Allow empty argument to `regexp-opt-charset'

* test/lisp/emacs-lisp/regexp-opt-tests.el (regexp-opt-charset):
Handle nil argument, and use regexp-quote for singletons.
* lisp/emacs-lisp/regexp-opt.el (regexp-opt-charset): Expand tests.

1 files changed, 12 insertions, 8 deletions

diff --git a/lisp/emacs-lisp/regexp-opt.el b/lisp/emacs-lisp/regexp-opt.el
index 00f72e284ad..b6104f22e7d 100644
--- a/lisp/emacs-lisp/regexp-opt.el
+++ b/lisp/emacs-lisp/regexp-opt.el
@@ -279,7 +279,9 @@ Merges keywords to avoid backtracking in Emacs's regexp matcher."
 
 (defun regexp-opt-charset (chars)
   "Return a regexp to match a character in CHARS.
-CHARS should be a list of characters."
+CHARS should be a list of characters.
+If CHARS is the empty list, the return value is a regexp that
+never matches anything."
   ;; The basic idea is to find character ranges.  Also we take care in the
   ;; position of character set meta characters in the character set regexp.
   ;;
@@ -326,13 +328,15 @@ CHARS should be a list of characters."
 	(while (>= end start)
 	  (setq charset (format "%s%c" charset start))
 	  (setq start (1+ start)))))
-    ;;
-    ;; Make sure a caret is not first and a dash is first or last.
-    (if (and (string-equal charset "") (string-equal bracket ""))
-	(if (string-equal dash "")
-            "\\^"                       ; [^] is not a valid regexp
-          (concat "[" dash caret "]"))
-      (concat "[" bracket charset caret dash "]"))))
+
+    ;; Make sure that ] is first, ^ is not first, - is first or last.
+    (let ((all (concat bracket charset caret dash)))
+      (pcase (length all)
+        (0 regexp-unmatchable)
+        (1 (regexp-quote all))
+        (_ (if (string-equal all "^-")
+               "[-^]"
+             (concat "[" all "]")))))))
 
 
 (defun regexp-opt--contains-prefix (strings)