Extract out regex from PhabricatorRemarkupRuleMention

Summary:
I'd like to use this regex elsewhere and copying and pasting is
bad.

Test Plan: none

Reviewers: casey, epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D2038

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

@@ -27,17 +27,17 @@ final class PhabricatorRemarkupRuleMention
 
   const KEY_MENTIONED = 'phabricator.mentioned-user-phids';
 
-  public function apply($text) {
 
   // 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". The negative lookahead prevents us from
   // matching "@joe.com" while allowing us to match "hey, @joe.".
-    $regexp = '/(?<!\w)@([a-zA-Z0-9]+)\b(?![.]\w)/';
+  const REGEX = '/(?<!\w)@([a-zA-Z0-9]+)\b(?![.]\w)/';
 
+  public function apply($text) {
     return preg_replace_callback(
-      $regexp,
+      self::REGEX,
       array($this, 'markupMention'),
       $text);
   }