bug#25529: diagnosis and one approach to a fix

[Tom Tromey]

Tom> Another way is to use a regexp, see appended.
Tom> I'll try to write a test for this soon.

Now with a test.

Dmitry, I'd appreciate your comments on this.

Tom

commit 0841c586b5a933773e770579b4a9cd6f86b2dcf7
Author: Tom Tromey <tom@tromey.com>
Date:   Sun Feb 5 11:40:18 2017 -0700

    Recognize JS regexp literals more correctly
    
    * lisp/progmodes/js.el (js--syntax-propertize-regexp-regexp): New
    constant.
    (js-syntax-propertize-regexp): Use it.
    * test/lisp/progmodes/js-tests.el (js-mode-regexp-syntax-bug-25529):
    New test.

diff --git a/lisp/progmodes/js.el b/lisp/progmodes/js.el
index e42e014..145aa6f 100644
--- a/lisp/progmodes/js.el
+++ b/lisp/progmodes/js.el
@@ -1694,22 +1694,33 @@ js--syntax-propertize-regexp-syntax-table
     (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 the body of 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 (looking-at js--syntax-propertize-regexp-regexp)
+        (put-text-property (match-beginning 1) (match-end 1)
+                           'syntax-table (string-to-syntax "\"/"))))))
 
 (defun js-syntax-propertize (start end)
   ;; JavaScript allows immediate regular expression objects, written /.../.
diff --git a/test/lisp/progmodes/js-tests.el b/test/lisp/progmodes/js-tests.el
index 7cb737c..d1a8db0 100644
--- a/test/lisp/progmodes/js-tests.el
+++ b/test/lisp/progmodes/js-tests.el
@@ -99,6 +99,15 @@
     (forward-line)
     (should (looking-at " \\* test"))))
 
+(ert-deftest js-mode-regexp-syntax-bug-25529 ()
+  (with-temp-buffer
+    (js-mode)
+    (insert "let x = /[^[]/;\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.
+    (should-not (nth 3 (syntax-ppss)))))
+
 (provide 'js-tests)
 
 ;;; js-tests.el ends here