1 files changed, 4 insertions, 1 deletions

diff --git a/lisp/emacs-lisp/regexp-opt.el b/lisp/emacs-lisp/regexp-opt.el
index de36f1d5446..e849cd60b96 100644
--- a/lisp/emacs-lisp/regexp-opt.el
+++ b/lisp/emacs-lisp/regexp-opt.el
@@ -205,7 +205,10 @@ so we can use character sets rather than grouping parenthesis."
             (if (> (length xiffus) 0)
                 ;; common suffix: take it and recurse on the prefixes.
                 (let* ((n (- (length xiffus)))
-                       (prefixes (mapcar (lambda (s) (substring s 0 n)) strings)))
+                       (prefixes
+                        ;; Sorting is necessary in cases such as ("ad" "d").
+                        (sort (mapcar (lambda (s) (substring s 0 n)) strings)
+                              'string-lessp)))
                   (concat open-group
                           (regexp-opt-group prefixes t t)
                           (regexp-quote