http://invalid##host.com

http://invalid-host.foo

http://host.foo

http://localhost

require 'uri'

def valid_url?(uri)
  uri = URI.parse(uri) && !uri.host.nil?
rescue URI::InvalidURIError
  false
end

require 'uri'

def valid_url?(url)
  uri = URI.parse(url)
  uri.is_a?(URI::HTTP) && !uri.host.nil?
rescue URI::InvalidURIError
  false
end

class HttpUrlValidator < ActiveModel::EachValidator

  def self.compliant?(value)
    uri = URI.parse(value)
    uri.is_a?(URI::HTTP) && !uri.host.nil?
  rescue URI::InvalidURIError
    false
  end

  def validate_each(record, attribute, value)
    unless value.present? && self.class.compliant?(value)
      record.errors.add(attribute, "is not a valid HTTP URL")
    end
  end

end

# in the model
validates :example_attribute, http_url: true

class UrlValidator < ActiveModel::EachValidator
  def validate_each(record, attribute, value)
    return if value.blank?
    begin
      uri = URI.parse(value)
      resp = uri.kind_of?(URI::HTTP)
    rescue URI::InvalidURIError
      resp = false
    end
    unless resp == true
      record.errors[attribute] << (options[:message] || "is not an url")
    end
  end
end

validates :url, :presence => true, :url => true

# app/models/my_model.rb
validates :website, :allow_blank => true, :uri => { :format => /(^$)|(^(http|https)://[a-z0-9]+([-.]{1}[a-z0-9]+)*.[a-z]{2,5}(([0-9]{1,5})?/.*)?$)/ix }

def website= url_str
  unless url_str.blank?
    unless url_str.split(':')[0] == 'http' || url_str.split(':')[0] == 'https'
        url_str = "http://" + url_str
    end
  end
  write_attribute :website, url_str
end

# app/validators/uri_vaidator.rb
require 'net/http'

# Thanks Ilya! http://www.igvita.com/2006/09/07/validating-url-in-ruby-on-rails/
# Original credits: http://blog.inquirylabs.com/2006/04/13/simple-uri-validation/
# HTTP Codes: http://www.ruby-doc.org/stdlib/libdoc/net/http/rdoc/classes/Net/HTTPResponse.html

class UriValidator < ActiveModel::EachValidator
  def validate_each(object, attribute, value)
    raise(ArgumentError, "A regular expression must be supplied as the :format option of the options hash") unless options[:format].nil? or options[:format].is_a?(Regexp)
    configuration = { :message => I18n.t('errors.events.invalid_url'), :format => URI::regexp(%w(http https)) }
    configuration.update(options)

    if value =~ configuration[:format]
      begin # check header response
        case Net::HTTP.get_response(URI.parse(value))
          when Net::HTTPSuccess then true
          else object.errors.add(attribute, configuration[:message]) and false
        end
      rescue # Recover on DNS failures..
        object.errors.add(attribute, configuration[:message]) and false
      end
    else
      object.errors.add(attribute, configuration[:message]) and false
    end
  end
end

class WebSite < ActiveRecord::Base
  validates :url, :url => true
end

before_validation :format_website
validate :website_validator

private

def format_website
  self.website = "http://#{self.website}" unless self.website[/^https?/]
end

def website_validator
  errors[:website] << I18n.t("activerecord.errors.messages.invalid") unless website_valid?
end

def website_valid?
  !!website.match(/^(https?://)?([da-z.-]+).([a-z.]{2,6})([/w .-=?]*)*/?$/)
end

validates_format_of :url, :with => /A(https?://)?([da-z.-]+).([a-z.]{2,6})([/w.-]*)*/?Z/i

Valid ones:
 'www.crowdint.com'
 'crowdint.com'
 'http://crowdint.com'
 'http://www.crowdint.com'

 Invalid ones:
  'http://www.crowdint. com'
  'http://fake'
  'http:fake'

require 'addressable/uri'

# Source: http://gist.github.com/bf4/5320847
# Accepts options[:message] and options[:allowed_protocols]
# spec/validators/uri_validator_spec.rb
class UriValidator < ActiveModel::EachValidator

  def validate_each(record, attribute, value)
    uri = parse_uri(value)
    if !uri
      record.errors[attribute] << generic_failure_message
    elsif !allowed_protocols.include?(uri.scheme)
      record.errors[attribute] << "must begin with #{allowed_protocols_humanized}"
    end
  end

private

  def generic_failure_message
    options[:message] || "is an invalid URL"
  end

  def allowed_protocols_humanized
    allowed_protocols.to_sentence(:two_words_connector => ' or ')
  end

  def allowed_protocols
    @allowed_protocols ||= [(options[:allowed_protocols] || ['http', 'https'])].flatten
  end

  def parse_uri(value)
    uri = Addressable::URI.parse(value)
    uri.scheme && uri.host && uri
  rescue URI::InvalidURIError, Addressable::URI::InvalidURIError, TypeError
  end

end

require 'spec_helper'

# Source: http://gist.github.com/bf4/5320847
# spec/validators/uri_validator_spec.rb
describe UriValidator do
  subject do
    Class.new do
      include ActiveModel::Validations
      attr_accessor :url
      validates :url, uri: true
    end.new
  end

  it "should be valid for a valid http url" do
    subject.url = 'http://www.google.com'
    subject.valid?
    subject.errors.full_messages.should == []
  end

  ['http://google', 'http://.com', 'http://ftp://ftp.google.com', 'http://ssh://google.com'].each do |invalid_url|
    it "#{invalid_url.inspect} is a invalid http url" do
      subject.url = invalid_url
      subject.valid?
      subject.errors.full_messages.should == []
    end
  end

  ['http:/www.google.com','<>hi'].each do |invalid_url|
    it "#{invalid_url.inspect} is an invalid url" do
      subject.url = invalid_url
      subject.valid?
      subject.errors.should have_key(:url)
      subject.errors[:url].should include("is an invalid URL")
    end
  end

  ['www.google.com','google.com'].each do |invalid_url|
    it "#{invalid_url.inspect} is an invalid url" do
      subject.url = invalid_url
      subject.valid?
      subject.errors.should have_key(:url)
      subject.errors[:url].should include("is an invalid URL")
    end
  end

  ['ftp://ftp.google.com','ssh://google.com'].each do |invalid_url|
    it "#{invalid_url.inspect} is an invalid url" do
      subject.url = invalid_url
      subject.valid?
      subject.errors.should have_key(:url)
      subject.errors[:url].should include("must begin with http or https")
    end
  end
end

http://google
http://.com
http://ftp://ftp.google.com
http://ssh://google.com

%r"A(https?://)?[a-zd-]+(.[a-zd-]+)*.[a-z]{2,6}(/.*)?Z"i

# Activate all the validators
ActiveValidators.activate(:all)

class Url < ActiveRecord::Base
   validates :url, :presence => true, :url => true
end

validates_format_of [:field1, :field2], with: URI.regexp(['http', 'https']), allow_nil: true

validates_format_of :url, :with => URI::regexp(%w(http https))
validate :validate_url
def validate_url

  unless self.url.blank?

    begin

      source = URI.parse(self.url)

      resp = Net::HTTP.get_response(source)

    rescue URI::InvalidURIError

      errors.add(:url,'is Invalid')

    rescue SocketError

      errors.add(:url,'is Invalid')

    end



  end

module UrlValidator
  extend ActiveSupport::Concern
  included do
    validates :url, presence: true, uniqueness: true
    validate :url_format
  end

  def url_format
    begin
      errors.add(:url, "Invalid url") unless URI(self.url).is_a?(URI::HTTP)
    rescue URI::InvalidURIError
      errors.add(:url, "Invalid url")
    end
  end
end

class UrlValidator < ActiveModel::Validator
  def validate(record)
    begin
      url = URI.parse(record.path)
      response = Net::HTTP.get(url)
      true if response.is_a?(Net::HTTPSuccess)
    rescue StandardError => error
      record.errors[:path] << 'Web address is invalid'
      false
    end
  end
end

class Url < ApplicationRecord

  # validations
  validates_presence_of :path
  validates_with UrlValidator

end

(^|[s.:;?-]<(])(ftp|https?://[-w;/?:@&=+$|_.!~*|'()[]%#,]+[w/#](())?)(?=$|[s',|().:;?-[]>)])

validates :some_field_expecting_url_value,
            format: {
              with: URI.regexp(%w[http https]),
              message: 'is not a valid URL'
            }