markdown_renderer.rb

require 'redcarpet'

class CustomMarkdownRenderer < Redcarpet::Render::Base
  @section_opened = false

  def header(text, header_level, options)
    s = ''
    if header_level == 1
      s << "</div>\n" if @section_opened
      s << "<div class=\"section\">\n<h1>"
      s << text
      s << "</h1>\n"
      @section_opened = true
    else
      s << "<h#{header_level}>#{text}</h#{header_level}>\n"
    end
    s
  end

  def postprocess(full_document)
    full_document << '</div>' if @section_opened
  end
end

renderer = CustomMarkdownRenderer.new
markdown = Redcarpet::Markdown.new(renderer, extensions = {})

s = markdown.render <<-EOF
# foo
## bar
# baz
## foobar
EOF

puts s