$re = '/[^\(]\(([^\(\)]+)\)[^\)]/m';
$str = 'ab(ab)ac
(ab)ndn
ab(ab(ac)an)
ab((ab)ab)ab
ab(ab(abb))ab
ab(ab(ab(ab))ab)
';
preg_match_all($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