Regeh

namespace ConsoleApp1
{
    using System;
    using System.Text;
    using System.Text.RegularExpressions;

    public class StartUp
    {
        public static void Main()
        {
            Regex regeh = new Regex(@"\[[^\[\s]+?<(\d+)REGEH(\d+)>[^\[\]\s]+?\]");

            StringBuilder result = new StringBuilder();

            string inputLine = Console.ReadLine();

            int totalIndex = 0;
            foreach (Match match in regeh.Matches(inputLine))
            {
                totalIndex += int.Parse(match.Groups[1].Value);
                result.Append(GetElementAtIndex(totalIndex, inputLine));

                totalIndex += int.Parse(match.Groups[2].Value);
                result.Append(GetElementAtIndex(totalIndex, inputLine));

            }

            Console.WriteLine(result);
        }

        private static char GetElementAtIndex(int index, string inputLine)
        {
            if (index >= 0 && index < inputLine.Length)
            {
                return inputLine[index];
            }
            else
            {
                return inputLine[index % (inputLine.Length - 1)];
            }
        }
    }
}