using System;

using System.Text.RegularExpressions;

​

public class Program

{

    public static void Main()

    {

        string registrationNumber = "RCZ12345678";

        var output = FormatCompanyRegistrationNumber(registrationNumber);

        Console.WriteLine(output);

    }

    private static string FormatCompanyRegistrationNumber(string registrationNumber)

    {

        int maxRegistrationNumberLength = 8;

        var parts = Regex.Split(registrationNumber, @"(\d{1,8})");

        var formattedRegNo = parts[0] + parts[1].PadLeft(maxRegistrationNumberLength-parts[0].Length,'0');

        return formattedRegNo.Substring(0,maxRegistrationNumberLength);

    }   

}