Fixed GFM autocomplete regex

Without this fix it is possible that the autocomplete popup will stay open with the very first match, no matter if there is a match further in the string.

Closes #25119

app/assets/javascripts/gfm_auto_complete.js.es6

@@ -59,12 +59,13 @@
         // Tweaked to commands to start without a space only if char before is a non-word character
         // https://github.com/ichord/At.js
         var _a, _y, regexp, match;
+        subtext = subtext.split(' ').pop();
         flag = flag.replace(/[\-\[\]\/\{\}\(\)\*\+\?\.\\\^\$\|]/g, "\\$&");
 
         _a = decodeURI("%C3%80");
         _y = decodeURI("%C3%BF");
 
-        regexp = new RegExp("(?:\\B|\\W|\\s)" + flag + "([A-Za-z" + _a + "-" + _y + "0-9_\'\.\+\-]*)|([^\\x00-\\xff]*)$", 'gi');
+        regexp = new RegExp("(?:\\B|\\W|\\s)" + flag + "(?!\\W)([A-Za-z" + _a + "-" + _y + "0-9_\'\.\+\-]*)|([^\\x00-\\xff]*)$", 'gi');
 
         match = regexp.exec(subtext);