Regex

Introduction#

Rust’s standard library does not contain any regex parser/matcher, but the regex crate (which is in the rust-lang-nursery and hence semi-official) provides a regex parser. This section of the documentation will provide an overview of how to use the regex crate in common situations, along with installation instructions and any other useful remarks which are needed while using the crate.

Simple match and search

Regular expression support for tust is provided by the regex crate, add it to your Cargo.toml:

[dependencies]
regex = "0.1"

The main interface of the regex crate is regex::Regex:

extern crate regex;
use regex::Regex;

fn main() {
    //"r" stands for "raw" strings, you probably
    // need them because rustc checks escape sequences,
    // although you can always use "\\" withour "r"
    let num_regex = Regex::new(r"\d+").unwrap();
    // is_match checks if string matches the pattern
    assert!(num_regex.is_match("some string with number 1"));

    let example_string = "some 123 numbers";
    // Regex::find searches for pattern and returns Option<(usize,usize)>,
    // which is either indexes of first and last bytes of match
    // or "None" if nothing matched
    match num_regex.find(example_string) {
        // Get the match slice from string, prints "123"
        Some(x) => println!("{}", &example_string[x.0 .. x.1]),
        None    => unreachable!()
    }
}

Capture groups

extern crate regex;
use regex::Regex;

fn main() {
    let rg = Regex::new(r"was (\d+)").unwrap();
    // Regex::captures returns Option<Captures>,
    // first element is the full match and others
    // are capture groups
    match rg.captures("The year was 2016") {
        // Access captures groups via Captures::at
        // Prints Some("2016")
        Some(x) => println!("{:?}", x.at(1)),
        None    => unreachable!()
    }

    // Regex::captures also supports named capture groups
    let rg_w_named = Regex::new(r"was (?P<year>\d+)").unwrap();
    match rg_w_named.captures("The year was 2016") {
        // Named capures groups are accessed via Captures::name
        // Prints Some("2016")
        Some(x) => println!("{:?}", x.name("year")),
        None    => unreachable!()
    }

}

Replacing

extern crate regex;
use regex::Regex;

fn main() {
    let rg = Regex::new(r"(\d+)").unwrap();

    // Regex::replace replaces first match 
    // from it's first argument with the second argument
    // => Some string with numbers (not really)
    rg.replace("Some string with numbers 123", "(not really)");

    // Capture groups can be accesed via $number
    // => Some string with numbers (which are 123)
    rg.replace("Some string with numbers 123", "(which are $1)");

    let rg = Regex::new(r"(?P<num>\d+)").unwrap();
    
    // Named capture groups can be accesed via $name
    // => Some string with numbers (which are 123)
    rg.replace("Some string with numbers 123", "(which are $num)");

    // Regex::replace_all replaces all the matches, not only the first
    // => Some string with numbers (not really) (not really)
    rg.replace_all("Some string with numbers 123 321", "(not really)");
}