/// <summary>
/// 给关键字加链接，同一关键字只加一次
/// </summary>
/// <param name="src">源字符串</param>
/// <param name="keys">关键字泛型</param>
/// <returns>替换后结果</returns>
private string keyAddUrl(string src, List<string> keys)
{
    Regex reg = new Regex(@"(?i)(?:^|(?<!<a\b(?>[^<>]*))>)(?>[^<>]*)(?:<|$)");
    int length = 0;
    string temp = string.Empty;
    return reg.Replace(src, delegate(Match m)
    {
        temp = m.Value;
        length = temp.Length;
        for (int i = keys.Count - 1; i >= 0; i--)
        {
            temp = Regex.Replace(temp, @"(?is)^((?:(?:(?!" + Regex.Escape(keys[i]) + @"|</?a\b).)*<a\b(?:(?!</?a\b).)*</a>)*(?:(?!" + Regex.Escape(keys[i]) + @"|</?a\b).)*)(?<tag>" + Regex.Escape(keys[i]) + @")", @"$1<a href=""http://www.21shipin.com"" target=""_blank"" title=""${tag}"">${tag}</a>");
            if (length != temp.Length)
            {
                keys.Remove(keys[i]);
            }
            length = temp.Length;
        }
        return temp;
    });
}
//调用
string str1 = "我想学习c语言教程，我想看的是C语言视频教程，其它什么C语言教程，我都不想看。我喜欢C语言";
List<string> keys = new List<string>(new string[] { "c语言", "C语言教程", "c语言视频教程" });
string result = keyAddUrl(str1, keys);
richTextBox2.Text = result;
/*------------输出------------
我想学习<a href="http://www.21shipin.com" target="_blank" title="c语言教程">c语言教程</a>，我想看的是<a href="http://www.21shipin.com" target="_blank" title="C语言视频教程">C语言视频教程</a>，其它什么<a href="http://www.21shipin.com" target="_blank" title="C语言">C语言</a>教程，我都不想看。我喜欢C语言
*/
 

来自：过客