using System;

using System.Linq;

public class Program

{

    public void Main()

    {

        string input = "123.45.678.90:00000/98765432109876541/[CLAN]Player/Na/me joined [windows/12345678901234567]";

        var content = input.Split(new string[]{" [windows/"}, StringSplitOptions.None);

        var tab = content[0].Split('/');

        var ip = tab[0];

        var groupId = tab[1];

        var groupName = String.Join("/", tab.Skip(2));

        var groupId1 = RemoveLast(content[1]);

        Console.WriteLine(groupName);

    }

​

    private static string RemoveLast(string s)

    {

        return s.Remove(s.Length - 1);

    }

}https://dotnetfiddle.net/0fZhIG