that the constructed match actually matches the pattern with
-	// correct case sensitivity
-	if (!vim_regexec_nl(&regmatch, match, (colnr_T)0))
+	// Construct a new match from completed word appended to pattern itself
+	match = concat_pattern_with_buffer_match(pat, pat_len, &end_match_pos,
+		FALSE);
+
+	// The regex pattern may include '\C' or '\c'. First, try matching the
+	// buffer word as-is. If it doesn't match, try again with the lowercase
+	// version of the word to handle smartcase behavior.
+	if (match == NULL || !is_regex_match(match, full_match))
 	{
 	    vim_free(match);
-	    continue;
+	    match = concat_pattern_with_buffer_match(pat, pat_len,
+		    &end_match_pos, TRUE);
+	    if (match == NULL || !is_regex_match(match, full_match))
+	    {
+		vim_free(match);
+		vim_free(full_match);
+		continue;
+	    }
 	}
-	vim_free(match);
-
-	// Construct a new match from completed word appended to pattern itself
-	line = ml_get(end_match_pos.lnum);
-	word_end = find_word_end(line + end_match_pos.col);  // col starts from 0
-	match_len = (int)(word_end - (line + end_match_pos.col));
-	match = alloc(match_len + pat_len + 1);  // +1 for NUL
-	if (match == NULL)
-	    goto cleanup;
-	mch_memmove(match, pat, pat_len);
-	if (match_len > 0)
-	    mch_memmove(match + pat_len, line + end_match_pos.col, match_len);
-	match[pat_len + match_len] = NUL;
+	vim_free(full_match);
 
 	// Include this match if it is not a duplicate
 	for (int i = 0; i < ga.ga_len; ++i)
@@ -4842,14 +4912,11 @@ expand_pattern_in_buf(
 	    cur_match_pos = word_end_pos;
     }
 
-    vim_regfree(regmatch.regprog);
-
     *matches = (char_u **)ga.ga_data;
     *numMatches = ga.ga_len;
     return OK;
 
 cleanup:
-    vim_regfree(regmatch.regprog);
     ga_clear_strings(&ga);
     return FAIL;
 }
diff --git a/src/testdir/test_cmdline.vim b/src/testdir/test_cmdline.vim
index b3d293639a..c5a7d85c76 100644
--- a/src/testdir/test_cmdline.vim
+++ b/src/testdir/test_cmdline.vim
@@ -4481,6 +4481,8 @@ func Test_search_complete()
   call assert_equal(['Foobar', 'FooBARR'], g:compl_info.matches)
   call feedkeys("gg/FO\<tab>\<f9>", 'tx')
   call assert_equal({},  g:compl_info)
+  call feedkeys("gg/\\cFo\<tab>\<f9>", 'tx')
+  call assert_equal(['\cFoobar', '\cFooBAr', '\cFooBARR'], g:compl_info.matches)
   set ignorecase
   call feedkeys("gg/f\<tab>\<f9>", 'tx')
   call assert_equal(['foobar', 'fooBAr', 'fooBARR'], g:compl_info.matches)
@@ -4488,13 +4490,19 @@ func Test_search_complete()
   call assert_equal(['Foobar', 'FooBAr', 'FooBARR'], g:compl_info.matches)
   call feedkeys("gg/FO\<tab>\<f9>", 'tx')
   call assert_equal(['FOobar', 'FOoBAr', 'FOoBARR'], g:compl_info.matches)
+  call feedkeys("gg/\\Cfo\<tab>\<f9>", 'tx')
+  call assert_equal(['\CfooBAr', '\Cfoobar'], g:compl_info.matches)
   set smartcase
   call feedkeys("gg/f\<tab>\<f9>", 'tx')
-  call assert_equal(['foobar', 'fooBAr', 'fooBARR'], g:compl_info.matches)
+  call assert_equal(['foobar', 'fooBAr', 'foobarr'], g:compl_info.matches)
   call feedkeys("gg/Fo\<tab>\<f9>", 'tx')
   call assert_equal(['Foobar', 'FooBARR'], g:compl_info.matches)
   call feedkeys("gg/FO\<tab>\<f9>", 'tx')
   call assert_equal({},  g:compl_info)
+  call feedkeys("gg/\\Cfo\<tab>\<f9>", 'tx')
+  call assert_equal(['\CfooBAr', '\Cfoobar'], g:compl_info.matches)
+  call feedkeys("gg/\\cFo\<tab>\<f9>", 'tx')
+  call assert_equal(['\cFoobar', '\cFooBAr', '\cFooBARR'], g:compl_info.matches)
 
   bw!
   call test_override("char_avail", 0)
diff --git a/src/version.c b/src/version.c
index 76c51ab75f..a892dcb4a2 100644
--- a/src/version.c
+++ b/src/version.c
@@ -719,6 +719,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1520,
 /**/
     1519,
 /**/