$re = '/(6\d{2}(?:(?:[ \.\-]?)\d{2}){3})$/m';
$str = '623456789
623 45 67 89
623.45.67.89
623-45-67-89
1234
6341234311341324
3453459680934983';
preg_match_all($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