extend redcarpet with custom video tag

video_markdown_renderer.rb

# create custom tags for redcarpet to insert youtube|vimeo iframes in 
# your document. 
# ::usage:: vid(youtube, 123456)

class VideoMarkdownRenderer < Redcarpet::Render::HTML
  include ActionView::Helpers::AssetTagHelper

  def preprocess(document)
    # scans for: vid(provider, id)
    # e.g. vid(youtube,12345)
    vidregex = Regexp.compile(/\bvid\((\S+)\,\s*(\S+)\)\s*/)
    document.to_enum(:scan, vidregex).map { Regexp.last_match }.each do |match|
      document.gsub!(match.to_s, video_div(match[1],match[2]))
    end
    document
  end

  def video_div(provider, vid)
    puts "video-div: looking for.."
    content_tag(:div, 
      iframe_for(provider, vid), class: 'embed-responsive embed-responsive-16by9' 
    )
  end

  def iframe_for(provider, vid)
    case provider 
    when 'youtube'
      return content_tag(:iframe, nil, 
                         width: '100%', 
                         height: 'auto', 
                         src: "//www.youtube.com/embed/#{vid}", 
                         class: 'embed-responsive-item')
    when 'vimeo'
      return content_tag(:iframe, nil, 
                         width: '100%', 
                         height: 'auto', 
                         src: "//player.vimeo.com/video/#{vid}", 
                         class: 'embed-responsive-item')
    end
  end
end

# usage:
md = Redcarpet::Markdown.new(VideoMarkdownRenderer, hard_wrap: true) 
md.render(document)