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.

Minimum Rust version policy

This crate's minimum supported rustc version is 1.60.0.

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).