ParkingValidation

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

namespace _22_05_ParkingValidation
{
    class ParkingValidation
    {
        static void Main(string[] args)
        {
            int n = int.Parse(Console.ReadLine());
            var database = new Dictionary<string, string>();

            for (int i = 0; i < n; i++)
            {
                var input = Console.ReadLine();
                var inputArray = input.Split(new[] { ' ' }, StringSplitOptions.RemoveEmptyEntries)
                                     .Select(s => s.Trim())
                                     .ToArray();
                var command = inputArray[0].ToLower();
                var userName = inputArray[1];
                var plate = inputArray[2];

                var paternPlate = new Regex(@"^[A - Z]{ 2}\d{ 4}[A-Z]{2}$");


                if (command == "register")
                {
                    var match = paternPlate.Match(plate);
                    if (!match.Success)
                    {
                        Console.WriteLine($"ERROR: invalid license plate {plate}");
                        continue;
                    }
                    else if (!database.ContainsKey(userName))
                    {
                        database.Add(userName, plate);
                        Console.WriteLine($"{userName} registered {plate} successfully");
                    }
                    else if (database.ContainsKey(userName) && database.ContainsValue(plate))
                    {
                        Console.WriteLine($"ERROR: already registered with plate number {database[userName]}");
                    }
                    else if (database.ContainsValue(plate))
                    {
                        Console.WriteLine($"ERROR: license plate {plate} is busy");
                    }

                }

                else if (command == "unregister")
                {
                    if (!database.ContainsKey(userName))
                    {
                        Console.WriteLine($"ERROR: user {userName} not found");
                    }
                    else
                    {
                        database.Remove(database[userName]);
                        Console.WriteLine($"user {userName} unregistered successfully");
                    }
                }

            }
            foreach (var pair in database)
            {
                Console.WriteLine($"{pair.Key} => {pair.Value}");
            }

        }
    }
}