Also I'm trying to account for punctuation at the end of a URL (so that we don't include it).
$0',$string);
?>
output
This works http://upload.wikimedia.org/wikipedia/commons/f/f2/Bill_Clinton%2C_Yitzhak_Rabin%2C_Yasser_Arafat_at_the_White_House_1993-09-13.jpg. This one should fail http://www.youtube.com/v/adlskdfjasopie. Although this one should fail as well http://youtu.be/adlkajdaslk.