//in c# // I am having problem with my regex fitting the requirments   using System; using System.Text.RegularExpressions; namespace UsernameProcessor.Question { public sealed class UsernameProcessorService {   ///   /// Requirements:   ///  - A valid username shall be at least 4 characters.

//in c#

// I am having problem with my regex fitting the requirments

using System;
using System.Text.RegularExpressions;

namespace UsernameProcessor.Question
{

public sealed class UsernameProcessorService
{

  /// <summary>
  /// Requirements:
  ///  - A valid username shall be at least 4 characters.
  ///  - A valid username shall contain only letters, numbers and an optional underscore.
  ///  - A valid username shall start with a letter, and shall not end with an underscore.
  /// </summary>
  /// <param name="username"></param>
  /// <returns>Whether or not the username is valid per the above requirements.</returns>
  private static Regex sUserNameAllowedRegEx = new Regex(@"^(?=[a-zA-Z])[-\w.]{0,23}([a-zA-Z\d])$", RegexOptions.Compiled);
  private static Regex sUserNameIllegalEndingRegEx = new Regex(@"(\-_)$", RegexOptions.Compiled);
  public static bool IsValidUsername(string username)
  {
        if (string.IsNullOrEmpty(username)
            || !sUserNameAllowedRegEx.IsMatch(username)
            || sUserNameIllegalEndingRegEx.IsMatch(username))
        {
            return false;
        }
    throw new NotImplementedException("TODO: please validate our usernames!!!");
    }
      

  /// <summary>
  /// Feel free to use the below main function to test and develop accordingly!
  /// </summary>
  public static void Main(string[] args)
  {
    Console.WriteLine(IsValidUsername("john_smith"));
    Console.WriteLine(IsValidUsername("jsim_"));
  }
}
}