Username

using System;
using System.Linq;

namespace Username
{
    class Program
    {
        static void Main(string[] args)
        {
            string text = Console.ReadLine();
            string command = Console.ReadLine();

            while (command != "Sign up")
            {
                string[] token = command.Split();
                if(token[0] == "Case")
                {
                    if(token[1] == "lower")
                    {
                        text = text.ToLower();
                        Console.WriteLine(text);
                    }
                    if (token[1] == "upper")
                    {
                        text = text.ToUpper();
                        Console.WriteLine();
                    }
                }
                else if(token[0] == "Reverse")
                {
                    int startIndex = int.Parse(token[1]);
                    int endIndex = int.Parse(token[2]);
                    // !!!
                    if(startIndex >= 0 && endIndex > startIndex && endIndex < text.Length)
                    {
                        int length = endIndex - startIndex +1 ;
                        string word = text.Substring(startIndex, length);
                        Console.WriteLine(String.Join("",word.Reverse()));
                    }

                }
                else if(token[0]== "Cut")
                {
                    string check = token[1];

                    if (text.Contains(check))
                    {
                        text = text.Remove(text.IndexOf(check), check.Length);
                        Console.WriteLine(text);
                    }
                    else
                    {
                        Console.WriteLine($"The word {text} doesn't contain {check}.");
                    }
                }
                else if(token[0] == "Replace")
                {
                    char currChar = char.Parse(token[1]);
                    text = text.Replace(currChar, '*');
                    Console.WriteLine(text);
                }
                else if(token[0] == "Check")
                {
                    char charToCheck = char.Parse(token[1]);
                    if (text.Contains(charToCheck))
                    {
                        Console.WriteLine("Valid");
                    }
                    else
                    {
                        Console.WriteLine($"Your username must contain {charToCheck}.");
                    }
                }
                command = Console.ReadLine();
            }

        }
    }
}