$re = '/(?<!\d)((0)?[1-9]|[12][0-9]|3[01])(?(2)\D?|\D)(0?[1-9]|1[0-2])(?(2)\D?|\D)([12][0-9]|20[12][0-9])(?!\d)/';
$str = '01-05-2015
01-05-15
01052015
010515
1-5-2015
152015';
preg_match_all($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