最佳实践:Ruby/Rails Rspec regex测试

你能告诉我如何正确测试regex代码吗?

我用以下代码测试我的用户登录属性:

# user.rb
class User < ActiveRecord::Base
  #...
  validates_format_of :login, :with => /^[a-zA-z0-9_.]{3,18}$/
end

# user_spec.rb
describe User do
  before(:each) do 
    @user = Factory.build(:user)
    @user.save
  end

  subject { @user }
  it { should be_valid }

  it { should_not allow_value("b lah").for(:login) }
  it { should_not allow_value("bälah").for(:login) }
  it { should_not allow_value("b@lah").for(:login) }
  it { should_not allow_value("bülah").for(:login) }
  it { should_not allow_value("bßlah").for(:login) }
  it { should_not allow_value("b!lah").for(:login) }
  it { should_not allow_value("b%lah").for(:login) }
  it { should_not allow_value("b)lah").for(:login) }
  # ....
  # Shall I test here every special sign????
end

但它看起来很多余而且不安全。。。。有最佳实践吗?谢谢!