$re = '/(?:\G(?!^)|STUFF)\s+\K(?:(?!end)[a-z0-9(),]){1,16}(?=.*?end)/im';
$str = 'THINGS H X F N AB end STUFF H X4 O 9D I,8U JK(S) XD LMJIK END OTHER_STUFF .AS VFD F123 SDF1 SDF4... end';
preg_match_all($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