$re = '/^(https:\/\/open.spotify.com\/user\/spotify\/playlist\/|spotify:user:spotify:playlist:)([a-zA-Z0-9]+)(.*)$/m';
$str = 'https://open.spotify.com/user/spotify/playlist/37i9dQZF1DZ06evO2ZpGiQ?si=-6S0EBlURrao__YyIOW6bg
spotify:user:spotify:playlist:37i9dQZF1DZ06evO2ZpGiQ';
preg_match_all($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