$re = '/^(\d? ?)?(\d{3}|\(\d{3}\))[ -]?\d{3}[ -]?\d+$/im';
$str = '1 (555) 555-5555
5555555555
1 555-555-5555
555-555-5555';
preg_match_all($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