public static string ClearHtmlExceptA(string html) {
    string acceptable = "a";
    string stringPattern = @"</?(?(?=" + acceptable + @")notag|[a-zA-Z0-9]+)(?:\s[a-zA-Z0-9\-]+=?(?:(["",']?).*?\1?)?)*\s*/?>";
    html = Regex.Replace(html, stringPattern, "");
    html = Regex.Replace(html, @"[\t\n]", "", RegexOptions.IgnoreCase);
    html = Regex.Replace(html, @"[\r]", "", RegexOptions.IgnoreCase);
    //html = Regex.Replace(html, @"[\t\n\r\s]","",RegexOptions.IgnoreCase);
    return html;
  }