diff --git a/lib/govuk_elements_form_builder/form_builder.rb b/lib/govuk_elements_form_builder/form_builder.rb
index 751bfaf..865f74b 100644
--- a/lib/govuk_elements_form_builder/form_builder.rb
+++ b/lib/govuk_elements_form_builder/form_builder.rb
@@ -192,9 +192,8 @@ def form_group_id attribute
     end
 
     def form_group_classes attributes
-      attributes = [attributes] if !attributes.respond_to? :count
       classes = 'form-group'
-      classes += ' error' if attributes.find { |a| error_for? a }
+      classes += ' error' if Array(attributes).find { |a| error_for? a }
       classes
     end
 
diff --git a/spec/lib/govuk_elements_form_builder/form_builder_spec.rb b/spec/lib/govuk_elements_form_builder/form_builder_spec.rb
index 6a254bd..f03d43f 100644
--- a/spec/lib/govuk_elements_form_builder/form_builder_spec.rb
+++ b/spec/lib/govuk_elements_form_builder/form_builder_spec.rb
@@ -13,7 +13,7 @@ class TestHelper < ActionView::Base; end
 
   let(:helper) { TestHelper.new }
   let(:resource)  { Person.new }
-  let(:builder) { described_class.new :person, resource, helper, {} }
+  subject(:builder) { described_class.new :person, resource, helper, {} }
 
   def expect_equal output, expected
     split_output = output.gsub(">\n</textarea>", ' />').split("<").join("\n<").split(">").join(">\n").squeeze("\n").strip + '>'
@@ -30,76 +30,42 @@ def type_for(method, type)
   end
 
   shared_examples_for 'input field' do |method, type|
-
-    def size(method, size)
-      method == :text_area ? '' : %'size="#{size}" '
-    end
+    let(:default_builder_options) { { resource: 'person' } }
 
     it 'outputs label and input wrapped in div' do
       output = builder.send method, :name
 
-      expect_equal output, [
-        '<div class="form-group">',
-        '<label class="form-label" for="person_name">',
-        'Full name',
-        '</label>',
-        %'<#{element_for(method)} class="form-control" #{type_for(method, type)}name="person[name]" id="person_name" />',
-        '</div>'
-      ]
+      expect(output).to match_form_group(default_builder_options.merge(field: :name, input_type: method))
+    end
+
+    it 'supports attributes defined as a string' do
+      output = builder.send method, 'name'
+
+      expect(output).to match_form_group(default_builder_options.merge(field: :name, input_type: method))
     end
 
     it 'adds custom class to input when passed class: "custom-class"' do
       output = builder.send method, :name, class: 'custom-class'
 
-      expect_equal output, [
-        '<div class="form-group">',
-        '<label class="form-label" for="person_name">',
-        'Full name',
-        '</label>',
-        %'<#{element_for(method)} class="form-control custom-class" #{type_for(method, type)}name="person[name]" id="person_name" />',
-        '</div>'
-      ]
+      expect(output).to match_form_group(default_builder_options.merge(field: :name, input_type: method, class: 'custom-class'))
     end
 
     it 'adds custom classes to input when passed class: ["custom-class", "another-class"]' do
       output = builder.send method, :name, class: ['custom-class', 'another-class']
 
-      expect_equal output, [
-        '<div class="form-group">',
-        '<label class="form-label" for="person_name">',
-        'Full name',
-        '</label>',
-        %'<#{element_for(method)} class="form-control custom-class another-class" #{type_for(method, type)}name="person[name]" id="person_name" />',
-        '</div>'
-      ]
+      expect(output).to match_form_group(default_builder_options.merge(field: :name, input_type: method, class: 'custom-class another-class'))
     end
 
     it 'passes options passed to text_field onto super text_field implementation' do
       output = builder.send method, :name, size: 100
-      expect_equal output, [
-        '<div class="form-group">',
-        '<label class="form-label" for="person_name">',
-        'Full name',
-        '</label>',
-        %'<#{element_for(method)} #{size(method, 100)}class="form-control" #{type_for(method, type)}name="person[name]" id="person_name" />',
-        '</div>'
-      ]
+
+      expect(output).to match_form_group(default_builder_options.merge(field: :name, input_type: method, size: 100))
     end
 
     context 'when hint text provided' do
       it 'outputs hint text in span inside label' do
         output = builder.send method, :ni_number
-        expect_equal output, [
-          '<div class="form-group">',
-          '<label class="form-label" for="person_ni_number">',
-          'National Insurance number',
-          '<span class="form-hint">',
-          'It’ll be on your last payslip. For example, JH 21 90 0A.',
-          '</span>',
-          '</label>',
-          %'<#{element_for(method)} class="form-control" #{type_for(method, type)}name="person[ni_number]" id="person_ni_number" />',
-          '</div>'
-        ]
+        expect(output).to match_form_group(default_builder_options.merge(field: :ni_number, input_type: method))
       end
     end
 
@@ -108,14 +74,7 @@ def size(method, size)
         output = builder.fields_for(:address, Address.new) do |f|
           f.send method, :postcode
         end
-        expect_equal output, [
-          '<div class="form-group">',
-          '<label class="form-label" for="person_address_attributes_postcode">',
-          'Postcode',
-          '</label>',
-          %'<#{element_for(method)} class="form-control" #{type_for(method, type)}name="person[address_attributes][postcode]" id="person_address_attributes_postcode" />',
-          '</div>'
-        ]
+        expect(output).to match_form_group(default_builder_options.merge(field: :postcode, input_type: method, fields_for: :address))
       end
     end
 
@@ -224,7 +183,7 @@ def expected_error_html method, type, attribute, name_value, label, error
   end
 
   describe '#text_area' do
-    include_examples 'input field', :text_area, nil
+    include_examples 'input field', :text_area
   end
 
   describe '#email_field' do
diff --git a/spec/rails_helper.rb b/spec/rails_helper.rb
index 241a969..f271a2b 100644
--- a/spec/rails_helper.rb
+++ b/spec/rails_helper.rb
@@ -21,6 +21,7 @@
 #
 # Dir[Rails.root.join('spec/support/**/*.rb')].each { |f| require f }
 require_relative 'support/translation_helper.rb'
+require_relative 'support/matcher_helpers.rb'
 
 # Checks for pending migrations before tests are run.
 # If you are not using ActiveRecord, you can remove this line.
@@ -49,4 +50,5 @@
   # The different available types are documented in the features, such as in
   # https://relishapp.com/rspec/rspec-rails/docs
   config.infer_spec_type_from_file_location!
+  config.include MatcherHelpers
 end
diff --git a/spec/support/matcher_helpers.rb b/spec/support/matcher_helpers.rb
new file mode 100644
index 0000000..ba561d8
--- /dev/null
+++ b/spec/support/matcher_helpers.rb
@@ -0,0 +1,7 @@
+require_relative 'matchers/form_group_matcher_helpers'
+
+module MatcherHelpers
+  def self.included(base)
+    base.send(:include, FormGroupMatcherHelpers)
+  end
+end
diff --git a/spec/support/matchers/form_group_matcher_helpers.rb b/spec/support/matchers/form_group_matcher_helpers.rb
new file mode 100644
index 0000000..405a6a9
--- /dev/null
+++ b/spec/support/matchers/form_group_matcher_helpers.rb
@@ -0,0 +1,115 @@
+require 'rspec/expectations'
+
+module FormGroupMatcherHelpers
+  extend RSpec::Matchers::DSL
+
+  class FormGroupObject
+    attr_reader :options, :resource, :field,
+      :label, :input_type, :input_class, :input_size,
+      :fields_for
+
+    def initialize(options)
+      @options = options
+      @resource = options[:resource]
+      @field = options.fetch(:field)
+      @label = options.fetch(:label) { @field.to_s.humanize }
+      @input_type = options.fetch(:input_type)
+      @input_class = options[:class]
+      @input_size = options[:size]
+      @fields_for = options[:fields_for]
+    end
+
+    def field_id
+      [resource, fields_for_attributes, field].compact.join('_')
+    end
+
+    def field_name
+      return "#{resource}[#{fields_for_attributes}][#{field}]" if resource && fields_for_attributes
+      return "#{resource}[#{field}]" if resource
+      field
+    end
+
+    def to_s
+      ['<div class="form-group">', label_tag, field_tag, '</div>'].join("\n")
+    end
+
+    private
+
+    def fields_for_attributes
+      return unless fields_for
+      "#{fields_for}_attributes"
+    end
+
+    def text_area_input?
+      input_type.to_s == 'text_area'
+    end
+
+    def input_tag
+      return '<textarea' if text_area_input?
+      '<input'
+    end
+
+    def input_tag_type
+      return if text_area_input?
+      type = {
+        text_field: 'text',
+        email_field: 'email',
+        number_field: 'number',
+        password_field: 'password',
+        phone_field: 'tel',
+        range_field: 'range',
+        search_field: 'search',
+        telephone_field: 'tel',
+        url_field: 'url'
+      }[input_type.to_sym]
+      %[type="#{type}"]
+    end
+
+    def input_tag_class
+      classes = 'form-control'
+      classes << ' ' << input_class if input_class
+      %[class="#{classes}"]
+    end
+
+    def input_tag_size
+      return if text_area_input?
+      return unless input_size
+      %[size="#{input_size}"]
+    end
+
+    def hint_tag
+      localized_path = ['helpers', 'hint', resource, field].compact.join('.')
+      localized_hint = I18n.t(localized_path, default: '')
+      return unless localized_hint.present?
+      ['<span class="form-hint">', localized_hint, '</span>'].join("\n")
+    end
+
+    def label_tag
+      localized_path = ['helpers', 'label', resource, field].compact.join('.')
+      localized_label = I18n.t(localized_path, default: field.to_s.humanize)
+      [%[<label class="form-label" for="#{field_id}">], localized_label, hint_tag, '</label>'].compact.join("\n")
+    end
+
+    def field_tag
+      [input_tag, input_tag_size, input_tag_class, input_tag_type, %[name="#{field_name}"], %[id="#{field_id}"], '/>'].compact.join(' ')
+    end
+  end
+
+  matcher :match_form_group do |expected|
+    define_method :formatted_expected do
+      FormGroupObject.new(expected).to_s
+    end
+
+    define_method :formatted_actual do |actual|
+      actual.gsub(">\n</textarea>", ' />').split("<").join("\n<").split(">").join(">\n").squeeze("\n").strip + '>'
+    end
+
+    match do |actual|
+      formatted_actual(actual) == formatted_expected
+		end
+
+    failure_message do |actual|
+      "expected\n#{formatted_actual(actual)}\nto match\n#{formatted_expected}"
+    end
+  end
+end