Markdown is pretty gnarly as a language. I wrote a markdown interpreter for Erlang https://github.com/hypernumbers/erlmarkdown

Initially I wrote it against the Daring Fireball spec but switched it over to track the output of the showdown javascript library.

Markdown is not a great language to implement. I wrote a hand parser because there are loads of places where you have to look ahead more than one token (not that I am a great lexer/parser expert).