$re = '/(?<=\d\/|\.be\/|v[=\/])([\w\-]{11,})|^([\w\-]{11})$/im';
$str = 'http://www.youtube.com/watch?v=vpiMAaPTze8
http://youtu.be/l_la5XiQJdk
l_la5XiQJdk
thisdoesnotmatch';
preg_match_all($re, $str, $matches, PREG_SET_ORDER, 0);
// Print the entire match result
var_dump($matches);
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php