$re = '/embed\/([\w+\-+]+)[\"\?]/';
$str = '<iframe width="560" height="315" src="//www.youtube.com/embed/0gugBiEkLwU?rel=0" frameborder="0" allowfullscreen></iframe>
<iframe width="910" height="500" src="https://www.youtube.com/embed/b8-dyNY-j_123-o0xM" frameborder="0" allowfullscreen></iframe>
<iframe width="910" height="500" src="https://www.youtube.com/embed/uMs_Rq4ideA" frameborder="0" allowfullscreen></iframe>
';
preg_match_all($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