regex
=====

A Rust library for parsing, compiling, and executing regular expressions.
This particular implementation of regular expressions guarantees execution
in linear time with respect to the size of the regular expression and
search text by using finite automata. In particular, it makes use of both
NFAs and DFAs when matching. Much of the syntax and implementation is inspired
by [RE2](https://github.com/google/re2).

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### Documentation

[Module documentation with examples](https://doc.rust-lang.org/regex).
The module documentation also include a comprehensive description of the syntax
supported.

Documentation with examples for the various matching functions and iterators
can be found on the
[`Regex` type](https://doc.rust-lang.org/regex/regex/enum.Regex.html).

### Usage

Add this to your `Cargo.toml`:

```toml
[dependencies]
regex = "0.1"
```

and this to your crate root:

```rust
extern crate regex;
```

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

```rust
extern crate regex;

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.name("year").unwrap());
    assert_eq!("03", caps.name("month").unwrap());
    assert_eq!("14", caps.name("day").unwrap());
}
```

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:

```rust
extern crate regex;

use regex::Regex;

const TO_SEARCH: &'static str = "
On 2010-03-14, foo happened. On 2014-10-14, bar happened.
";

fn main() {
    let re = Regex::new(r"(\d{4})-(\d{2})-(\d{2})").unwrap();

    for caps in re.captures_iter(TO_SEARCH) {
        // Note that all of the unwraps are actually OK for this regex
        // because the only way for the regex to match is if all of the
        // capture groups match. This is not true in general though!
        println!("year: {}, month: {}, day: {}",
                 caps.at(1).unwrap(),
                 caps.at(2).unwrap(),
                 caps.at(3).unwrap());
    }
}
```

This example outputs:

```
year: 2010, month: 03, day: 14
year: 2014, month: 10, day: 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
[`lazy_static`](https://crates.io/crates/lazy_static) crate to ensure that
regular expressions are compiled exactly once.

For example:

```rust
#[macro_use] extern crate lazy_static;
extern crate regex;

use regex::Regex;

fn some_helper_function(text: &str) -> bool {
    lazy_static! {
        static ref RE: Regex = Regex::new("...").unwrap();
    }
    RE.is_match(text)
}
```

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`. By default, `.` will match any *byte* using
`regex::bytes::Regex`, while `.` will match any encoded Unicode *codepoint*
using the main API.

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

```rust
use regex::bytes::Regex;

let re = Regex::new(r"(?P<cstr>[^\x00]+)\x00").unwrap();
let text = b"foo\x00bar\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())
      .collect();
assert_eq!(vec![&b"foo"[..], &b"bar"[..], &b"baz"[..]], cstrs);
```

Notice here that the `[^\x00]+` will match any *byte* except for `NUL`. 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:

```rust
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!` compiler plugin

**WARNING**: The `regex!` compiler plugin is orders of magnitude slower than
the normal `Regex::new(...)` usage. You should not use the compiler plugin
unless you have a very special reason for doing so. The performance difference
may be the temporary, but the path forward at this point isn't clear.

The `regex!` compiler plugin will compile your regexes at compile time. **This
only works with a nightly compiler.**

Here is a small example:

```rust
#![feature(plugin)]

#![plugin(regex_macros)]
extern crate regex;

fn main() {
    let re = regex!(r"(\d{4})-(\d{2})-(\d{2})");
    let caps = re.captures("2010-03-14").unwrap();

    assert_eq!("2010", caps.at(1).unwrap());
    assert_eq!("03", caps.at(2).unwrap());
    assert_eq!("14", caps.at(3).unwrap());
}
```

Notice that we never `unwrap` the result of `regex!`. This is because your
*program* won't compile if the regex doesn't compile. (Try `regex!("(")`.)


### Usage: a regular expression parser

This repository contains a crate that provides a well tested regular expression
parser and abstract syntax. 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` with
examples](https://doc.rust-lang.org/regex/regex_syntax/index.html).

# License

`regex` is primarily distributed under the terms of both the MIT license and
the Apache License (Version 2.0), with portions covered by various BSD-like
licenses.

See LICENSE-APACHE, and LICENSE-MIT for details.