Recognize JS regexp literals more correctly

Bug#25529
* lisp/progmodes/js.el (js--syntax-propertize-regexp-regexp): New
constant.
(js-syntax-propertize-regexp): Use it.  Remove "end" argument.
(js--syntax-propertize-regexp-syntax-table): Remove.
(js-syntax-propertize): Update.
* test/lisp/progmodes/js-tests.el (js-mode-regexp-syntax-bug-25529):
New test.

lisp/progmodes/js.el

@@ -1687,29 +1687,36 @@ This performs fontification according to `js--class-styles'."
                                    js--font-lock-keywords-3)
   "Font lock keywords for `js-mode'.  See `font-lock-keywords'.")
 
-(defconst js--syntax-propertize-regexp-syntax-table
-  (let ((st (make-char-table 'syntax-table (string-to-syntax "."))))
-    (modify-syntax-entry ?\[ "(]" st)
-    (modify-syntax-entry ?\] ")[" st)
-    (modify-syntax-entry ?\\ "\\" st)
-    st))
+(defconst js--syntax-propertize-regexp-regexp
+  (rx
+   ;; Start of regexp.
+   "/"
+   (0+ (or
+        ;; Match characters outside of a character class.
+        (not (any ?\[ ?/ ?\\))
+        ;; Match backslash quoted characters.
+        (and "\\" not-newline)
+        ;; Match character class.
+        (and
+         "["
+         (0+ (or
+              (not (any ?\] ?\\))
+              (and "\\" not-newline)))
+         "]")))
+   (group "/"))
+  "Regular expression matching a JavaScript regexp literal.")
 
 (defun js-syntax-propertize-regexp (end)
   (let ((ppss (syntax-ppss)))
     (when (eq (nth 3 ppss) ?/)
       ;; A /.../ regexp.
-      (while
-          (when (re-search-forward "\\(?:\\=\\|[^\\]\\)\\(?:\\\\\\\\\\)*/"
-                                   end 'move)
-            (if (nth 1 (with-syntax-table
-                           js--syntax-propertize-regexp-syntax-table
-                         (let ((parse-sexp-lookup-properties nil))
-                           (parse-partial-sexp (nth 8 ppss) (point)))))
-                ;; A / within a character class is not the end of a regexp.
-                t
-              (put-text-property (1- (point)) (point)
-                                 'syntax-table (string-to-syntax "\"/"))
-              nil))))))
+      (goto-char (nth 8 ppss))
+      (when (and (looking-at js--syntax-propertize-regexp-regexp)
+                 ;; Don't touch text after END.
+                 (<= (match-end 1) end))
+        (put-text-property (match-beginning 1) (match-end 1)
+                           'syntax-table (string-to-syntax "\"/"))
+        (goto-char (match-end 0))))))
 
 (defun js-syntax-propertize (start end)
   ;; JavaScript allows immediate regular expression objects, written /.../.

test/lisp/progmodes/js-tests.el

@@ -23,6 +23,7 @@
 
 (require 'ert)
 (require 'js)
+(require 'syntax)
 
 (ert-deftest js-mode-fill-bug-19399 ()
   (with-temp-buffer
@@ -99,6 +100,22 @@ if (!/[ (:,='\"]/.test(value)) {
     (forward-line)
     (should (looking-at " \\* test"))))
 
+(ert-deftest js-mode-regexp-syntax-bug-25529 ()
+  (dolist (regexp-contents '("[^[]"
+                             "[/]"
+                             ;; A comment with the regexp on the next
+                             ;; line.
+                             "*comment*/\n/regexp"))
+    (with-temp-buffer
+      (js-mode)
+      (insert "let x = /" regexp-contents "/;\n")
+      (save-excursion (insert "something();\n"))
+      ;; The failure mode was that the regexp literal was not
+      ;; recognized, causing the next line to be given string syntax;
+      ;; but check for comment syntax as well to prevent an
+      ;; implementation not recognizing the comment example.
+      (should-not (syntax-ppss-context (syntax-ppss))))))
+
 (provide 'js-tests)
 
 ;;; js-tests.el ends here