I recently wrote something just like this for Node.js:
https://github.com/joehewitt/markdom
It's used on http://uponahill.com to extract Flickr urls so I can query the Flickr API and get complete metadata about each photo. My gf, who does most of the writing/photography on the site, has found writing in Markdown and copy/pasting Flickr urls a pretty frictionless experience compared to the Wordpress CMS.