Create a folder app/validators. Add this to it. Now you can do "validate :some_field, url: true" in your model validations. Works in Rails 3, 4, and 5.

url_validator.rb

class UrlValidator < ActiveModel::EachValidator

  def validate_each(record, attribute, value)
    valid = begin
      URI.parse(value.to_s).kind_of?(URI::HTTP)
    rescue URI::InvalidURIError
      false
    end
    unless valid
      record.errors[attribute] << (options[:message] || "is an invalid URL")
    end
  end

end

This is an oldie from my personal snippet collection. I really should start a gem for this.