Parsing Expression Grammar vs. Regexes: Building Org Parser in Lisp, Export HTML

Over the years, I have implemented parsers numerous times, both at work and for side projects, so writing recursive-descent parsers from scratch has become second nature. Once you understand the mechanics, writing a parser by hand is straightforward and offers distinct advantages, particularly much greater flexibility with error handling and reporting. Because of that, I had always viewed PEGs and parser generators as tools primarily for people who couldn’t hand-roll their own because of their circumstances or skill level. (If you look at projects that are neither understaffed nor underskilled you'll find that hand-written recursive-descent parsers are very common: Clang, Go, Rust, TypeScript, Swift and Lua all have hand-written parsers.)

LLMs have changed the equation. It used to take me 2-3 hours to write a parser for a moderately complex grammar. Now, if I hand an LLM a loosely written, BNF-ish grammar and ask for a recursive-descent parser, it finishes the job in five minutes. At this point, writing them by hand is hard to justify. The model does it substantially faster, and lately, often better than I do.

Which makes me wonder: what’s the appeal of PEGs or parser generators now? They used to make sense when hand-writing wasn't practical, but what compelling reasons are left to use them today?

Ha, that's interesting.

I started exactly such a project, for org syntax, a while ago. Didn't get very far though because it seems to me that things in org cannot be parsed and understood in one go. For example the TODO keywords can be specified at the top of an org file and then would influence how headings in the document are understood. This cannot be done with merely a PEG.

Another tricky point is nested inline markup. Bold inside italic? Verbatim inside bold, inside italic? And so on. Would be great to support all meaningful combinations via recursive rules. Org has many inline markup things. Even programming language specific inline markup elements.

Another issue is, that Guile's PEG library, when using the non-string form of grammar rules, does not allow mutually recursive grammar rules, due to being buggy (in that way it is not excellent, but in other ways it is!). One has to use the string form or rules, which makes it a lot less nice to use, unfortunately. I hope that will be fixed at some point, because I like the library except for that. And it is in the standard lib, no need to install any dependencies. Like to use it for AoC puzzle inputs for example.

Will be interesting to read how far they got.

EDIT: Also it's going to be great to have an actual grammar based parser for org. This will ease creating one for other tools, like for example parsers of readmes in repos on git hosters.

One cool factoid about PEGs is that it is an open problem if they can parse all context free languages.

“Org” in this context is emacs org mode. Seems odd in this day and age that anyone would see hierarchical text format and think “regex”!

Due to personal failings, my main takeaway from this piece is that Regex is under attack -- I will not stand by as CS theory blorbo is maligned so! Of course there's an undeniable elegance to Lisp in all cases, and seeing Lisp in use today feels like seeing someone unsheathe Valerian steel. But still, the fact remains: regex is vastly underappreciated by the standard engineer, largely because a few of the most common implementations drop some of the most important features. Namely, composition.

I have no easy way to express this other than to plug my recently-released OSS, namely a library I wrote to implement "RegexStores" in Python[1] to compile complex, composed regexes upfront from a custom DSL. I'll link a representative usecase below[2], which I think drives home two things:

1. Regex deserves to be composed using named groups, at the very least! Most people aren't even aware that you can define named groups upfront and then reference them by name throughout, even 'capturing' them multiple times in one match.[3] Even when you do write patterns that use subroutines, the ergonomics of actually accessing, say, the three matched substrings for the `username` group in your single match is finnicky at best (without some magic[4]...).

2. Regex needs high quality syntax highlighting. The linked page is hopefully skimmable in a python sense, but trying to decipher the individual patterns in GitLab would be tough even for me, and I wrote the darn things. It's kinda awkwardly sized, but this screenshot drives home the basics of the point -- note the bright yellow named groups, which are basically subroutine invocations as discussed above: https://i.imgur.com/NllqM6S.png

Sorry to quasihijack the thread. Hopefully the fact that all this stuff has never been announced or published anywhere is proof enough of my good intentions -- that is, to defend my one n' only :)

Most of this functionality was conceived and implemented in a rage after I found out that Rust's main (only?) regex library avoids the possibility of infinite loops by just dropping half the features of the language, so excuse the jank. Even the halting problem seems solvable when you're willing to make moves like that!

[1]: https://gitlab.com/doering-ai/libs/basis/#regular-expression...

[2]: https://gitlab.com/doering-ai/apps/wiki-parse/-/blob/main/wi...

[3]: I wrote up an overview of advanced regexes in Python a while back, but was hit by the agential engineering bus before I had the time to polish and release it. Some may find it interesting, if overly long -- the aforementioned subroutines are covered under `3.1`, and repeated captures under `6.2`: https://gitlab.com/doering-ai/libs/basis/-/blob/main/docs/re...

[4]: https://gitlab.com/doering-ai/libs/basis/-/blob/main/my/rege...