It is lookbehind, meaning it looks before the match for something, it seemed to work for me locally.

```diff
diff --git a/grammars/gfm.cson b/grammars/gfm.cson
index e09dab6..fadd1ec 100644
--- a/grammars/gfm.cson
+++ b/grammars/gfm.cson
@@ -394,7 +394,7 @@
         'name': 'comment.quote.gfm'
   }
   {
-    'match': '(?:^|\\s)(@)(\\w+)'
+    'match': '(?<=^|\\s)(@)(\\w+)'
     'captures':
       '1':
         'name': 'variable.mention.gfm'
```

![screen shot 2014-03-26 at 4 47 03 pm](https://cloud.githubusercontent.com/assets/671378/2532378/f5a54508-b540-11e3-90da-40bfcb6a3241.png)