$re = '/([0-9]{1,5})(_|__)([a-zA-Z0-9-]{1,99})/m';
$str = '42_foo-bar-baz
11111_qux
42__foo-bar-baz
11111__qux';
preg_match_all($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