$re = '/^(ht{2}ps?:\/{2})?(w{3}.)?regex101\.com\/r\/\w+$/m';
$str = 'http://regex101.com/r/fW0vZ2
https://regex101.com/r/mH1fN5
regex101.com/r/121fa5
www.regex101.com/r/mH1fN5
http://www.regex101.com/r/fW0vZ2
https://www.regex101.com/r/mH1fN5
http://www.regex101.com/r/';
preg_match_all($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