PHP正则表达式如何反向引用？

Question

<?php
$str = "201309/22/120711ndvh1pkk1mmevy18.jpg";
$mode = "/(\d+)\/(\d+)\/(\w+)\.(\w+)/i";
$replace = "\1";
echo preg_replace($mode,$replace,$str);
?>
正确是写法应该是？