CubicMessages

using System;
using System.Text;
using System.Text.RegularExpressions;

namespace CubicMessages
{
    public class CubicMessages
    {
        public static void Main()
        {
            StringBuilder result = new StringBuilder();

            Regex validateMessage = null;
            string messageInfo = Console.ReadLine();
            while (messageInfo != "Over!")
            {
                int lenOfMessage = int.Parse(Console.ReadLine());

                validateMessage = new Regex(
                    @"^([0-9]+)([a-zA-Z]{" + lenOfMessage + @"})([^a-zA-Z]*)$");

                if (!validateMessage.IsMatch(messageInfo))
                {
                    messageInfo = Console.ReadLine();

                    continue;
                }

                Match validMessage = validateMessage.Match(messageInfo);

                string message = validMessage.Groups[2].ToString();
                string verificationCode = string.Concat(validMessage.Groups[1].ToString(),
                                                            validMessage.Groups[3].ToString());

                StringBuilder currResult = new StringBuilder();
                for (int currSymbol = 0; currSymbol < verificationCode.Length; currSymbol++)
                {
                    char symbol = verificationCode[currSymbol];

                    if (!char.IsDigit(symbol))
                    {
                        continue;
                    }

                    int index = int.Parse(symbol.ToString());

                    try
                    {
                        currResult.Append(message[index]);
                    }
                    catch (IndexOutOfRangeException)
                    {
                        currResult.Append(" ");
                    }
                }

                result.AppendLine($"{message} == {currResult}");

                messageInfo = Console.ReadLine();
            }

            Console.WriteLine(result.ToString().TrimEnd('\r', '\n'));
        }
    }
}