Get Youtube ID from URL string in C#

public static string _pegarIDYoutube(string URLYoutube)
{
    var regexString = @"^(http(s?):\/\/)?(www\.)?youtu(be)?\.([a-z])+\/(watch(.*?)(\?|\&)v=)?(.*?)(&(.)*)?$";

    Match match = Regex.Match(URLYoutube, regexString,
    RegexOptions.IgnoreCase);

    if (match.Success)
        return match.Groups[9].Value;
    else
        return null;
}