Parsimonious aims to be the fastest arbitrary-lookahead parser written 
in pure Python and the most usable. It's based on parsing expression 
grammars (PEGs), which means you feed it a simplified sort of EBNF 
notation. Parsimonious was designed to undergird a MediaWiki parser 
that wouldn't take 5 seconds or a GB of RAM to do one page, but it's 
applicable to all sorts of languages.

WWW: https://pypi.python.org/pypi/parsimonious