protected void Button1_Click(object sender, EventArgs e)
{
    string htmlcode = GetHTML("http://www.imooc.com/learn/348");
    Regex reg = new Regex("<strong><i class=\"state-expand\"></i>.*</strong>");
    MatchCollection ms = reg.Matches(htmlcode);
    foreach (Match m in ms)
    {
       Response.Write(m.ToString()+"<br>");
    }
}

public string GetHTML(string url)
{
    WebClient web = new WebClient();
    byte[] buffer = web.DownloadData(url);
    return Encoding.UTF8.GetString(buffer);
}