Code
 #region 过滤关键字
        public static string FilterKeyWord(string content, string regexstr)
        {
            Regex regex = new Regex(regexstr, RegexOptions.IgnoreCase);
            MatchEvaluator evaluator = new MatchEvaluator(ReplaceContent);
            return regex.Replace(content, evaluator);
        }
        public static string ReplaceContent(Match m)
        {
            string connStr = ConfigurationManager.ConnectionStrings["easthomedbConnectionString"].ConnectionString;
            LinkSourseBusiness linkSourseBusiness = new LinkSourseBusiness(connStr);
            List<LinkSourse> linkSourseList = linkSourseBusiness.GetListPagedCustom(" content='" + m.Value + "'", " id desc ", 1, 1);
            if (linkSourseList.Count > 0 && linkSourseList[0].Url != "")
            {
                return "<a href='" + linkSourseList[0].Url + "' target='" + linkSourseList[0].Target + "'>" + linkSourseList[0].Content + "</a>";
            }
            return m.Value;
        }
        #endregion

Regex regex = = new Regex("");//要过滤的正则表达式
string result = regex.Replace(source, new MatchEvaluator(MatchHelper.CapText));



public static class MatchHelper
{
        public static string CapText(Match m)
        {
            // Get the matched string.
            string matchString = m.ToString();

            matchString = "<span style='color:red;'>" + matchString + "</span>";

            return matchString;
        }
}