$re = '/^((\()(?1)?(\))(?1)?)$/m';
$str = '//SHOULD
()
(())
((()))
()()
(())()
()()()
()()()()()
(((())))
()((()))
()()()()((()))(())((()))(((((((())))))))
//NOT
(
)
)(
))((
)()(
(()
))
())
(()
()()())
()())
)))))))))
((((((((
';
preg_match_all($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