我这个截取网页源代码之间怎么一直截取的不对呢？

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.Net;
using System.IO;

namespace WindowsApplication2
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            System.Net.HttpWebRequest req;
            System.Net.HttpWebResponse res;
            string url = "http://fanyi.yahoo.com.cn/translate_txt?trtext="+richTextBox2.Text+"&lp=en_zh";
            req = (System.Net.HttpWebRequest)System.Net.WebRequest.Create(url);
            res = (System.Net.HttpWebResponse)req.GetResponse();
            System.IO.StreamReader strm = new System.IO.StreamReader(res.GetResponseStream(), Encoding.GetEncoding("UTF-8"));
            string str = strm.ReadToEnd();
            str = str.Substring(str.IndexOf("resultcon")+65, str.IndexOf("div"));
            richTextBox1.Text = str;
        }
    }
}

结果为：你好，怎么样您  </div>
</div>
</div>
</div>
</form>

<div class="clr"></div>
</div>


<div class="parea">

<div class="pform">
<h2>翻译网页</h2>

但是我指向要“你好，怎么样您”
这个要怎么弄呢？ --------------------编程问答-------------------- 分析完整的返回的整体 html ，用正则匹配

你这样肯定不精确的，页面肯定多个 div

补充：.NET技术 ,  C#