$re = '/^(?=(?:\/[a-z0-9s_@\-/.]{1,255})+).{1,4095}$/i';
$str = '/whatever/thisisnotapornstash/StillNot255CharactersButTheNextFolderIs/BPLrmwQRjmtwIGEMDcgGk1BCRY6ZkKzsHoWqJNzGxCzlGTSZkfOei0QD2S3bGfqSMJMPxuvgHhUJotNgh3hGDYD01n56JiZy32JygaHHDLQbGWtkbFJy5BrMP5s6eL6V8Kcft71CxHZUMEEJ2LbYExYtPxaWuQ9USUCxbt7wTIjALoLN6aHW0GovD5euXWsYuOsqvyGuzJqjaohM9FFNmMz7ul0R4HxzTWWQqCZ8hp6O2yipRTs5k4RmGCTLfdsadsanY/ ';
preg_match($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