The 1960's elegance behind Go's regexp | Hacker News Reader

The 1960's elegance behind Go's regexp

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32 pointsDawny3326 comments

The article by Russ Cox [0] (from which the initial Perl vs. Thompson NFA graph is taken) is a much more informative read. Also, I'm not sure why this always gets brought up as being a big deal: CS is all about making the right tradeoffs and I agree that sometimes there are cool tricks that let you _almost_ have your cake an eat it. But here, there is no such trick: backtracking has always been exponential and finite state automata have always been linear.

No surprises.

[0] https://swtch.com/~rsc/regexp/regexp1.html

Respectively exponential or linear in the input size. The NFA in exchange is exponential in the states (or rather, it's deterministic counterpart is).

There are some more gotchas.

The finite state automata can potentially be exponential in the size of the regular expression you feed it. (Its states are set of states in the NFA.) Subexpression matching is more complex to implement and can make that exponential superexponential. (You have to go from sets of states to ordered sets of states.)

And these catastrophic expressions get much easier when you add in lookaheads, look behinds, and so on.

The lack of lookahead is a pain in Go's regexp engine, and this cascades to the various apps that use it, such as Prometheus and Kubernetes.

Mainly, it means it's super awkward — arguably so awkward it's completely impractical, because the result ends up being unreadable and probably slow — to do negative matching, i.e. exclusion.

Full disclosure - I work on the Hyperscan project at Intel github.com/01org/hyperscan

Regular expression implementation is fun and many interesting things have been done in this area. We are partial to the work of Gonzalo Navarro (in terms of summary papers) and the Glushkov construction, which predates the Thompson NFA construction and IMO is better in a number of ways for fast implementation.

I quite enjoyed the Russ Cox posts, but they are a very partial and idiosyncratic picture of regular expression implementation. RE2 is one point in the automata-style regex implementation space; Hyperscan is another, and there a bunch of other distinct and interesting approaches (e.g. the work from the Parabix guys, various hardware and GPGPU implementations, etc).

Interesting. The Glushkov construction is also used to decide ambiguousness of content models in SGML and XML. Moreover, it naturally extends to partial derivative-based automata as was used by Antimirov (might be interesting to the algebraically-minded FP folks on HN).

See, now _this_ is something interesting. When I get some time, I'll take a look at hyperscan. Looks pretty neat.

As the author of Rust's regex crate, Hyperscan is a work of art, both in terms of the algorithms it employs and its performance. (I don't think Hyperscan has been part of any publicized benchmark yet, so I'm only speaking with some limited experience I've had poking at it.)

I stopped reading when a post in 2017 compared anything to perl 5.8.7 which is older than what CentOS 5 shipped.

Perl 5.8.x was EOL in 2008. 5.10 was EOL in 2009.

The currently maintained versions are 5.22.3 and 5.24.1 and major redesign, refactoring, and rework has been done in the system including the regex parser. It went in the meantime from being a primarily recursive, backtracking NFA to using a DFA when possible, being primarily iterative, and only recursing when necessary.

Anyone intentionally picking a more than decade old version whose major version line was end-of-life nearly a decade ago to compare to their new hotness has completely invalidated the evidence for their arguments. All the great things the slides might say may still be true, but they are unsupported by these charts.

I don't think the author 'intentionally' picked it. Like all the other graphics in the presentation, it looks to have been sourced from somewhere else.

Edit: https://swtch.com/~rsc/regexp/regexp1.html

All the graphics come from the above, and no mention of the original article it was taken from. Nice.

Plagiarism v. cherry-picking (or is that pit-picking in this case)... It's difficult to say which is the bigger ethical issue. Both are extremely dishonest and misleading.

Slightly off-topic: BurntSushi wrote a Go binding[1] for the Rust regex engine[2] (he is also the author) which shares the same approach (finite automata) but has a more polished, highly optimized implementation. It could be useful in some scenario for people facing performance issues with Go's regex engine.

[1] https://github.com/BurntSushi/rure-go [2] https://github.com/rust-lang/regex

"[A regex is] a style of describing character strings. If a string successfully describes a regex, then it is called a match"

Shouldn't that be "if a regex successfully describes a string"? I'm confused.

I think the Thompson NFA matcher could be pushed further if someone would bother to do it. One example is that since it finds all solutions to a given regular expression, it could be used within a larger backtracking matcher to support back-references.

Consider:

  (a+)(a+)=\1

With an input like: aaaaa=aa

The matcher will find these strings by the time it gets to the '=':

    (a)(aaaa)=
    (aa)(aaa)=
    (aaa)(aa)=
    (aaaa)(a)=

Each match will end up as a separate still-existing thread in the Thompson matcher. Instead of pruning them as soon as possible, keep them and recursively match the rest of the string with the back-reference set in turn to each until a match is found.

If I understand your idea right, then this is actually being done! https://github.com/google/fancy-regex

TL;DR we use a subset of today's common regexps which allowed us to go lightning fast (as long as you don't need lookahead). Oh, and I'll neglect to be clear about that.

Nothing wrong in making the tradeoff but I didn't see it in the text.

I do not get why there still is so much of a hype for regular expressions. They are hard to read and maintain. They have a relatively large syntax. Edge-cases are easy to get wrong with them.

Looks like Tcl has been doing it right for a while...

Seriously, can we stop fetishising old approaches and papers? Yes, there's some tricks we've missed along the way, but there's a thread of alchemical thinking in our community that is just plain unproductive.

> Seriously, can we stop fetishising old approaches and papers?

The entire existence of Go is predicated on fetishizing old approaches, unfortunately.

> there's a thread of alchemical thinking in our community

You write that as if Lisp isn't the Philosopher's Stone. :)

LOL. Yeah, The Clojure community has a bit of that as well, but it's married to a genuine interest in stealing great ideas from new papers and other languages.

I think your argument would have more weight if this presentation wasn't sharing the front page with the story about a 9.6 inch IPad that has 100s of comments. What Apple does next is a tech fetish.

Personally, I'd rather read about regexps. We have a word for reading about old techniques where I am from -- we call it "learning".

It would be easier to take this comment seriously if Go wasn't a modern re-implementation of ALGOL 68.

60s regex with 60s language. Hmm, I wonder which area (regular language parsing or programming theory) has seen more research since then :D.

I'm not sure if I've ever seen someone fetishizing old approaches without ample justification for it, whereas unnecessarily reinventing the wheel happens constantly.

If our community has a problem in this area, it's that we don't pay enough attention to the old ways.

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