http://blog.bmonkeys.net/2013/simple-html-tag-parsing-with-regular-expressions-and-shellwords

content_renderer.rb

require "shellwords"

class ContentRenderer
  def initialize str, opts
    @str  = str.dup
    @opts = opts.reverse_merge markdown: true, youtube: true
  end

  def to_str
    @_rendered ||= @opts.inject(@str) do |text, (opt, perform)|
      perform ? try("render_#{opt}", text) || text : text
    end
  end
  alias_method :to_s, :to_str

  def render_markdown str
    renderer = HTMLwithPygments.new(hard_wrap: true, with_toc_data: true)
    Redcarpet::Markdown.new(renderer, {
      autolink: true,
      fenced_code_blocks: true,
      highlight: true,
      lax_spacing: true,
      no_intra_emphasis: false,
      strikethrough: true,
      superscript: true,
      tables: true,
    }).render(str)
  end

  def render_youtube str
    process_tags(str, :youtube) do |args|
      defaults = {
        ratio:          16/9.0,  # ratio to calculate width/height
        autohide:       2,       # [YT] controls behaviour 2 = fade progress, 1 = fade controls, 0 = fade nothing
        autoplay:       false,   # [YT] autoplay on load
        color:          "red",   # [YT] progress color (red or white)
        disablekb:      false,   # [YT] disable player keyboard controls
        enablejsapi:    false,   # [YT] enable JS API
        end:            nil,     # [YT] end position in seconds (from 0:00)
        iv_load_policy: 1,       # [YT] annotations 1 = load, 2 = ?, 3 = don't load
        loop:           false,   # [YT] loop video
        modestbranding: false,   # [YT] Hide YT logo in controls
        origin:         nil,     # [YT] origin for JS API
        playlist:       nil,     # [YT] list of video IDs to play (after if ID given)
        rel:            false,   # [YT] show related videos at the end
        showinfo:       true,    # [YT] Show video info (title bar)
        start:          nil,     # [YT] start position in seconds
        theme:          "dark",  # [YT] theme (dark or light)
        vq:             "large", # [YT] video quality (small, medium, large, hd720, hd1080)
      }
      args.reverse_merge!(defaults)
      params = youtube_params(args.slice(*defaults.keys - [:ratio]))

      # calculate dimensions if only one is given
      args[:width]  = 420 if !args[:width] && !args[:height]
      args[:height] = args[:width]  / args[:ratio] if args[:width]  && !args[:height]
      args[:width]  = args[:height] * args[:ratio] if args[:height] && !args[:width]

      %{
        <iframe
          frameborder="0"
          allowfullscreen="allowfullscreen"
          width="#{args[:width]}"
          height="#{args[:height]}"
          src="//www.youtube.com/embed/#{args[:id]}?#{params.to_query}"
        ></iframe>
      }.gsub("\n", "").squeeze(" ")
    end
    str
  end

  protected

  # Searches for occurences of tags <tag> and
  # replaces them with the result of &block.
  def process_tags str, tag, &block
    str.gsub!(/<#{tag}\s(.*?)>/im) do
      args = {}.with_indifferent_access
      Shellwords.shellsplit($1).each do |key_val|
        key, val = *key_val.split("=", 2)
        val = false if val.in? %w[false no]
        val = true if val.in? %w[true yes]
        val = parse_number(val)
        args[key] = val
      end
      block.call(args)
    end
    str
  end

  # parse numbers from text value
  def parse_number str
    return str.to_i if str =~ /\A\d+\z/
    return Float(str) if Float(str) rescue nil
    str
  end

  # youtubeify params (true = 1, false = 0)
  def youtube_params params
    params.each_with_object({}) do |(k, v), r|
      r[k] = case v
        when FalseClass then "0"
        when TrueClass then "1"
        else v
      end
    end
  end
end

# create a custom renderer that allows highlighting of code blocks
class HTMLwithPygments < Redcarpet::Render::HTML
  def block_code(code, language)
    Pygments.highlight(code, lexer: language, options: { linespans: Digest::SHA1.base64digest(code) })
  end
end

post.rb

class Post < ActiveRecord::Base
  def content opts = {}
    ContentRenderer.new(body, opts)
  end
end

# <youtube id="foobar">
# <youtube id="foobar" autostart="yes" start="30" end="60">