Parsing Expression Grammar vs. Regexes: Building Org Parser in Lisp, Export HTML
jointhefreeworld.org72 points by jjba23 2 days ago
72 points by jjba23 2 days ago
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.
I had done some work in this direction in my org parser for racket [0]. That work is stalled at the moment (though I still intend to come back to it), but many of the interactions between org and traditional formal grammars have been explored there.
Aha! Interesting to hear you were going in the same direction. There are indeed quite some tricky situations to handle since Org (and Markdown) are complex in many ways.
the approach I am taking is to combine the PEG with a step of AST processing in order to then produce a correct and descriptive tree structure (for later rendering).
I do agree the Guile library has some spots to improve but honestly, I am already enjoying it so much, and it's far better for the Lisper in me than other things I've tried. I also find S-expressions much more comprehensible and self-documenting.
Looking forward to seeing what you do with it :)
I am indeed hoping that this library is useful for more people and that perhaps things like Haunt blog generator can use it (and other interesting uses)
As for nested markup, and nested lists it has been tricky, but so far I've really promising results:
(assert-equal #:got (inline-org->html (inline-org->ast "*bold with /italic/*")) #:expect "<strong>bold with <em>italic</em></strong>")
One cool factoid about PEGs is that it is an open problem if they can parse all context free languages.
If I understand correctly, one of the recent cool generative AI-assisted results is actually supposed to have closed this problem, complete with a Lean proof!
“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”!
Indeed. Regexes are _regular_, which is non hierarchical by definition.
The two are useful for different layers of abstraction: regex is for lexing and PEG is for parsing.
Speaking of the Chomsky hierarchy, last I checked, it is still unproven whether or not PEGs can parse all context free languages. Intuitively, they're _probably_ weaker than CFGs, but no one has yet provided a counter example.
PEGs aren't contained within the context-free languages, so it's not really intuitive that they're weaker, particularly as nobody has yet come up with a context-free language that a PEG cannot parse. They're capable of parsing things context-free grammars cannot.
Indeed you make a good point. My naive self thought it could work out, but actually the more you zoom out, the more tree-structures you see in many formats. And in my opinion, then it's time to sheathe Valyrian Steel of Lisp, IMHO the language to rule them all when it comes to working with tree based structures (Org, HTML, SXML, many more)
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...
I think you make a great point in highlighting the value of Regex. I don't want to undermine it at all, although some use-cases are just not a good fit for it (even if you can horseshoe things). I think a higher-level abstraction for regex is super welcome, and I quite admire what you are doing there, thanks a lot for sharing. I do think regex's indeed quite a lot more difficult to grasp when complexity rises compared to grammars or other approaches.
Worth looking into nested words aka visibly pushdown languages.
They're a proper superset of regular languages and a proper subset of deterministic context-free languages, but they retain many of the nice properties of regular languages that DCFLs don't - they're closed under intersection, union, concatenation, Kleene Star and reversal.
They can parse more languages that Regular Expressions (non PCRE), but fewer than deterministic CFG subsets like LL/LR. They're expressive enough to parse languages which have a regular tree structure like S-expressions, JSON, XHTML.
