C# - How To Highlight Complete Words With Keywords With Regexp

I have a longer text and some keywords. I want to highlight these keywords in my text. That is no problem with this code:

private static string HighlightKeywords2(string keywords, string text)
{
// Swap out the ,<space> for pipes and add the braces
Regex r = new Regex(@", ?");
keywords = "(" + r.Replace(keywords, @"|") + ")";
// Get ready to replace the keywords
r = new Regex(keywords, RegexOptions.Singleline | RegexOptions.IgnoreCase);
// Do the replace
return r.Replace(text, new MatchEvaluator(MatchEval2));
}
private static string MatchEval2(Match match)
{
if (match.Groups[1].Success)
{
return "<b>" + match.ToString() + "</b>";
}
return ""; //no match
}

But when the word"tournament" is in the text and the keyword "tour" it becomes <b>tour</b>nament. I want to it to highlight the complete word: <b>tournament</b>.