ValidUsernames*

using System;
using System.Text;

namespace ValidUsernames
{
    class MainClass
    {
        public static void Main(string[] args)
        {
            string[] usernames = Console.ReadLine().Split(", ",StringSplitOptions.RemoveEmptyEntries);
            StringBuilder sb = new StringBuilder();

            foreach (var name in usernames)
            {
                bool isValid = true;
                if (name.Length>=3 && name.Length <= 16)
                {
                    foreach (var symbol in name)
                    {
                        if(!(char.IsLetter(symbol) || char.IsDigit(symbol) || symbol=='-' || symbol == '_'))
                        {
                            isValid = false;
                            break;
                        }
                    }

                    if (isValid == true)
                    {
                        sb.AppendLine(name);
                    }
                }
            }

            Console.WriteLine(sb);
        }
      }
    }