Username

using System;
using System.Text;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Playing
{
    class Program
    {
        static void Main(string[] args)
        {
            string username = Console.ReadLine();
            string command = Console.ReadLine();
            while (command != "Sign up")
            {
                string[] tokens = command.Split().ToArray();
                if (tokens[0] == "Case")
                {
                    if (tokens[1] == "lower")
                    {
                        username = username.ToLower();
                        Console.WriteLine(username);
                    }
                    else if (tokens[1] == "upper")
                    {
                        username = username.ToUpper();
                        Console.WriteLine(username);
                    }
                }
                else if (tokens[0] == "Reverse")
                {
                    string reversed = string.Empty;
                    int startIndex = int.Parse(tokens[1]);
                    int endIndex = int.Parse(tokens[2]);
                    for (int i = endIndex; i > startIndex - 1; i--)
                    {
                        reversed += username[i];
                    }
                    Console.WriteLine(reversed);
                }
                else if (tokens[0] == "Cut")
                {
                    if (username.Contains(tokens[1]))
                    {
                        int index = username.IndexOf(tokens[1]);
                        username = username.Remove(index,tokens[1].Length);
                        Console.WriteLine(username);
                    }
                    else
                    {
                        Console.WriteLine($"The word {username} doesn't contain {tokens[1]}.");
                    }
                }
                else if (tokens[0] == "Replace")
                {
                    char oldChar = char.Parse(tokens[1]);
                    username = username.Replace(oldChar, '*');
                    Console.WriteLine(username);
                }
                else if (tokens[0] == "Check")
                {
                    if (username.Contains(tokens[1]))
                    {
                        Console.WriteLine("Valid");
                    }
                    else
                    {
                        Console.WriteLine($"Your username must contain {tokens[1]}.");
                    }
                }
                command = Console.ReadLine();
            }
        }
    }
}