How can I restrict Markdown syntax in Ruby?

I wish to implement Markdown in a Rails CMS comments system using a Ruby library such as Maraku or Kramdown. I need to restrict which Markdown features the users can submit. In this system users aren't allowed to insert to images, html, or perform any heavy editing, but emphasis and hyperlinks are okay.

Essentially, I wish to create something similar to this Textile filter, but for Markdown syntax.

I have been using a second step after the markdown trasformation to sanitize the data using the sanitize gem. Its white-list based and very configurable, you could easily achieve what you are after with it.

To save you some time, here is my text formatter module, hope it helps you out. The built-in relaxed rule was a bit too strict for me.

module TextFormatter
  require 'sanitize'

  module Formatters
    MARKDOWN = 1
    TEXTILE = 2
  end

  RELAXED = {
      :elements => [
        'a', 'b', 'blockquote', 'br', 'caption', 'cite', 'code', 'col',
        'colgroup', 'dd', 'dl', 'dt', 'em', 'i', 'img', 'li', 'ol', 'p', 'pre',
        'q', 'small', 'strike', 'strong', 'sub', 'sup', 'table', 'tbody', 'td',
        'tfoot', 'th', 'thead', 'tr', 'u', 'ul', 'del', 'ins', 'h1', 'h2', 'h3', 'h4', 'h5', 'h5', 'hr', 'kbd'],

      :attributes => {
        'a'          => ['href', 'title'],
        'blockquote' => ['cite'],
        'col'        => ['span', 'width'],
        'colgroup'   => ['span', 'width'],
        'img'        => ['align', 'alt', 'height', 'src', 'title', 'width'],
        'ol'         => ['start', 'type'],
        'q'          => ['cite'],
        'table'      => ['summary', 'width'],
        'td'         => ['abbr', 'axis', 'colspan', 'rowspan', 'width'],
        'th'         => ['abbr', 'axis', 'colspan', 'rowspan', 'scope',
                         'width'],
        'ul'         => ['type']
      },

      :protocols => {
        'a'          => {'href' => ['ftp', 'http', 'https', 'mailto',
                                    :relative]},
        'blockquote' => {'cite' => ['http', 'https', :relative]},
        'img'        => {'src'  => ['http', 'https', :relative]},
        'q'          => {'cite' => ['http', 'https', :relative]}
      }
    }



  def self.to_html(text, formatter = Formatters::MARKDOWN)
    return "" unless text

    html = case formatter 
           when Formatters::MARKDOWN then
             RDiscount.new(text, :smart).to_html
           when Formatters::TEXTILE then
             RedCloth.new(text).to_html
           end

    Sanitize.clean(html, RELAXED) 
  end
end