created_at2019-01-02 22:11:41.647566
updated_at2024-03-04 12:49:20.813775
descriptionAutomata construction and matching using regular expressions.
Andrew Gallant (BurntSushi)




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



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.

use regex_automata::{meta::Regex, PatternID};

let re = Regex::new_many(&[
let hay = r#"
best_album="Blow Your Face Out"
best_quote='"then as it was, then again it will be"'
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"),


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.


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!

Commit count: 1490

cargo fmt