using System;

public class Program

{

    public static void Main()

    {

        var bMark = "https://eu21.springcm.com/atlas/Link/Document/19490/4d082bb4-bfa6-ea11-9c35-30e17160d15b?aid=19490";

        var lastIndex = bMark.LastIndexOf('/');

        string fileId = bMark.Substring(lastIndex + 1);

        lastIndex = fileId.LastIndexOf('?');

        fileId = fileId.Substring(0,lastIndex);

​

        Console.WriteLine("Bookmark :" + bMark + "  FileID : " + fileId);

        Console.WriteLine(bMark.Split('/')[2]);

        var arr = bMark.Split('/');

        Console.WriteLine(arr.Split('/').IndexOf("eu21.springcm.com"));

    }

}