GitHub HTML filter to add target="_blank" to all <a>s linking to an external domain

target_blank_external_links_filter.rb

module HTML
  class Pipeline
    class TargetBlankExternalLinksFilter < Filter
      def call
        doc.css('a').each do |node|
          if !node.get_attribute("href").start_with?('/') && !node.get_attribute("href").start_with?('#')
            node.set_attribute("target", '_blank')
          end
        end

        doc
      end
    end
  end
end