$re = '/(?<=\()(?:[^\(\)]*|\((?R)\))*(?=\))/';
$str = '$upper(string1)
$lower(string2)
$lower(artist (feat other artist))
$upper(This(is (why PHP is (awesome) (feat other artist))))
$nothing(nothing)';
preg_match_all($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