Validate or get the check digit of a Windows activation installation ID / confirmation ID, C#

Validate or get the check digit of a Windows activation installation ID / confirmation ID

// Validate or get the check digit of a segment of a Windows activation installation ID / confirmation ID

using System;
using System.Text.RegularExpressions;

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
            string idSegment = Console.ReadLine();
            if (!Regex.IsMatch(idSegment, "^[0-9]+$"))
            {
                Console.Error.WriteLine("The ID segment should contain digits only");
                return;
            }
            if (idSegment.Length == 6)
            {
                int[] intIdSegment = new int[6];
                for (int i = 0; i < idSegment.Length; i++)
                {
                    intIdSegment[i] = int.Parse(new string(idSegment[i], 1));
                }
                if (intIdSegment[5] < 7 && (intIdSegment[0] + 2 * intIdSegment[1] + intIdSegment[2] + 2 * intIdSegment[3] + intIdSegment[4] - intIdSegment[5]) % 7 == 0)
                {
                    Console.WriteLine("The ID segment is valid");
                }
                else
                {
                    Console.WriteLine("The ID segment is invalid");
                }
            }
            else if (idSegment.Length == 5)
            {
                int[] intIdSegment = new int[5];
                for (int i = 0; i < idSegment.Length; i++)
                {
                    intIdSegment[i] = int.Parse(new string(idSegment[i], 1));
                }
                int checkDigit = (intIdSegment[0] + 2 * intIdSegment[1] + intIdSegment[2] + 2 * intIdSegment[3] + intIdSegment[4]) % 7;
                Console.WriteLine("The check digit is " + checkDigit.ToString());
                Console.WriteLine("The full ID segment is " + idSegment + checkDigit.ToString());
            }
            else
            {
                Console.Error.WriteLine("Enter 5 digits to get the check digit, or enter 6 digits to validate the ID segment");
            }
        }
    }
}


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