[gfm mode] Fix exponential running time in regexp

mode/gfm/gfm.js

@@ -75,7 +75,7 @@ CodeMirror.defineMode("gfm", function(config, modeConfig) {
           return "link";
         }
       }
-      if (stream.match(/^((?:[a-z][\w-]+:(?:\/{1,3}|[a-z0-9%])|www\d{0,3}[.]|[a-z0-9.\-]+[.][a-z]{2,4}\/)(?:[^\s()<>]+|\([^\s()<>]*\))+(?:\([^\s()<>]*\)|[^\s`!()\[\]{};:'".,<>?«»“”‘’]))/i) &&
+      if (stream.match(/^((?:[a-z][\w-]+:(?:\/{1,3}|[a-z0-9%])|www\d{0,3}[.]|[a-z0-9.\-]+[.][a-z]{2,4}\/)(?:[^\s()<>]|\([^\s()<>]*\))+(?:\([^\s()<>]*\)|[^\s`!()\[\]{};:'".,<>?«»“”‘’]))/i) &&
          stream.string.slice(stream.start - 2, stream.start) != "](") {
         // URLs
         // Taken from http://daringfireball.net/2010/07/improved_regex_for_matching_urls