与其依赖不可靠的正则表达式，不如使用 System.Uri 为您进行解析.使用这样的代码:

Rather than relying on unreliable regex use System.Uri to do the parsing for you. Use a code like this:

string uriStr = "www.foo.com";
if (!uriStr.Contains(Uri.SchemeDelimiter)) {
    uriStr = string.Concat(Uri.UriSchemeHttp, Uri.SchemeDelimiter, uriStr);
}
Uri uri = new Uri(uriStr);
string domain = uri.Host; // will return www.foo.com

现在获取您可以使用的***域:

string tld = uri.GetLeftPart( UriPartial.Authority ); // will return foo.com