using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        var sData = "20231231";

        Match m = Regex.Match(sData, @"^(20[0-9]{2})((?:1[0-2]|0[1-9]))((?:[0-2][0-9]|[3][0-1]))$"); // circa

        if (m.Success)

        {

            var sYear = m.Groups[1].Value.Trim();

            var sMonth = m.Groups[2].Value.Trim();

            var sDay = m.Groups[3].Value.Trim();

            Console.WriteLine($"{sYear} - {sMonth} - {sDay}");

        }

        Console.WriteLine("Hello World");

    }

}