using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Hello World");

        var pattern = @"FPS001.*_[A-Z]{2}_";

        Regex regex = new Regex(pattern);

        var fileName = "fs90.sqstone1.xf00.FPS001__QV__20210927__20210927090219.xlsx";

        Console.WriteLine(regex.Match(fileName).Value);

        Console.WriteLine(regex.Match(fileName).Value.GetType());

        var matches = new Regex(@"(FPS001)|(_[A-Z]{2}_)").Matches(fileName);

         foreach (Match match in matches) {

            Console.WriteLine(match);            

         }

        Console.WriteLine("trustCode:");

        Console.WriteLine(Regex.Match(fileName, @"FPS001.*(_)(?<trustCode>[A-Z]{2})(_)").Groups["trustCode"].Value);

    }

}