过滤外链！

protected void Page_Load(object sender, EventArgs e) 
{
 string s = @"<a href=""http://www.qq.com"">腾讯</a> <a href=""http://www.163.com"">网易</a> <a href=""http://www.sina.com.cn"">新浪</a>"; 
MatchEvaluator me = new MatchEvaluator(ABC); 
string r = Regex.Replace(s, @"(?is)<a[^>]*href=([""'])(?<href>.*?)\1>.*?</a>", me); 
Response.Write(r); Response.End(); }

 private string ABC(Match match) 
{ 

if (!match.Groups["href"].Value.ToLower().Contains("www.sina.com.cn")) 
return "";
 else return match.Value; 
}