var validator = new UsernameValidator();
Console.WriteLine($"'Gamer123': {validator.IsValid("Gamer123")}");
Console.WriteLine($"'Pro Gamer': {validator.IsValid("Pro Gamer")}");
Console.WriteLine($"'user@name': {validator.IsValid("user@name")}");
Console.WriteLine($"'#champion': {validator.IsValid("#champion")}");
Console.WriteLine($"'': {validator.IsValid("")}");