using System;

using System.Text.RegularExpressions;

using System.Linq;

using System.Text;

​

public class Program

{

    public static void Main()

    {

        var masked = GetMaskedIban("DE12500123456789012345");

        //masked.Dump();

        masked = GetMaskedIban("BE71096123456769");

        //masked.Dump();

        string input = "DE12500123456789012345";

        string pattern = @"^([A-Z]{2})(?:.{5})(\d{5})(.*)(\d{3})$";

        var match = Regex.Matches(input, pattern)[0];

        masked = $"{match.Groups[1].Value}*****{match.Groups[2].Value}{"".PadRight(match.Groups[3].Value.Length, '*')}{match.Groups[4].Value}";

        masked.Dump();

        var groupPattern = @"^([A-Z]{2})(?<Ast>\d{5})(\d{5})(?<Ast>.*)(\d{3})$";

        var groupRegex = new Regex(groupPattern);

        var x = groupRegex.ReplaceGroup(input,"Ast", '*');

        x.Dump();

    }

​

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

}

​

public static class X

{

    public static string ReplaceGroup(this Regex regex, string input, string groupName, char replacement)

    {

        return regex.Replace(input, m =>

        {

            var group = m.Groups[groupName];

            var sb = new StringBuilder();

            var previousCaptureEnd = 0;

            foreach (var capture in group.Captures.Cast<Capture>())

            {

                var currentCaptureEnd = capture.Index + capture.Length - m.Index;

                var currentCaptureLength = capture.Index - m.Index - previousCaptureEnd;

                sb.Append(m.Value.Substring(previousCaptureEnd, currentCaptureLength));

                sb.Append("".PadRight(currentCaptureLength,replacement));

                previousCaptureEnd = currentCaptureEnd;

            }

​

            sb.Append(m.Value.Substring(previousCaptureEnd));

            return sb.ToString();

        });

    }

}