就直接上代码了：

using System;
using System.Text.RegularExpressions;

/// <summary>
/// NoFollow contains the functionality to add rel=nofollow to unstusted links
/// </summary>
public static class NoFollow
{
  // the white list of domains (in lower case)
  private static string[] whitelist =    
     { "seoasp", "seoegghead.com">www.seoegghead.com", "www.cristiandarie.ro" };

  // finds all the links in the input string and processes them using fixLink
  public static string FixLinks(string input)
  {
    // define the match evaluator
    MatchEvaluator fixThisLink = new MatchEvaluator(NoFollow.fixLink);

    // fix the links in the input string
    string fixedInput = Regex.Replace(input,
                                      "(<a.*?>)",
                                      fixThisLink,
                                      RegexOptions.IgnoreCase);

    // return the "fixed" input string
    return fixedInput;
  }

  // receives a Regex match that contains a link such as
  // <a href="http://too.much.spam/"> and adds ref=nofollow if needed
  private static string fixLink(Match linkMatch)
  {
    // retrieve the link from the received Match
    string singleLink = linkMatch.Value;

    // if the link already has rel=nofollow, return it back as it is
    if (Regex.IsMatch(singleLink,
                      @"rel\s*?=\s*?['""]?.*?nofollow.*?['""]?",
                      RegexOptions.IgnoreCase))
    {
      return singleLink;
    }

    // use a named group to extract the URL from the link
    Match m = Regex.Match(singleLink,
                          @"href\s*?=\s*?['""]?(?<url>[^'""]*)['""]?",
                          RegexOptions.IgnoreCase);
    string url = m.Groups["url"].Value;

    // if URL doesn't contain http://, assume it's a local link
    if (!url.Contains("http://"))
    {
      return singleLink;
    }

    // extract the host name (such as www.cristiandarie.ro) from the URL
    Uri uri = new Uri(url);
    string host = uri.Host.ToLower();

    // if the host is in the whitelist, don't alter it
    if (Array.IndexOf(whitelist, host) >= 0)
    {
      return singleLink;
    }

    // if the URL already has a rel attribute, change its value to nofollow
    string newLink = Regex.Replace(singleLink,
             @"(?<a>rel\s*=\s*(?<b>['""]?))((?<c>[^'""\s]*|[^'""]*))(?<d>['""]?)?",
             "${a}nofollow${d}",
             RegexOptions.IgnoreCase);

    // if the string had a rel attribute that we changed, return the new link
    if (newLink != singleLink)
    {
      return newLink;
    }

    // if we reached this point, we need to add rel=nofollow to our link
    newLink = Regex.Replace(singleLink, "<a", @"<a rel=""nofollow""",
                            RegexOptions.IgnoreCase);
    return newLink;
  }
}