[DependencyInjection] made a small optimization

src/Symfony/Components/DependencyInjection/Container.php

@@ -400,6 +400,6 @@ class Container implements ContainerInterface, \ArrayAccess, \Iterator
 
   static public function underscore($id)
   {
-    return strtolower(preg_replace(array('/_/', '/([A-Z]+)([A-Z][a-z])/', '/([a-z\d])([A-Z])/'), array('.', '\\1_\\2', '\\1_\\2'), $id));
+    return strtolower(preg_replace(array('/([A-Z]+)([A-Z][a-z])/', '/([a-z\d])([A-Z])/'), array('\\1_\\2', '\\1_\\2'), strtr($id, '_', '.')));
   }
 }