diff --git a/lib/gitlab/incoming_email.rb b/lib/gitlab/incoming_email.rb
index a386d9b36fbcdb2ea751367d2a10e8f758e35cde..0ea42148c5844368624326591194db45ca2f9f69 100644
--- a/lib/gitlab/incoming_email.rb
+++ b/lib/gitlab/incoming_email.rb
@@ -35,7 +35,9 @@ module Gitlab
       end
 
       def key_from_fallback_message_id(mail_id)
-        mail_id[/\Areply\-(.+)@#{Gitlab.config.gitlab.host}\z/, 1]
+        message_id_regexp = /\Areply\-(.+)@#{Gitlab.config.gitlab.host}\z/
+
+        mail_id[message_id_regexp, 1]
       end
 
       def scan_fallback_references(references)