regex

This crate provides routines for searching strings for matches of a regular expression (aka "regex"). The regex syntax supported by this crate is similar to other regex engines, but it lacks several features that are not known how to implement efficiently. This includes, but is not limited to, look-around and backreferences. In exchange, all regex searches in this crate have worst case O(m * n) time complexity, where m is proportional to the size of the regex and n is proportional to the size of the string being searched.

Documentation

Module documentation with examples. The module documentation also includes a comprehensive description of the syntax supported.

Documentation with examples for the various matching functions and iterators can be found on the Regex type.

Usage

To bring this crate into your repository, either add regex to your Cargo.toml, or run cargo add regex.

Here's a simple example that matches a date in YYYY-MM-DD format and prints the year, month and day:

use regex::Regex;

fn main() {
    let re = Regex::new(r"(?x)
(?P<year>\d{4})  # the year
-
(?P<month>\d{2}) # the month
-
(?P<day>\d{2})   # the day
").unwrap();

    let caps = re.captures("2010-03-14").unwrap();
    assert_eq!("2010", &caps["year"]);
    assert_eq!("03", &caps["month"]);
    assert_eq!("14", &caps["day"]);
}

If you have lots of dates in text that you'd like to iterate over, then it's easy to adapt the above example with an iterator:

use regex::Regex;

fn main() {
    let re = Regex::new(r"(\d{4})-(\d{2})-(\d{2})").unwrap();
    let hay = "On 2010-03-14, foo happened. On 2014-10-14, bar happened.";

    let mut dates = vec![];
    for (_, [year, month, day]) in re.captures_iter(hay).map(|c| c.extract()) {
        dates.push((year, month, day));
    }
    assert_eq!(dates, vec![
      ("2010", "03", "14"),
      ("2014", "10", "14"),
    ]);
}

Usage: Avoid compiling the same regex in a loop

It is an anti-pattern to compile the same regular expression in a loop since compilation is typically expensive. (It takes anywhere from a few microseconds to a few milliseconds depending on the size of the regex.) Not only is compilation itself expensive, but this also prevents optimizations that reuse allocations internally to the matching engines.

In Rust, it can sometimes be a pain to pass regular expressions around if they're used from inside a helper function. Instead, we recommend using the once_cell crate to ensure that regular expressions are compiled exactly once. For example:

use {
    once_cell::sync::Lazy,
    regex::Regex,
};

fn some_helper_function(haystack: &str) -> bool {
    static RE: Lazy<Regex> = Lazy::new(|| Regex::new(r"...").unwrap());
    RE.is_match(haystack)
}

fn main() {
    assert!(some_helper_function("abc"));
    assert!(!some_helper_function("ac"));
}

Specifically, in this example, the regex will be compiled when it is used for the first time. On subsequent uses, it will reuse the previous compilation.

Usage: match regular expressions on &[u8]

The main API of this crate (regex::Regex) requires the caller to pass a &str for searching. In Rust, an &str is required to be valid UTF-8, which means the main API can't be used for searching arbitrary bytes.

To match on arbitrary bytes, use the regex::bytes::Regex API. The API is identical to the main API, except that it takes an &[u8] to search on instead of an &str. The &[u8] APIs also permit disabling Unicode mode in the regex even when the pattern would match invalid UTF-8. For example, (?-u:.) is not allowed in regex::Regex but is allowed in regex::bytes::Regex since (?-u:.) matches any byte except for \n. Conversely, . will match the UTF-8 encoding of any Unicode scalar value except for \n.

This example shows how to find all null-terminated strings in a slice of bytes:

use regex::bytes::Regex;

let re = Regex::new(r"(?-u)(?<cstr>[^\x00]+)\x00").unwrap();
let text = b"foo\xFFbar\x00baz\x00";

// Extract all of the strings without the null terminator from each match.
// The unwrap is OK here since a match requires the `cstr` capture to match.
let cstrs: Vec<&[u8]> =
    re.captures_iter(text)
      .map(|c| c.name("cstr").unwrap().as_bytes())
      .collect();
assert_eq!(vec![&b"foo\xFFbar"[..], &b"baz"[..]], cstrs);

Notice here that the [^\x00]+ will match any byte except for NUL, including bytes like \xFF which are not valid UTF-8. When using the main API, [^\x00]+ would instead match any valid UTF-8 sequence except for NUL.

Usage: match multiple regular expressions simultaneously

This demonstrates how to use a RegexSet to match multiple (possibly overlapping) regular expressions in a single scan of the search text:

use regex::RegexSet;

let set = RegexSet::new(&[
    r"\w+",
    r"\d+",
    r"\pL+",
    r"foo",
    r"bar",
    r"barfoo",
    r"foobar",
]).unwrap();

// Iterate over and collect all of the matches.
let matches: Vec<_> = set.matches("foobar").into_iter().collect();
assert_eq!(matches, vec![0, 2, 3, 4, 6]);

// You can also test whether a particular regex matched:
let matches = set.matches("foobar");
assert!(!matches.matched(5));
assert!(matches.matched(6));

Usage: regex internals as a library

The regex-automata directory contains a crate that exposes all of the internal matching engines used by the regex crate. The idea is that the regex crate exposes a simple API for 99% of use cases, but regex-automata exposes oodles of customizable behaviors.

Documentation for regex-automata.

Usage: a regular expression parser

This repository contains a crate that provides a well tested regular expression parser, abstract syntax and a high-level intermediate representation for convenient analysis. It provides no facilities for compilation or execution. This may be useful if you're implementing your own regex engine or otherwise need to do analysis on the syntax of a regular expression. It is otherwise not recommended for general use.

Documentation for regex-syntax.

Crate features

This crate comes with several features that permit tweaking the trade off between binary size, compilation time and runtime performance. Users of this crate can selectively disable Unicode tables, or choose from a variety of optimizations performed by this crate to disable.

When all of these features are disabled, runtime match performance may be much worse, but if you're matching on short strings, or if high performance isn't necessary, then such a configuration is perfectly serviceable. To disable all such features, use the following Cargo.toml dependency configuration:

[dependencies.regex]
version = "1.3"
default-features = false
# Unless you have a specific reason not to, it's good sense to enable standard
# library support. It enables several optimizations and avoids spin locks. It
# also shouldn't meaningfully impact compile times or binary size.
features = ["std"]

This will reduce the dependency tree of regex down to two crates: regex-syntax and regex-automata.

The full set of features one can disable are in the "Crate features" section of the documentation.

Performance

One of the goals of this crate is for the regex engine to be "fast." What that is a somewhat nebulous goal, it is usually interpreted in one of two ways. First, it means that all searches take worst case O(m * n) time, where m is proportional to len(regex) and n is proportional to len(haystack). Second, it means that even aside from the time complexity constraint, regex searches are "fast" in practice.

While the first interpretation is pretty unambiguous, the second one remains nebulous. While nebulous, it guides this crate's architecture and the sorts of the trade offs it makes. For example, here are some general architectural statements that follow as a result of the goal to be "fast":

• When given the choice between faster regex searches and faster Rust compile times, this crate will generally choose faster regex searches.
• When given the choice between faster regex searches and faster regex compile times, this crate will generally choose faster regex searches. That is, it is generally acceptable for Regex::new to get a little slower if it means that searches get faster. (This is a somewhat delicate balance to strike, because the speed of Regex::new needs to remain somewhat reasonable. But this is why one should avoid re-compiling the same regex over and over again.)
• When given the choice between faster regex searches and simpler API design, this crate will generally choose faster regex searches. For example, if one didn't care about performance, we could like get rid of both of the Regex::is_match and Regex::find APIs and instead just rely on Regex::captures.

There are perhaps more ways that being "fast" influences things.

While this repository used to provide its own benchmark suite, it has since been moved to rebar. The benchmarks are quite extensive, and there are many more than what is shown in rebar's README (which is just limited to a "curated" set meant to compare performance between regex engines). To run all of this crate's benchmarks, first start by cloning and installing rebar:

$ git clone https://github.com/BurntSushi/rebar
$ cd rebar
$ cargo install --path ./

Then build the benchmark harness for just this crate:

$ rebar build -e '^rust/regex$'

Run all benchmarks for this crate as tests (each benchmark is executed once to ensure it works):

$ rebar measure -e '^rust/regex$' -t

Record measurements for all benchmarks and save them to a CSV file:

$ rebar measure -e '^rust/regex$' | tee results.csv

Explore benchmark timings:

$ rebar cmp results.csv

See the rebar documentation for more details on how it works and how to compare results with other regex engines.

Hacking

The regex crate is, for the most part, a pretty thin wrapper around the meta::Regex from the regex-automata crate. Therefore, if you're looking to work on the internals of this crate, you'll likely either want to look in regex-syntax (for parsing) or regex-automata (for construction of finite automata and the search routines).

My blog on regex internals goes into more depth.

Minimum Rust version policy

This crate's minimum supported rustc version is 1.65.0.

The policy is that the minimum Rust version required to use this crate can be increased in minor version updates. For example, if regex 1.0 requires Rust 1.20.0, then regex 1.0.z for all values of z will also require Rust 1.20.0 or newer. However, regex 1.y for y > 0 may require a newer minimum version of Rust.

License

This project is licensed under either of

• Apache License, Version 2.0, (LICENSE-APACHE or https://www.apache.org/licenses/LICENSE-2.0)
• MIT license (LICENSE-MIT or https://opensource.org/licenses/MIT)

at your option.

The data in regex-syntax/src/unicode_tables/ is licensed under the Unicode License Agreement (LICENSE-UNICODE).