<?

$domain="www.naum.kr";

preg_match('/[^\.]+\.([^\.]{4}|[^\.]{3}|(co|or|pe|ac)\.[^\.]{2}|[^\.]{2})$/i', $domain, $matches);

print_r($matches);

?>

결과 : 

Array ( [0] => naum.kr [1] => kr )