using System;

using System.Text.RegularExpressions;

​

​

​

public class Program

{

    public static void Main()

    {

        //var pattern = "[^.]*(safeframe.googlesyndication.com)";

        var pattern="2###(.+).(safeframe.googlesyndication.com)(/?)(.*)";

        var tokens = pattern.Split("###");

        var groupToReplace=1;       

        if(tokens.Length > 1)

        {

            groupToReplace = Convert.ToInt32(tokens[0]);    

            pattern = tokens[1];    

        }   

        var urls = new []{

            "https://0e1723b07ef98ee3d002766790cdb606.safeframe.googlesyndication.com/",

            "https://0e1723b07ef98ee3d002766790cdb606.safeframe.googlesyndication.com",

            "https://safeframe.googlesyndication.com",

            "https://googlesyndication.com",

            "https://0e1723b07ef98ee3d002766790cdb606.safeframe.googlesyndication.com/safeframe/1-0-37/html/container.html"

        };

        foreach(string url in urls){

            Regex regex = new Regex(pattern);

            Match match = regex.Match(url);

            Console.WriteLine(match.Success + " " + url);

            if (match.Success)

            {

                Console.WriteLine(Regex.Replace(url, pattern, $"${groupToReplace}").Trim('/'));

            }

            Console.WriteLine("---------------");

        }                           

    }

}