I'd like to point out that there are libraries (PyPEG, PyParsing, and PyMeta) which take EBNF-style descriptions of grammars and produce decently fast parsers, PEG-style. PyMeta's my personal favorite, since it both takes a straight text description, and also uses OMeta instead of ENBF, turning parse and lex into a single operation.

Of course, the downside is that formal grammars are often a bad fit for custom languages of the web, which tend to have incomplete/undecideable/context-dependent grammars. This would work for Markdown, though, since it does have a parseable grammar.