具体代码如下：
function get_domain($url){
$pattern = "/[w-] .(com|net|org|gov|cc|biz|info|cn)(.(cn|hk))*/";
preg_match($pattern, $url, $matches);
if(count($matches) > 0) {
return $matches[0];
}else{
$rs = parse_url($url);
$main_url = $rs["host"];
if(!strcmp(long2ip(sprintf("%u",ip2long($main_url))),$main_url)) {
return $main_url;
}else{
$arr = explode(".",$main_url);
$count=count($arr);
$endArr = array("com","net","org","3322");//com.cn net.cn 等情况
if (in_array($arr[$count-2],$endArr)){
if(isset($arr[$count-4])){
$domain = "http://".$arr[$count-4].".".$arr[$count-3].".".$arr[$count-2].".".$arr[$count-1];
}else{
$domain = $arr[$count-3].".".$arr[$count-2].".".$arr[$count-1];
}
}else{
if(isset($arr[$count-3])){
$domain = "http://".$arr[$count-3].".".$arr[$count-2].".".$arr[$count-1];
}else{
$domain = "http://".$arr[$count-2].".".$arr[$count-1];
}
}
return $domain;
}// end if(!strcmp...)
}// end if(count...)
}// end function
echo get_domain("http://blog.csd.com.hk")."<br/>";
echo get_domain("http://www.csd.net/www")."<br/>";
echo get_domain("http://csd.net/")."<br/>";
echo get_domain("http://csd.net/fdfdfd")."<br/>";
