$re = '/(?<=v(\=|\/))([-a-zA-Z0-9_]+)|(?<=youtu\.be\/)([-a-zA-Z0-9_]+)/';
$str = 'http://youtu.be/iwGFalTRHDA
http://www.youtube.com/watch?v=iwGFalTRHDA
https://www.youtube.com/watch?v=iwGFalTRHDA
http://www.youtube.com/watch?v=iwGFalTRHDA&feature=related
http://youtu.be/iwGFalTRHDA
http://www.youtube.com/embed/watch?feature=player_embedded&v=iwGFalTRHDA
http://www.youtube.com/embed/watch?v=iwGFalTRHDA
http://www.youtube.com/embed/v=iwGFalTRHDA
http://www.youtube.com/watch?feature=player_embedded&v=iwGFalTRHDA
http://www.youtube.com/watch?v=iwGFalTRHDA
www.youtube.com/watch?v=iwGFalTRHDA
www.youtu.be/iwGFalTRHDA
youtu.be/iwGFalTRHDA
youtube.com/watch?v=iwGFalTRHDA
http://www.youtube.com/watch/iwGFalTRHDA
http://www.youtube.com/v/iwGFalTRHDA
http://www.youtube.com/v/i_GFalTRHDA
http://www.youtube.com/watch?v=i-GFalTRHDA&feature=related
http://www.youtube.com/attribution_link?u=/watch?v=aGmiw_rrNxk&feature=share&a=9QlmP1yvjcllp0h3l0NwuA
http://www.youtube.com/attribution_link?a=fF1CWYwxCQ4&u=/watch?v=qYr8opTPSaQ&feature=em-uploademail
http://www.youtube.com/attribution_link?a=fF1CWYwxCQ4&feature=em-uploademail&u=/watch?v=qYr8opTPSaQ';
preg_match_all($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