- var_dump: documentation ( source)
- preg_replace: documentation ( source)
<?php
var_dump(preg_replace('#_([a-zA-Z])+(\.jpg|\.gif|\.png|\.jpeg){1}$#','_n$2','http://photos-h.ak.fbcdn.net/hphotos-ak-frc3/t1.0-0/1538683_705144079517314_1930601228_n.jpg'));