using System;

using System.Net;

using System.Text.Encodings.Web;

using System.Text.RegularExpressions;

​

public class Example

{

   public static void Main()

   {

      string pattern = @"(?:(?:^|.*;\s*)XSRF-TOKEN\s*\=\s*([^;]*).*$)|^.*$";

       string pattern2 = @"__";

      string name = "__atuvc=1%7C29; XSRF-TOKEN=CfDJ8IK3vTjpvWNGgLJ8KfGU2c9Xv4_1NpMeumZV78170i0teFnAIiIdIZa_SmHHDEP95juAiCBK6ve7gDTgM9zKeXTfqp7sbNTWSPYoffxzt8Ufu_fBGskRl3cspGeyBIg9mJqPHD1olntU_c5w6js5Ko4";

      Console.WriteLine(Regex.Replace(name, pattern, "$1"));

   }

}