CHchubot·Jun 30, 2020
Hm I don't really think RE2 is even that influential (unfortunately IMO). Rust is the only new language whose engine borrows its design AFAIK. For example, I was disappointed to learn that Julia uses PCRE.
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I think backtracking is firmly in the past because it causes too many problems in practice [1], but I think there's scope for a text matching engine that's built on top of regular languages, but more expressive.
As mentioned here and on lobste.rs recently, regular languages aren't expressive enough for syntax highlighting (or lexing most programming languages in their entirety).
But it doesn't take much to make them more expressive, WHILE meaning these properties:
1. linear-time and constant space
2. Composition by OR
Backtracking fails at both properties. Backtracking is bad. Like you say with backreferences, there should be s
Read the rest
omething more expressive than regular languages that maintains both properties (and it's likely to be useful).
Example of tiny mechanism on top of regular languages:
When Are Lexer Modes Useful? http://www.oilshell.org/blog/2017/12/17.html
https://lobste.rs/s/m24zv1/xi_editor_retrospective#c_p3zadg
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Here's an implementation idea for such a text matching engine:
(1) Start with re2c, because it has some different scaling properties than other engines https://news.ycombinator.com/item?id=23665569
The compile time and code size are linear with respect to the number of patterns. What was very surprising to me is that the compiled C code starts off slower than interpreters, but scales better as you increase the number of patterns.
(2) Write a WebAssembly backend for it. Compiling its C output to WebAssembly is not likely to be great, because it uses gotos on every line, and gotos in WebAssembly need the Relooper algorithm last time I checked.
(3) Now add a linear-space constant-time VM on top. As a test of this property, it should be able to express LL(1) parsing and LALR(1) parsing. Something like "lexer modes" should also fall out trivially.
I believe this strategy saves a bunch of work, and has some desirable properties:
- an entire RE front end, handling captures and such in DFAs: https://arxiv.org/abs/1907.08837 . I'm sure re2c is slower than Hyperscan, but as you hint, I think it's also more general and widely used.
- wasm could be more palatable to embed than other VMs. There are multiple implementations of it.
- wasm avoids dealing with C code and assembly code. It has a few high quality JITs.
- The first version of wasm has a ton of limitations (lack of GC, rich interfaces to the host), but this application hits none of them. It's a perfect problem for it.
(I'm also pretty sure this hasn't been done, but I could be wrong. I googled "re2c wasm" and found that it is used in some wasm tools, but I think that's unrelated: https://chromium.googlesource.com/external/github.com/WebAss... )
And it's also significantly different than RE2 or rust/regex, if that's your complaint :)
If any of that is interesting feel free to contact me at any method listed here: http://www.oilshell.org/ (email, zulip, etc.)
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[1] https://lobste.rs/s/m24zv1/xi_editor_retrospective#c_pyfpv1