Slightly loosen the mention regexp to allow for "Go ask @tomo."

Summary: Currently, we incorrectly flag these under the "@lists.com" rule.
Test Plan:
https://secure.phabricator.com/file/view/PHID-FILE-887c2bc66eb9d47d02f4/
Reviewed By: tomo
Reviewers: tomo
CC: aran, epriestley, tomo
Differential Revision: 708

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

@@ -29,8 +29,9 @@ class PhabricatorRemarkupRuleMention
     // NOTE: Negative lookahead for period prevents us from picking up email
     // 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(?![.])/';
+    // while allowing "@tomo/@mroch". The negative lookahead prevents us from
+    // matching "@joe.com" while allowing us to match "hey, @joe.".
+    $regexp = '/(?<!\w)@([a-zA-Z0-9]+)\b(?![.]\w)/';
 
     $matches = null;
     $ok = preg_match_all($regexp, $text, $matches);