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.org/project/parsimonious/