$re = '/^[A-Za-z0-9]{15}(?:[A-Za-z0-9]{3})?$/mi';
$str = '001i000001AWbWuGTA
001i000001AWbWugta
50130000000014c
50130000000014C
50130000000014ca
50130000000014cab
50130000000014cabde';
preg_match_all($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