string htmltext = "我们的<a href=\"http://www.kaila.cn\">祖国</a>是花园。请访问。我们的<a href=\"http://www.kaila.cn\">祖国</a>是花园。请访问。";
System.Text.RegularExpressions.Regex r = new Regex(@"(<[aA]).*?>.*?(</[aA]>)");
System.Text.RegularExpressions.MatchCollection mc=r.Matches(htmltext);
/*System.Text.RegularExpressions.Match m = r.Match(htmltext);
if(m.Success) {
    Response.Write(m.Captures[0].ToString());
}*/
for(int i = 0; i < mc.Count; i++) {
    Response.Write(mc[i].Captures[0].ToString()+"<br>");
}

------解决方案--------------------
http://blog.csdn.net/precipitant/archive/2007/10/12/1821510.aspx