$re = '/(?m)(?:^.*\R?){1,20}/';
$str = 'Clip 08.jpg
Clip 31.jpg
Clip 31b.jpg
Clip 32.jpg
Clip 40.jpg
Clip 40b.jpg
Clip 53.jpg
Clip 54.jpg
Maui Clip 53b.jpg';
preg_match($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