$re = '/^\s*<p>\s*(?:</?\w+/?>\s*)*<a href=\"(?:(?:[a-z]+:)?//(?:[^\" >/.]*\.)*(?:youtube\.com|youtu\.be)[/\"])\W*(?![\W\w]*?</(?:code|blockquote)>)(?#Separate start with youtube watch; lower TP rate, but most TP are not otherwise detected)/m';
$str = '';
preg_match_all($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