Youtube link to Embed code in C#

StringExtensions.cs

using System.Text.RegularExpressions;
namespace YourNamespace
{
    internal static class StringExtensions
    {
        //http://stackoverflow.com/questions/3652046/c-sharp-regex-to-get-video-id-from-youtube-and-vimeo-by-url
        static readonly Regex YoutubeVideoRegex = new Regex(@"youtu(?:\.be|be\.com)/(?:(.*)v(/|=)|(.*/)?)([a-zA-Z0-9-_]+)", RegexOptions.IgnoreCase);
        static readonly Regex VimeoVideoRegex = new Regex(@"vimeo\.com/(?:.*#|.*/videos/)?([0-9]+)", RegexOptions.IgnoreCase | RegexOptions.Multiline);
        
        // Use as
        // string youtubeLink = "https://www.youtube.com/watch?v=dQw4w9WgXcQ";
        // var embedCode = youtubeLink.UrlToEmbedCode();
        static internal string UrlToEmbedCode(this string url)
        {
            if (!string.IsNullOrEmpty(url))
            {
                var youtubeMatch = YoutubeVideoRegex.Match(url);

                if (youtubeMatch.Success)
                {
                    return getYoutubeEmbedCode(youtubeMatch.Groups[youtubeMatch.Groups.Count - 1].Value);
                }

                var vimeoMatch = VimeoVideoRegex.Match(url);
                if (vimeoMatch.Success)
                {
                    return getVimeoEmbedCode(vimeoMatch.Groups[1].Value);
                }


            }

            return null;
        }

        const string youtubeEmbedFormat = "<iframe type=\"text/html\" class=\"embed-responsive-item\" src=\"https://www.youtube.com/embed/{0}\"></iframe>";

        private static string getYoutubeEmbedCode(string youtubeId)
        {
            return string.Format(youtubeEmbedFormat, youtubeId);
        }

        const string vimeoEmbedFormat = "<iframe src=\"https://player.vimeo.com/video/{0}\" class=\"embed-responsive-item\" webkitallowfullscreen mozallowfullscreen allowfullscreen></iframe>";
        private static string getVimeoEmbedCode(string vimeoId)
        {
            return string.Format(vimeoEmbedFormat, vimeoId);
        }
    }
}

This doesn't make sense as an extension method for the string class