$re = '/^Hex Nut(?!.*\bNylon\b).*$/m';
$str = 'Hex Nut M16x1.5 Nylon 9121600-HI Carbon Steel MF
Hex Nut 5/8 9161590 Carbon Steel UNF 9161590
Hex Nut 1/2 9141270 Carbon Steel UNF 9141270
Hex Nut M20x1.5 9122000-HI Carbon Steel MF';
preg_match_all($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