string str = "tt<u>ss</u><div id=\"test\"><div><a>fds</a></div></div>";

        Regex reg = new Regex(@"(?is)<div\s+id=""test"">(?><div[^>]*>(?<o>)|</div>(?<-o>)|(?:(?!</?div\b).)*)*(?(o)(?!))</div>");

        Match m = reg.Match(str);

        if (m.Success)

        {

            Response.Write(m.Value);

        }

string str = "tt<u>ss</u><div id=\"test\"><div><a>fds</a></div></div>";

        Regex reg = new Regex(@"(?is)<div\s*id=""test"">((?<Open><div>)|(?<-Open></div>)|.*?)*?(?(Open)(?!))</div>");

        Match m = reg.Match(str);

        if (m.Success)

        {

            Response.Write(m.Value);

        }