string test = @"<div id=""0"" >

                            0

                        </div>

                        <div id=""1"" class=""a"">

                            1

                            <div id=""2"">

                                2

                            </div>

                        </div>";
            //根据class=a来匹配
            Regex reg = new Regex(@"(?isx)<div[^>]*class=(['""]?)a\1[^>]*?>(?><div[^>]*>  (?<Open>)|</div>  (?<-Open>)|(?:(?!</?div\b).)*)*(?(Open)(?!))</div>");
            string match_str = reg.Match(test).Value;
            /*
              <div id=""1"" class=""a"">

                            1

                            <div id=""2"">

                                2

                            </div>

                        </div>
             */

------解决方案--------------------
坐等大婶，试问一下一定要用正则表达式吗？jquery不行吗？

------解决方案--------------------
C# code

    using System.Text.RegularExpressions;      //需要引用


    public string HtmlFilter(string strHtml)   //从字符串里删除HTML
    {
        Regex regex = new Regex("<[^>]*>", RegexOptions.IgnoreCase);
        string strOutput = regex.Replace(strHtml, "");
        return strOutput;
    }