Adding an attribute to all html tags in C#

My use case for this requirement was that users can add text on a web site and could include links (a tags). I want all these links
to open a new window when clicked on. So I needed to add a target="_blank" to all the a tags in the text.

My first idea was to use pure Regular Expressions, but I couldn't figure out how to use the negative lookahead feature, I needed
this because I can't simple add a target="_blank" to each a tag, what if it already has one?

So instead I am using a simple Regex to find the appropriate tags and then use a MatchEvaluator, which is a very cool
feature of the .Net regular expression class. It allows you to compute the replacement string in a method for each
match.

public class HtmlHelper
{
    string _attrib = string.Empty;

    public string AddAttribute(string source, string tagName, string attrib)
    {
        _attrib = attrib;

        string term = "<" + tagName + " [^>]+>";

        Regex r = new Regex(term, RegexOptions.IgnoreCase);

        MatchEvaluator myEvaluator = new MatchEvaluator(this.ProcessMatch);

        return r.Replace(source, myEvaluator);

    }

    private string ProcessMatch(Match m)
    {
        string tag = m.Value;
        if (tag.IndexOf(_attrib) == -1)
        {
            tag = tag.Replace(">", " " + _attrib + ">");
        }

        return tag;
    }
}

HtmlHelper helper = new HtmlHelper();
htmlText = helper.AddAttribute(htmlText, "a", "target=\"_blank\"");