$re = '/(?= #lookahead
(\(
(?>[^()]+|(?1))* #more comments
\))
)/x';
$str = '(+ (+ 4 3) ( - 3 2) 5)';
preg_match_all($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