C#实现的html内容截取.

/// <summary>
        /// 按文本内容长度截取HTML字符串(支持截取带HTML代码样式的字符串)
        /// </summary>
        /// <param name="html">将要截取的字符串参数</param>
        /// <param name="len">截取的字节长度</param>
        /// <param name="endString">字符串末尾补上的字符串</param>
        /// <returns>返回截取后的字符串</returns>
        public static string HTMLSubstring(string html, int len, string endString)
        {
            if (string.IsNullOrEmpty(html) || html.Length <= len) return html;
            MatchCollection mcentiry, mchtmlTag;
            ArrayList inputHTMLTag = new ArrayList();
            string r = "",tmpValue;
            int rWordCount = 0, wordNum = 0, i = 0;
            Regex rxSingle = new Regex("^<(br|hr|img|input|param|meta|link)", RegexOptions.Compiled | RegexOptions.IgnoreCase)//是否单标签正则
                , rxEndTag = new Regex("</[^>]+>", RegexOptions.Compiled)//是否结束标签正则
                , rxTagName = new Regex("<([a-z]+)[^>]*>", RegexOptions.Compiled | RegexOptions.IgnoreCase)//获取标签名正则
                , rxHtmlTag = new Regex("<[^>]+>", RegexOptions.Compiled)//html标签正则
                , rxEntity = new Regex("&[a-z]{1,9};", RegexOptions.Compiled | RegexOptions.IgnoreCase)//实体正则
                , rxEntityReverse = new Regex("§", RegexOptions.Compiled)//反向替换实体正则
                ;
            html = html.Replace("§", "§");//替换字符§为他的实体“§”，以便进行下一步替换
            mcentiry = rxEntity.Matches(html);//收集实体对象到匹配数组中
            html = rxEntity.Replace(html, "§");//替换实体为特殊字符§，这样好控制一个实体占用一个字符
            mchtmlTag = rxHtmlTag.Matches(html);//收集html标签到匹配数组中
            html = rxHtmlTag.Replace(html, "__HTMLTag__");//替换为特殊标签
            string []arrWord = html.Split(new string[] { "__HTMLTag__" }, StringSplitOptions.None);//通过特殊标签进行拆分
            wordNum = arrWord.Length;
            //获取指定内容长度及HTML标签
            for (; i < wordNum; i++) {
                if (rWordCount + arrWord[i].Length >= len) r += arrWord[i].Substring(0, len - rWordCount) + endString;
                else r += arrWord[i];
                rWordCount += arrWord[i].Length;//计算已经获取到的字符长度
                if (rWordCount >= len) break;
                //搜集已经添加的非单标签，以便封闭HTML标签对
                if (i < wordNum - 1) {
                    tmpValue=mchtmlTag[i].Value;
                    if (!rxSingle.IsMatch(tmpValue))
                    { //不是单标签
                        if (rxEndTag.IsMatch(tmpValue)&&inputHTMLTag.Count>0) inputHTMLTag.RemoveAt(inputHTMLTag.Count - 1);
                        else inputHTMLTag.Add(tmpValue);
                    }
                    r += tmpValue;
                }
 
            }
            //替换回实体
            for (i = 0; i < mcentiry.Count; i++) r = rxEntityReverse.Replace(r, mcentiry[i].Value, 1);
            //封闭标签
            for (i = inputHTMLTag.Count - 1; i >= 0; i--) r += "</" + rxTagName.Match(inputHTMLTag[i].ToString()).Groups[1].Value + ">";
            return r;
        }