protected string str = "<table><tr><td>sdasasdsdd</td></tr></table><br><p>sds</p><img id='img1' src='http://www.baidu.com/img/baidu_logo.gif' width='100' height='50' alt=''>aaassss<br><img src='http://www.baidu.com/img/baidu_logo.gif' width='100' height='50' alt=''> 说是道 ";

    protected void Page_Load(object sender,EventArgs e)
    {

        //string regexstr = @"<[^>]*>";    //去除所有的标签

        //@"<script[^>]*?>.*?</script>" //去除所有脚本，中间部分也删除
        
        // string regexstr = @"<img[^>]*>";   //去除图片的正则

       // string regexstr = @"<(?!br).*?>";   //去除所有标签，只剩br

        // string regexstr = @"<table[^>]*?>.*?</table>";   //去除table里面的所有内容

        string regexstr = @"<(?!img|br|p|/p).*?>";   //去除所有标签，只剩img,br,p
  
        str = Regex.Replace(str,regexstr,string.Empty,RegexOptions.IgnoreCase);

    }