public static string GetYouTubeUserChannelId(this string str)
        {
            if (str == null)
            {
                return(null);
            }
            var match = YouTubeUserChannelIdRegex.Match(str);

            if (!match.Success || match.Groups["id"].Captures.Count != 1)
            {
                return(null);
            }
            return(match.Groups["id"].Captures[0].Value);
        }
 public static bool IsValidYouTubeUserChannelUrl(this string str)
 {
     return(YouTubeUserChannelIdRegex.IsMatch(str));
 }