private void btnShowCode_Click(object sender, System.EventArgs e)
        {
            System.Net.WebRequest myWebRequest=System.Net.WebRequest.Create(this.txtURL.Text);
            myWebRequest.Timeout=5000;
            string _htmlCode="";

            try
            {
                System.Net.WebResponse myWebR=myWebRequest.GetResponse();
                System.IO.Stream resStream=myWebR.GetResponseStream();
                System.IO.StreamReader sr=new System.IO.StreamReader(resStream,System.Text.Encoding.Default);

                _htmlCode=sr.ReadToEnd();

                resStream.Close();
                sr.Close();
                this.txtCode.Text=_htmlCode;
            }
            catch(System.Net.WebException ex)
            {
                this.txtCode.Text=ex.Message;
            }

            getValidateCode(_htmlCode);
        }

        private void getValidateCode(string htmlCode)
        {
            string pattern=@"[>]\d{4}[<]";

            System.Text.RegularExpressions.Regex regex=new System.Text.RegularExpressions.Regex(pattern);
            System.Text.RegularExpressions.Match match=regex.Match(htmlCode);

            if(match.Success)
            {
                this.txtValidateCode.Text=match.Value.Substring(1,4);
            }
            else
            {
                this.txtValidateCode.Text="null";
            }

        }