$re = '/\[youtube\].*(?:(?:youtu\.be\/|v\/|vi\/|u\/\w\/|embed\/)|(?:(?:watch)?\?v(?:i)?=|\&v(?:i)?=))([^#\&\?]*).*\[\/youtube\]/';
$str = '[youtube]https://www.youtube.com/watch?v=ikyBLVda00I[/youtube]
[youtube]https://www.youtube.com/watch?v=PT2_F-1esPk&list=PLMC9KNkIncKtPzgY-5rmhvj7fax8fdxoj&index=5[/youtube]
[youtube]https://youtu.be/kOkQ4T5WO9E?list=PLMC9KNkIncKtPzgY-5rmhvj7fax8fdxoj[/youtube]
[youtube]https://www.youtube.com/watch?v=ikyBLVda00I[/youtube]
[youtube]https://www.youtube.com/watch?v=PT2_F-1esPk&list=PLMC9KNkIncKtPzgY-5rmhvj7fax8fdxoj&index=5[/youtube]
[youtube]https://www.youtube.com/watch?v=ikyBLVda00I[/youtube]
';
preg_match($re, $str, $matches, PREG_OFFSET_CAPTURE, 0);
// Print the entire match result
var_dump($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