A Little About PEG Parsers

PEG parsers don't draw a distinction between lexing and parsing; everything's done at once. As a result, there is no lookahead limit, as there is with, for instance, Yacc. And, due to both of these properties, PEG grammars are easier to write: they're basically just EBNF.

Optimizing Grammars

Don't repeat expressions. If you need a Regex('such-and-such') at some point in your grammar, don't type it twice; make it a rule of its own, and reference it from wherever you need it. You'll get the most out of the caching this way, since cache lookups are by expression object identity (for speed). Even if you have an expression that's very simple, not repeating it will save RAM, as there can, at worst, be a cached int for every char in the text you're parsing.

How much should you shove into one Regex, versus how much should you break them up to not repeat yourself? That's a fine balance and worthy of benchmarking. More stuff jammed into a regex will execute faster, because it doesn't have to run any Python between pieces, but a broken up one will give better cache performance if the individual pieces are re-used elsewhere. If the pieces of a regex aren't used anywhere else, by all means keep the whole thing together.