$re = '/^(.*?)(?:;(.+?))?(?:;(.+?))?(?:;(.+?))?(?:;(.+?))?(?:;(.+?))?(?:;(.+?))?(?:;(.+?))?(?:;(.+?))?(?:;(.+?))?(?:;(.+?))?(?:;(.+?))?(?:;(.+?))?(?:;(.+?))?(?:;(.+?))?(?:;(.+?))?(?:;(.+?))?(?:;(.+?))?(?:;(.+?))?(?:;(.+?))?$/mix';
$str = '1.jpg;2.jpg;3.jpg;4.jpg;5.jpg;6.jpg;7.jpg;8.jpg;9.jpg;10.jpg;11.jpg;12.jpg;13.jpg;14.jpg;15.jpg;16.jpg;17.jpg;18.jpg;19.jpg;20.jpg';
preg_match_all($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