If that is too big for emacs-27, the absolute mimimum fix would be:

modified   lisp/emacs-lisp/re-builder.el
@@ -779,7 +779,9 @@ reb-mark-non-matching-parenthesis
   ;; We have a small string, check the whole of it, but wait until
   ;; everything else is fontified.
   (when (>= bound (point-max))
-    (let (left-pars
+    (defvar n-reb)
+    (let ((n-reb 0)
+          left-pars
           faces-here)
       (goto-char (point-min))
       (while (and (reb-while 100 'n-reb "mark-par")

[1: c1234ca9c3]: 2019-06-27 19:08:42 +0200
  Add more fontification to regexp builder mode
  https://git.savannah.gnu.org/cgit/emacs.git/commit/?id=c1234ca9c3703cd8bae3912f3e0a1948bae3aed1