wired up a framework for testing with rspec, including 2 POC specs

.rspec

@@ -0,0 +1,2 @@
+--color
+--order random

spec/spec_helper.rb

@@ -0,0 +1,22 @@
+require 'active_model' if Gem.loaded_specs.keys.include?('activemodel')
+
+RSpec::Matchers.define :have_errors_on_email do
+  match do |actual|
+    expect(actual).not_to be_nil, "#{actual} should not be nil"
+    expect(actual).not_to be_empty, "#{actual} should not be empty"
+    expect(actual.size).to eq(@reasons.size), "#{actual} should have #{@reasons.size} elements"
+    @reasons.each do |reason|
+      reason = "#{"Email " if defined?(ActiveModel)}#{reason}"
+      expect(actual).to include(reason), "#{actual} should contain #{reason}"
+    end
+  end
+  chain :because do |reason|
+    (@reasons ||= []) << reason
+  end
+  chain :and_because do |reason|
+    (@reasons ||= []) << reason
+  end
+  match_when_negated do |actual|
+    expect(actual).to (defined?(ActiveModel) ? be_empty : be_nil)
+  end
+end

spec/validates_email_format_of_spec.rb

@@ -0,0 +1,33 @@
+require_relative "spec_helper"
+require "validates_email_format_of"
+
+describe ValidatesEmailFormatOf do
+  before(:all) do
+    described_class.load_i18n_locales
+    I18n.enforce_available_locales = false
+  end
+  subject do |example|
+    if defined?(ActiveModel)
+      user = Class.new do
+        def initialize(email)
+          @email = email
+        end
+        attr_reader :email
+        include ActiveModel::Validations
+        validates_email_format_of :email, example.example_group_instance.options
+      end
+      example.example_group_instance.class::User = user
+      user.new(example.example_group_instance.email).tap(&:valid?).errors.full_messages
+    else
+      ValidatesEmailFormatOf::validate_email_format(email, options)
+    end
+  end
+  let(:options) { {} }
+  let(:email) { |example| example.example_group.description }
+  describe "no_at_symbol" do
+    it { should have_errors_on_email.because("does not appear to be a valid e-mail address") }
+  end
+  describe "user1@gmail.com" do
+    it { should_not have_errors_on_email }
+  end
+end

validates_email_format_of.gemspec

@@ -16,4 +16,5 @@ spec = Gem::Specification.new do |s|
   s.require_paths = ['lib']
 
   s.add_dependency 'i18n'
+  s.add_development_dependency 'rspec'
 end