using System;

using System.Text.RegularExpressions;

​

public class Program

{

    public static void Main()

    {

        //String St = "super exemple of string key : text I want to keep - end of my string";

String St = "https://checkmarx.upmc.com/CxWebClient/ViewerMain.aspx?scanid=1010021&projectid=132344&pathid=196";

        //get the index of second =

        int pFrom = St.IndexOf('=', St.IndexOf('=')+1) + 1;

//int pFrom = St.IndexOf("=") + "&pa".Length;

int pTo = St.LastIndexOf("&pa");

Console.WriteLine("pfrom = "+ pFrom +" " +"pTo " + pTo + "pTo - pFrom = " + ( pTo - pFrom));

String result = St.Substring(pFrom, pTo - pFrom);

        Console.WriteLine(result);

    }

}