$re = '/^(?:[\dA-Z-]+\.(?:jpe?g|png|gif))?(?:, (?:[\dA-Z-]+\.(?:jpe?g|png|gif)))*$/mi';
$str = 'bad_image.jpg
2017-06-01-03-55-00-5930782c5c0b1.jpg
2017-06-01-03-55-00-5930782c5c0b1.jpeg, bad_image.jpg
2017-06-01-03-55-00-5930782c5c0b1.png, 2017-06-01-03-55-00-5930782c5c0b1.gif
';
preg_match_all($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