using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        string format = "CST-000000";

​

        var match = Regex.Match(format, "(.*?)(0+)$");

        if (!match.Success)

        {

            throw new ArgumentException("Input must end with one or more zeros");   

        }

        string prefix = match.Groups[1].Value;

        string zeros = match.Groups[2].Value;

        int value = 123;

        Console.WriteLine($"{prefix}{value:zeros}");

    }

}