user.rb

# app/models/user.rb
class User < ActiveRecord::Base
  validates :zipcode, presence: true, zipcode: true
end

zip_code_validator.rb

# lib/validators/zipcode_validator.rb
# Usage:
#
# class User << AR
#   validates :postalcode, zipcode: true
#
#   # Required country_alpha2 attribute
#   def country_alpha2
#     country.alpha2
#   end
# end
#
class ZipcodeValidator < ActiveModel::EachValidator
  # using country alpha2 code as key
  ZIPCODES_REGEX = {
    ES: /\A\d{5}\z/,
    AR: /\A([A-HJ-TP-Z]{1}\d{4}[A-Z]{3}|[a-z]{1}\d{4}[a-hj-tp-z]{3})\z/,
    NZ: /\A\d{4}\z/,
    AU: /\A\d{4}\z/,
    US: /\A\d{5}(-\d{4})?\z/,
    CA: /\A[ABCEGHJKLMNPRSTVXY]\d[ABCEGHJKLMNPRSTWVXYZ]\d[ABCEGHJKLMNPRSTWVXYZ]\d\z/
  }

  def validate_each(record, attribute, value)
    alpha2 = record.country_alpha2.try(:to_sym)

    unless alpha2 && ZIPCODES_REGEX[alpha2].match(value)
      record.errors.add(attribute, "#{value} is not a valid zipcode")
    end
  end
end

zip_code_validator_spec.rb

# spec/lib/validators/zipcode_validator_spec.rb
require 'spec_helper'

describe ZipcodeValidator, '#validate_each' do
  context "Spain" do
    it 'does not add errors with a valid zipcode' do
      record = build_record('93108', "ES")
      zipcode_should_be_valid(record)
    end

    it 'adds errors with an invalid Zipcode' do
      ['1234', '12345-12345', 'D0D0D0', 'invalid_zip'].each do |zipcode|
        record = build_record(zipcode, 'ES')
        zipcode_should_be_invalid(record, zipcode)
      end
    end
  end

  context "USA" do
    it 'does not add errors with a valid zipcode' do
      record = build_record('93108', "US")
      zipcode_should_be_valid(record)
      ['12345', '12345-1234'].each do |zipcode|
        record = build_record(zipcode, 'US')
        zipcode_should_be_valid(record)
      end

    end

    it 'adds errors with an invalid Zipcode' do
      ['1234', '12345-12345', 'D0D0D0', 'invalid_zip'].each do |zipcode|
        record = build_record(zipcode, 'US')
        zipcode_should_be_invalid(record, zipcode)
      end
    end
  end

  context "Argentina" do
    it 'does not add errors with a valid zipcode' do
      record = build_record('C1424CHN', "AR")
      zipcode_should_be_valid(record)
    end

    it 'adds errors with an invalid Zipcode' do
      ['1234', '12345', 'D0D0D0', 'invalid_zip'].each do |zipcode|
        record = build_record(zipcode, 'AR')
        zipcode_should_be_invalid(record, zipcode)
      end
    end
  end

  context "Australia and New Zeland" do
    it 'does not add errors with a valid zipcode' do
      ["AU", "NZ"].each do |code|
        record = build_record('9310', code)
        zipcode_should_be_valid(record)
      end
    end

    it 'adds errors with an invalid Zipcode' do
      ["AU", "NZ"].each do |code|
        ['C1424CHN', '12345', 'invalid_zip'].each do |zipcode|
          record = build_record(zipcode, code)
          zipcode_should_be_invalid(record, zipcode)
        end
      end
    end
  end

  context "Canada" do
    it 'does not add errors with a valid zipcode' do
      record = build_record('A1J2Z9', 'CA')
      zipcode_should_be_valid(record)
    end

    it 'adds errors with an invalid Zipcode' do
      ['C1424CHN', '12345', 'D0D0D0', 'invalid_zip'].each do |zipcode|
        record = build_record(zipcode, 'CA')
        zipcode_should_be_invalid(record, zipcode)
      end
    end
  end

  def zipcode_should_be_valid(record)
    ZipcodeValidator.new(attributes: :zipcode).validate(record)

    record.errors.should be_empty
  end

  def zipcode_should_be_invalid(record, zipcode = "invalid_zip")
    ZipcodeValidator.new(attributes: :zipcode).validate(record)

    record.errors.full_messages.should eq [
      "Zipcode #{zipcode} is not a valid zipcode"
    ]
  end

  def build_record(zipcode, country_alpha2)
    test_class.new.tap do |object|
      object.country_alpha2 = country_alpha2
      object.zipcode = zipcode
    end
  end

  def test_class
    Class.new do
      include ActiveModel::Validations
      attr_accessor :zipcode, :country_alpha2

      def self.name
        'TestClass'
      end
    end
  end
end