$re = '/parent_folder\/05\/(1[1-9]|20)\//';
$str = 'parent_folder/05/01/
parent_folder/05/02/
parent_folder/05/03/
parent_folder/05/04/
parent_folder/05/05/
parent_folder/05/06/
parent_folder/05/07/
parent_folder/05/08/
parent_folder/05/09/
parent_folder/05/10/
parent_folder/05/11/
parent_folder/05/12/
parent_folder/05/13/
parent_folder/05/14/
parent_folder/05/15/
parent_folder/05/16/
parent_folder/05/17/
parent_folder/05/18/
parent_folder/05/19/
parent_folder/05/20/
parent_folder/05/21/
...
parent_folder/05/29/
parent_folder/05/30/';
preg_match_all($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