Regular expression to convert mark down to HTML

This would be *italicized* text and this would be **bold** text

This would be <em>italicized</em> text and this would be <strong>bold</strong> text

private string DoItalicsAndBold (string text)
{
    // <strong> must go first:
    text = Regex.Replace (text, @"(**|__) (?=S) (.+?[*_]*) (?<=S) 1",
                          new MatchEvaluator (BoldEvaluator),
                          RegexOptions.IgnorePatternWhitespace | RegexOptions.Singleline);

    // Then <em>:
    text = Regex.Replace (text, @"(*|_) (?=S) (.+?) (?<=S) 1",
                          new MatchEvaluator (ItalicsEvaluator),
                          RegexOptions.IgnorePatternWhitespace | RegexOptions.Singleline);
    return text;
}

private string ItalicsEvaluator (Match match)
{
    return string.Format ("<em>{0}</em>", match.Groups[2].Value);
}

private string BoldEvaluator (Match match)
{
    return string.Format ("<strong>{0}</strong>", match.Groups[2].Value);
}