It's not ruby, but one of the formats Pandoc now writes is 'plain'. Here's some arbitrary markdown:
# My Great Work
## First Section
Here we discuss my difficulties with [Markdown](http://wikipedia.org/Markdown)
## Second Section
We begin with a quote:
> We hold these truths to be self-evident ...
then some code:
#! /usr/bin/bash
That's *all*.
(Not sure how to turn off the syntax highlighting!) Here's the associated 'plain':
My Great Work
=============
First Section
-------------
Here we discuss my difficulties with Markdown
Second Section
--------------
We begin with a quote:
We hold these truths to be self-evident ...
then some code:
#! /usr/bin/bash
That's all.
You can get an idea what it does with the different elements it parses out of documents from the definition of plainify
in pandoc/blob/master/src/Text/Pandoc/Writers/Markdown.hs in the Github repository; there is also a tutorial that shows how easy it is to modify the behavior.