diff -u emacs-src/lisp/emacs-lisp/re-builder.el emacs-src/lisp/emacs-lisp/re-builder-new.el
--- /lisp/emacs-lisp/re-builder.el	2021-08-02 20:47:39.226669281 -0700
+++ /lisp/emacs-lisp/re-builder-new.el	2021-08-02 20:37:27.442020958 -0700
@@ -642,16 +642,19 @@
 	 (submatches 0)
 	 firstmatch
          here
+         start end
          firstmatch-after-here)
     (with-current-buffer reb-target-buffer
         (setq here
               (if reb-target-window
                   (with-selected-window reb-target-window (window-point))
-                (point)))
+                (point))
+              start (if (region-active-p) (region-beginning) (point-min))
+              end   (if (region-active-p) (region-end) (point-max)))
       (reb-delete-overlays)
-      (goto-char (point-min))
+      (goto-char start)
       (while (and (not (eobp))
-		  (re-search-forward re (point-max) t)
+		  (re-search-forward re end t)
 		  (or (not reb-auto-match-limit)
 		      (< matches reb-auto-match-limit)))
 	(when (and (= 0 (length (match-string 0)))

Diff finished.  Mon Aug  2 20:48:01 2021