init.rb

# encoding: utf-8
module ValidatesUrlFormatOf
  IPv4_PART = /\d|[1-9]\d|1\d\d|2[0-4]\d|25[0-5]/  # 0-255

  # First regexp doesn't work in Ruby 1.8 and second has a bug in 1.9.2:
  # https://github.com/henrik/validates_url_format_of/issues/issue/4/#comment_760674
  ALNUM = "ä".match(/[[:alnum:]]/) ? /[[:alnum:]]/ : /[^\W_]/

  REGEXP = %r{
    \A
    https?://                                                          # http:// or https://
    ([^\s:@]+:[^\s:@]*@)?                                              # optional username:pw@
    ( ((#{ALNUM}+\.)*xn---*)?#{ALNUM}+([-.]#{ALNUM}+)*\.[a-z]{2,6}\.? |  # domain (including Punycode/IDN)...
        #{IPv4_PART}(\.#{IPv4_PART}){3} )                              # or IPv4
    (:\d{1,5})?                                                        # optional port
    ([/?]\S*)?                                                         # optional /whatever or ?whatever
    \Z
  }iux

  DEFAULT_MESSAGE     = 'does not appear to be a valid URL'
  DEFAULT_MESSAGE_URL = 'does not appear to be valid'

  def validates_url_format_of(*attr_names)
    options = { :allow_nil => false,
                :allow_blank => false,
                :with => REGEXP }
    options = options.merge(attr_names.pop) if attr_names.last.is_a?(Hash)

    attr_names.each do |attr_name|
      message = attr_name.to_s.match(/(_|\b)URL(_|\b)/i) ? DEFAULT_MESSAGE_URL : DEFAULT_MESSAGE
      validates_format_of(attr_name, { :message => message }.merge(options))
    end
  end

end

ActiveRecord::Base.extend(ValidatesUrlFormatOf)