MDEV-6965 non-captured group \2 in regexp_replace

mysql-test/r/func_regexp_pcre.result

@@ -839,3 +839,9 @@ SELECT REGEXP_REPLACE('abc','^(.*)(.*)$','\\1/\\2');
 REGEXP_REPLACE('abc','^(.*)(.*)$','\\1/\\2')
 /abc
 SET default_regex_flags=DEFAULT;
+#
+# MDEV-6965 non-captured group \2 in regexp_replace
+#
+SELECT REGEXP_REPLACE('1 foo and bar', '(\\d+) foo and (\\d+ )?bar', '\\1 this and \\2that');
+REGEXP_REPLACE('1 foo and bar', '(\\d+) foo and (\\d+ )?bar', '\\1 this and \\2that')
+1 this and that

mysql-test/t/func_regexp_pcre.test

@@ -397,3 +397,8 @@ SET default_regex_flags='UNGREEDY';
 SELECT REGEXP_SUBSTR('abc','.+');
 SELECT REGEXP_REPLACE('abc','^(.*)(.*)$','\\1/\\2');
 SET default_regex_flags=DEFAULT;
+
+--echo #
+--echo # MDEV-6965 non-captured group \2 in regexp_replace
+--echo #
+SELECT REGEXP_REPLACE('1 foo and bar', '(\\d+) foo and (\\d+ )?bar', '\\1 this and \\2that');

sql/item_strfunc.cc

@@ -1379,13 +1379,16 @@ bool Item_func_regexp_replace::append_replacement(String *str,
       break; /* End of line */
     beg+= cnv;
 
-    if ((n= ((int) wc) - '0') >= 0 && n <= 9 && n < re.nsubpatterns())
+    if ((n= ((int) wc) - '0') >= 0 && n <= 9)
+    {
+      if (n < re.nsubpatterns())
       {
         /* A valid sub-pattern reference found */
         int pbeg= re.subpattern_start(n), plength= re.subpattern_end(n) - pbeg;
         if (str->append(source->str + pbeg, plength, cs))
           return true;
       }
+    }
     else
     {
       /*