New Post: why ignore four specified string

    {
        if (html == null)
        {
            throw new ArgumentNullException("html");
        }
        // replace & by & but only once!
        Regex rx = new Regex("&(?!(amp;)|(lt;)|(gt;)|(quot;))", RegexOptions.IgnoreCase);
        return rx.Replace(html, "&amp;").Replace("<", "&lt;").Replace(">", "&gt;").Replace("\"", "&quot;");
    }