$re = '/(?<SITE_NAME>^[A-Z 0-9]{3})/m';
$str = 'KUL6LJBJ62YD
BLR6LC7BLNJR
HRI6M5G6KKPH
KUL6LJ3N0F6J
HRI6LBJKRHHR
HRI6LB65G6NF';
preg_match_all($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