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<?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'));

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38.76 ms | 402 KiB | 5 Q