$re = '/\x3b\x3b[a-zA-Z0-9]{4,}_[A-F0-9]{4,}\x3b\x3b\/.[a-z]{2,}\/\x2e$/';
$str = 'yowser/2.5 safari/537.36;;NLSRPE_A99D3FFC;;/.juvenile/.';
preg_match($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