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diff --git a/vendor/regex-automata/README.md b/vendor/regex-automata/README.md index 23e0bffe0..c12b07012 100644 --- a/vendor/regex-automata/README.md +++ b/vendor/regex-automata/README.md @@ -1,15 +1,13 @@ regex-automata ============== -A low level regular expression library that uses deterministic finite automata. -It supports a rich syntax with Unicode support, has extensive options for -configuring the best space vs time trade off for your use case and provides -support for cheap deserialization of automata for use in `no_std` environments. +This crate exposes a variety of regex engines used by the `regex` crate. +It provides a vast, sprawling and "expert" level API to each regex engine. +The regex engines provided by this crate focus heavily on finite automata +implementations and specifically guarantee worst case `O(m * n)` time +complexity for all searches. (Where `m ~ len(regex)` and `n ~ len(haystack)`.) -[![Build status](https://github.com/BurntSushi/regex-automata/workflows/ci/badge.svg)](https://github.com/BurntSushi/regex-automata/actions) +[![Build status](https://github.com/rust-lang/regex/workflows/ci/badge.svg)](https://github.com/rust-lang/regex/actions) [![Crates.io](https://img.shields.io/crates/v/regex-automata.svg)](https://crates.io/crates/regex-automata) -![Minimum Supported Rust Version 1.41](https://img.shields.io/badge/rustc-1.41-green) - -Dual-licensed under MIT or the [UNLICENSE](https://unlicense.org/). ### Documentation @@ -17,206 +15,103 @@ Dual-licensed under MIT or the [UNLICENSE](https://unlicense.org/). https://docs.rs/regex-automata -### Usage - -Add this to your `Cargo.toml`: - -```toml -[dependencies] -regex-automata = "0.1" -``` - -**WARNING**: The `master` branch currently contains code for the `0.2` release, -but this README still targets the `0.1` release. Namely, it is recommended to -stick with the `0.1` release. The `0.2` release was made prematurely in order -to unblock some folks. +### Example - -### Example: basic regex searching - -This example shows how to compile a regex using the default configuration -and then use it to find matches in a byte string: +This example shows how to search for matches of multiple regexes, where each +regex uses the same capture group names to parse different key-value formats. ```rust -use regex_automata::Regex; - -let re = Regex::new(r"[0-9]{4}-[0-9]{2}-[0-9]{2}").unwrap(); -let text = b"2018-12-24 2016-10-08"; -let matches: Vec<(usize, usize)> = re.find_iter(text).collect(); -assert_eq!(matches, vec![(0, 10), (11, 21)]); +use regex_automata::{meta::Regex, PatternID}; + +let re = Regex::new_many(&[ + r#"(?m)^(?<key>[[:word:]]+)=(?<val>[[:word:]]+)$"#, + r#"(?m)^(?<key>[[:word:]]+)="(?<val>[^"]+)"$"#, + r#"(?m)^(?<key>[[:word:]]+)='(?<val>[^']+)'$"#, + r#"(?m)^(?<key>[[:word:]]+):\s*(?<val>[[:word:]]+)$"#, +]).unwrap(); +let hay = r#" +best_album="Blow Your Face Out" +best_quote='"then as it was, then again it will be"' +best_year=1973 +best_simpsons_episode: HOMR +"#; +let mut kvs = vec![]; +for caps in re.captures_iter(hay) { + // N.B. One could use capture indices '1' and '2' here + // as well. Capture indices are local to each pattern. + // (Just like names are.) + let key = &hay[caps.get_group_by_name("key").unwrap()]; + let val = &hay[caps.get_group_by_name("val").unwrap()]; + kvs.push((key, val)); +} +assert_eq!(kvs, vec![ + ("best_album", "Blow Your Face Out"), + ("best_quote", "\"then as it was, then again it will be\""), + ("best_year", "1973"), + ("best_simpsons_episode", "HOMR"), +]); ``` -For more examples and information about the various knobs that can be turned, -please see the [docs](https://docs.rs/regex-automata/0.1). - - -### Support for `no_std` - -This crate comes with a `std` feature that is enabled by default. When the -`std` feature is enabled, the API of this crate will include the facilities -necessary for compiling, serializing, deserializing and searching with regular -expressions. When the `std` feature is disabled, the API of this crate will -shrink such that it only includes the facilities necessary for deserializing -and searching with regular expressions. - -The intended workflow for `no_std` environments is thus as follows: - -* Write a program with the `std` feature that compiles and serializes a - regular expression. Serialization should only happen after first converting - the DFAs to use a fixed size state identifier instead of the default `usize`. - You may also need to serialize both little and big endian versions of each - DFA. (So that's 4 DFAs in total for each regex.) -* In your `no_std` environment, follow the examples above for deserializing - your previously serialized DFAs into regexes. You can then search with them - as you would any regex. - -Deserialization can happen anywhere. For example, with bytes embedded into a -binary or with a file memory mapped at runtime. - -Note that the -[`ucd-generate`](https://github.com/BurntSushi/ucd-generate) -tool will do the first step for you with its `dfa` or `regex` sub-commands. - - -### Cargo features - -* `std` - **Enabled** by default. This enables the ability to compile finite - automata. This requires the `regex-syntax` dependency. Without this feature - enabled, finite automata can only be used for searching (using the approach - described above). -* `transducer` - **Disabled** by default. This provides implementations of the - `Automaton` trait found in the `fst` crate. This permits using finite - automata generated by this crate to search finite state transducers. This - requires the `fst` dependency. - - -### Differences with the regex crate - -The main goal of the [`regex`](https://docs.rs/regex) crate is to serve as a -general purpose regular expression engine. It aims to automatically balance low -compile times, fast search times and low memory usage, while also providing -a convenient API for users. In contrast, this crate provides a lower level -regular expression interface that is a bit less convenient while providing more -explicit control over memory usage and search times. - -Here are some specific negative differences: - -* **Compilation can take an exponential amount of time and space** in the size - of the regex pattern. While most patterns do not exhibit worst case - exponential time, such patterns do exist. For example, `[01]*1[01]{N}` will - build a DFA with `2^(N+1)` states. For this reason, untrusted patterns should - not be compiled with this library. (In the future, the API may expose an - option to return an error if the DFA gets too big.) -* This crate does not support sub-match extraction, which can be achieved with - the regex crate's "captures" API. This may be added in the future, but is - unlikely. -* While the regex crate doesn't necessarily sport fast compilation times, the - regexes in this crate are almost universally slow to compile, especially when - they contain large Unicode character classes. For example, on my system, - compiling `\w{3}` with byte classes enabled takes just over 1 second and - almost 5MB of memory! (Compiling a sparse regex takes about the same time - but only uses about 500KB of memory.) Conversly, compiling the same regex - without Unicode support, e.g., `(?-u)\w{3}`, takes under 1 millisecond and - less than 5KB of memory. For this reason, you should only use Unicode - character classes if you absolutely need them! -* This crate does not support regex sets. -* This crate does not support zero-width assertions such as `^`, `$`, `\b` or - `\B`. -* As a lower level crate, this library does not do literal optimizations. In - exchange, you get predictable performance regardless of input. The - philosophy here is that literal optimizations should be applied at a higher - level, although there is no easy support for this in the ecosystem yet. -* There is no `&str` API like in the regex crate. In this crate, all APIs - operate on `&[u8]`. By default, match indices are guaranteed to fall on - UTF-8 boundaries, unless `RegexBuilder::allow_invalid_utf8` is enabled. - -With some of the downsides out of the way, here are some positive differences: - -* Both dense and sparse DFAs can be serialized to raw bytes, and then cheaply - deserialized. Deserialization always takes constant time since searching can - be performed directly on the raw serialized bytes of a DFA. -* This crate was specifically designed so that the searching phase of a DFA has - minimal runtime requirements, and can therefore be used in `no_std` - environments. While `no_std` environments cannot compile regexes, they can - deserialize pre-compiled regexes. -* Since this crate builds DFAs ahead of time, it will generally out-perform - the `regex` crate on equivalent tasks. The performance difference is likely - not large. However, because of a complex set of optimizations in the regex - crate (like literal optimizations), an accurate performance comparison may be - difficult to do. -* Sparse DFAs provide a way to build a DFA ahead of time that sacrifices search - performance a small amount, but uses much less storage space. Potentially - even less than what the regex crate uses. -* This crate exposes DFAs directly, such as `DenseDFA` and `SparseDFA`, - which enables one to do less work in some cases. For example, if you only - need the end of a match and not the start of a match, then you can use a DFA - directly without building a `Regex`, which always requires a second DFA to - find the start of a match. -* Aside from choosing between dense and sparse DFAs, there are several options - for configuring the space usage vs search time trade off. These include - things like choosing a smaller state identifier representation, to - premultiplying state identifiers and splitting a DFA's alphabet into - equivalence classes. Finally, DFA minimization is also provided, but can - increase compilation times dramatically. - - -### Future work - -* Look into being smarter about generating NFA states for large Unicode - character classes. These can create a lot of additional work for both the - determinizer and the minimizer, and I suspect this is the key thing we'll - want to improve if we want to make DFA compile times faster. I *believe* - it's possible to potentially build minimal or nearly minimal NFAs for the - special case of Unicode character classes by leveraging Daciuk's algorithms - for building minimal automata in linear time for sets of strings. See - https://blog.burntsushi.net/transducers/#construction for more details. The - key adaptation I think we need to make is to modify the algorithm to operate - on byte ranges instead of enumerating every codepoint in the set. Otherwise, - it might not be worth doing. -* Add support for regex sets. It should be possible to do this by "simply" - introducing more match states. I think we can also report the positions at - each match, similar to how Aho-Corasick works. I think the long pole in the - tent here is probably the API design work and arranging it so that we don't - introduce extra overhead into the non-regex-set case without duplicating a - lot of code. It seems doable. -* Stretch goal: support capturing groups by implementing "tagged" DFA - (transducers). Laurikari's paper is the usual reference here, but Trofimovich - has a much more thorough treatment here: - https://re2c.org/2017_trofimovich_tagged_deterministic_finite_automata_with_lookahead.pdf - I've only read the paper once. I suspect it will require at least a few more - read throughs before I understand it. - See also: https://re2c.org -* Possibly less ambitious goal: can we select a portion of Trofimovich's work - to make small fixed length look-around work? It would be really nice to - support ^, $ and \b, especially the Unicode variant of \b and CRLF aware $. -* Experiment with code generating Rust code. There is an early experiment in - src/codegen.rs that is thoroughly bit-rotted. At the time, I was - experimenting with whether or not codegen would significant decrease the size - of a DFA, since if you squint hard enough, it's kind of like a sparse - representation. However, it didn't shrink as much as I thought it would, so - I gave up. The other problem is that Rust doesn't support gotos, so I don't - even know whether the "match on each state" in a loop thing will be fast - enough. Either way, it's probably a good option to have. For one thing, it - would be endian independent where as the serialization format of the DFAs in - this crate are endian dependent (so you need two versions of every DFA, but - you only need to compile one of them for any given arch). -* Experiment with unrolling the match loops and fill out the benchmarks. -* Add some kind of streaming API. I believe users of the library can already - implement something for this outside of the crate, but it would be good to - provide an official API. The key thing here is figuring out the API. I - suspect we might want to support several variants. -* Make a decision on whether or not there is room for literal optimizations - in this crate. My original intent was to not let this crate sink down into - that very very very deep rabbit hole. But instead, we might want to provide - some way for literal optimizations to hook into the match routines. The right - path forward here is to probably build something outside of the crate and - then see about integrating it. After all, users can implement their own - match routines just as efficiently as what the crate provides. -* A key downside of DFAs is that they can take up a lot of memory and can be - quite costly to build. Their worst case compilation time is O(2^n), where - n is the number of NFA states. A paper by Yang and Prasanna (2011) actually - seems to provide a way to character state blow up such that it is detectable. - If we could know whether a regex will exhibit state explosion or not, then - we could make an intelligent decision about whether to ahead-of-time compile - a DFA. - See: https://www.researchgate.net/profile/Xu-Shutu/publication/229032602_Characterization_of_a_global_germplasm_collection_and_its_potential_utilization_for_analysis_of_complex_quantitative_traits_in_maize/links/02bfe50f914d04c837000000/Characterization-of-a-global-germplasm-collection-and-its-potential-utilization-for-analysis-of-complex-quantitative-traits-in-maize.pdf + +### Safety + +**I welcome audits of `unsafe` code.** + +This crate tries to be extremely conservative in its use of `unsafe`, but does +use it in a few spots. In general, I am very open to removing uses of `unsafe` +if it doesn't result in measurable performance regressions and doesn't result +in significantly more complex code. + +Below is an outline of how `unsafe` is used in this crate. + +* `util::pool::Pool` makes use of `unsafe` to implement a fast path for +accessing an element of the pool. The fast path applies to the first thread +that uses the pool. In effect, the fast path is fast because it avoid a mutex +lock. `unsafe` is also used in the no-std version of `Pool` to implement a spin +lock for synchronization. +* `util::lazy::Lazy` uses `unsafe` to implement a variant of +`once_cell::sync::Lazy` that works in no-std environments. A no-std no-alloc +implementation is also provided that requires use of `unsafe`. +* The `dfa` module makes extensive use of `unsafe` to support zero-copy +deserialization of DFAs. The high level problem is that you need to get from +`&[u8]` to the internal representation of a DFA without doing any copies. +This is required for support in no-std no-alloc environments. It also makes +deserialization extremely cheap. +* The `dfa` and `hybrid` modules use `unsafe` to explicitly elide bounds checks +in the core search loops. This makes the codegen tighter and typically leads to +consistent 5-10% performance improvements on some workloads. + +In general, the above reflect the only uses of `unsafe` throughout the entire +`regex` crate. At present, there are no plans to meaningfully expand the use +of `unsafe`. With that said, one thing folks have been asking for is cheap +deserialization of a `regex::Regex`. My sense is that this feature will require +a lot more `unsafe` in places to support zero-copy deserialization. It is +unclear at this point whether this will be pursued. + + +### Motivation + +I started out building this crate because I wanted to re-work the `regex` +crate internals to make it more amenable to optimizations. It turns out that +there are a lot of different ways to build regex engines and even more ways to +compose them. Moreover, heuristic literal optimizations are often tricky to +get correct, but the fruit they bear is attractive. All of these things were +difficult to expand upon without risking the introduction of more bugs. So I +decided to tear things down and start fresh. + +In the course of doing so, I ended up designing strong boundaries between each +component so that each component could be reasoned and tested independently. +This also made it somewhat natural to expose the components as a library unto +itself. Namely, folks have been asking for more capabilities in the regex +crate for a long time, but these capabilities usually come with additional API +complexity that I didn't want to introduce in the `regex` crate proper. But +exposing them in an "expert" level crate like `regex-automata` seemed quite +fine. + +In the end, I do still somewhat consider this crate an experiment. It is +unclear whether the strong boundaries between components will be an impediment +to ongoing development or not. De-coupling tends to lead to slower development +in my experience, and when you mix in the added cost of not introducing +breaking changes all of the time, things can get quite complicated. But, I +don't think anyone has ever release the internals of a regex engine as a +library before. So it will be interesting to see how it plays out! |