$re = '/^(?:(?<track>\d{1,3}) \s*[\s-]\s*)?
(?:(?<artist>.+?) \s*-\s*)?
(?<title>.+)
(?<ext> \.mp3)/mx';
$str = '01 Example Artist - Example Song.mp3
01 Example Song.mp3 (In this example, there\'s no artist so that group should be null)
Example Artist - Example Song.mp3
Example Song.mp3 (Again, no artist)';
preg_match_all($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