# add this to your Gemfile
gem "validate_url"

# and  run
sudo gem install validate_url

class Pony < ActiveRecord::Base
  # standard validation
  validates :homepage, :url => true

  # with allow_nil
  validates :homepage, :url => {:allow_nil => true}

  # with allow_blank
  validates :homepage, :url => {:allow_blank => true}
end

class Unicorn
  include ActiveModel::Validations

  attr_accessor :homepage

  # with legacy syntax (the syntax above works also)
  validates_url :homepage, :allow_blank => true
end