$re = '/\((?:\([^\(\)]*?\)|[^\)])*(?=\.\w+$)/m';
$str = '02 Back for Good (Radio Mix).mp3
02 Back for Good (Radio Mi(x).mp3';
preg_match($re, $str, $matches, PREG_OFFSET_CAPTURE, 0);
// Print the entire match result
var_dump($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