en.yml

en:
  activemodel:
    errors:
      messages:
        invalid_url: "must be a valid URL"

url_validator.rb

class UrlValidator < ActiveModel::EachValidator
  def validate_each(record, attribute, value)
    unless url_valid?(value)
      record.errors.add(attribute, options[:message] || :invalid_url)
    end
  end

  # a URL may be technically well-formed but may not actually be
  # valid, so this checks for both.
  def url_valid?(url)
    url = URI.parse(url) rescue false
    url.kind_of?(URI::HTTP) || url.kind_of?(URI::HTTPS)
  end
end

url_validator_spec.rb

describe UrlValidator do
  before { @validator = UrlValidator.new(attributes: {}) }

  context 'invalid input' do
    it 'should return false for a poorly formed URL' do
      expect(@validator.url_valid? 'something.com').to be false
    end

    it 'should return false for garbage input' do
      pi = 3.14159265
      expect(@validator.url_valid? pi).to be false
    end

    it 'should return false for URLs without an HTTP protocol' do
      expect(@validator.url_valid? 'ftp://secret-file-stash.net').to be false
    end
  end

  context 'valid input' do
    it 'should return true for a correctly formed HTTP URL' do
      expect(@validator.url_valid? 'http://nooooooooooooooo.com').to be true
    end

    it 'should return true for a correctly formed HTTPS URL' do
      expect(@validator.url_valid? 'https://google.com').to be true
    end
  end
end