Prevent mentions from matching "derp@derp"

Summary: See task / comment.
Test Plan:
https://secure.phabricator.com/file/view/PHID-FILE-f337d112ffe053fdea7d/
Reviewed By: tomo
Reviewers: mroch, tomo
CC: aran, tomo
Differential Revision: 614

src/infrastructure/markup/remarkup/markuprule/mention/PhabricatorRemarkupRuleMention.php

@@ -27,8 +27,10 @@ class PhabricatorRemarkupRuleMention
   public function apply($text) {
 
     // NOTE: Negative lookahead for period prevents us from picking up email
-    // addresses, while allowing constructs like "@tomo, lol".
-    $regexp = '/@([a-zA-Z0-9]+)\b(?![.])/';
+    // addresses, while allowing constructs like "@tomo, lol". The negative
+    // lookbehind for a word character prevents us from matching "mail@lists"
+    // while allowing "@tomo/@mroch".
+    $regexp = '/(?<!\w)@([a-zA-Z0-9]+)\b(?![.])/';
 
     $matches = null;
     $ok = preg_match_all($regexp, $text, $matches);