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.

2 files changed, 36 insertions, 13 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)
diff --git a/test/lisp/emacs-lisp/regexp-opt-tests.el b/test/lisp/emacs-lisp/regexp-opt-tests.el
index 1fc49909d3e..927de8c6a5f 100644
--- a/test/lisp/emacs-lisp/regexp-opt-tests.el
+++ b/test/lisp/emacs-lisp/regexp-opt-tests.el
@@ -1,4 +1,4 @@
-;;; regexp-tests.el --- Test suite for regular expression handling.
+;;; regexp-opt-tests.el --- Tests for regexp-opt.el
 
 ;; Copyright (C) 2013-2019 Free Software Foundation, Inc.
 
@@ -25,9 +25,28 @@
 
 (require 'regexp-opt)
 
-(ert-deftest regexp-test-regexp-opt ()
-  "Test the `compilation-error-regexp-alist' regexps.
-The test data is in `compile-tests--test-regexps-data'."
-  (should (string-match (regexp-opt-charset '(?^)) "a^b")))
+(ert-deftest regexp-opt-charset ()
+  (should (equal (regexp-opt-charset '(?a ?b ?a)) "[ab]"))
+  (should (equal (regexp-opt-charset '(?D ?d ?B ?a ?b ?C ?7 ?a ?c ?A))
+                 "[7A-Da-d]"))
+  (should (equal (regexp-opt-charset '(?a)) "a"))
+
+  (should (equal (regexp-opt-charset '(?^)) "\\^"))
+  (should (equal (regexp-opt-charset '(?-)) "-"))
+  (should (equal (regexp-opt-charset '(?\])) "]"))
+  (should (equal (regexp-opt-charset '(?^ ?\])) "[]^]"))
+  (should (equal (regexp-opt-charset '(?^ ?-)) "[-^]"))
+  (should (equal (regexp-opt-charset '(?- ?\])) "[]-]"))
+  (should (equal (regexp-opt-charset '(?- ?\] ?^)) "[]^-]"))
+
+  (should (equal (regexp-opt-charset '(?^ ?a)) "[a^]"))
+  (should (equal (regexp-opt-charset '(?- ?a)) "[a-]"))
+  (should (equal (regexp-opt-charset '(?\] ?a)) "[]a]"))
+  (should (equal (regexp-opt-charset '(?^ ?\] ?a)) "[]a^]"))
+  (should (equal (regexp-opt-charset '(?^ ?- ?a)) "[a^-]"))
+  (should (equal (regexp-opt-charset '(?- ?\] ?a)) "[]a-]"))
+  (should (equal (regexp-opt-charset '(?- ?\] ?^ ?a)) "[]a^-]"))
+
+  (should (equal (regexp-opt-charset '()) regexp-unmatchable)))
 
 ;;; regexp-tests.el ends here.