Going postal

converter.rb

class Converter
  def postalize(text)
    if text.length == 5
      "#{text[0..1]}-#{text[2..4]}"
    else
      text[2] = "-"
      text
    end
  end
end

test.rb

 test 'postalize' do
    converter = Converter.new

    ['12-345', '12345', '12d345', '12 345', '12_345'].each do |code|
      assert_equal '12-345', converter.postalize, code
    end
  end

class Converter
  def postalize(text)
    text.gsub(/[^\d]/, '')[0..4].insert(2, '-') rescue raise Exception.new('Bad code')
  end
end

+1 for sodercober