parse youtube video id using preg_match

I am attempting to parse the video ID of a youtube URL using preg_match. I found a regular expression on this site that appears to work;

(?<=v=)[a-zA-Z0-9-]+(?=&)|(?<=[0-9]/)[^&\n]+|(?<=v=)[^&\n]+

As shown in this pic:

My PHP is as follows, but it doesn't work (gives Unknown modifier '[' error)...

<?
 $subject = "http://www.youtube.com/watch?v=z_AbfPXTKms&NR=1";

 preg_match("(?<=v=)[a-zA-Z0-9-]+(?=&)|(?<=[0-9]/)[^&\n]+|(?<=v=)[^&\n]+", $subject, $matches);

 print "<pre>";
 print_r($matches);
 print "</pre>";

?>

Cheers