using System;

​

public class Program

{

    public static void Main()

    {

        var masked = GetMaskedIban( "DE12500123456789012345");

        masked.Dump();

        masked = GetMaskedIban("BE71096123456769");

        masked.Dump();

    }

    public static string GetMaskedIban(string iban) => $"{iban[0..2]}{"".PadRight(5, '*')}{iban[7..12]}{"".PadRight(iban.Length - 15, '*')}{iban[^3..]}";

}