Crate lazy_regex

source ·
Expand description

With lazy-regex macros, regular expressions

  • are checked at compile time, with clear error messages
  • are wrapped in once_cell lazy static initializers so that they’re compiled only once
  • can hold flags as suffix: let case_insensitive_regex = regex!("ab*"i);
  • are defined in a less verbose way

The regex! macro returns references to normal instances of regex::Regex or regex::bytes::Regex so all the usual features are available.

Other macros are specialized for testing a match, replacing with concise closures, or capturing groups as substrings in some common situations:

All of them support the B flag for the regex::bytes::Regex variant.

Some structs of the regex crate are reexported to ease dependency managment.

Build Regexes

use lazy_regex::regex;

// build a simple regex
let r = regex!("sa+$");
assert_eq!(r.is_match("Saa"), false);

// build a regex with flag(s)
let r = regex!("sa+$"i);
assert_eq!(r.is_match("Saa"), true);

// you can use a raw literal
let r = regex!(r#"^"+$"#);
assert_eq!(r.is_match("\"\""), true);

// or a raw literal with flag(s)
let r = regex!(r#"^\s*("[a-t]*"\s*)+$"#i);
assert_eq!(r.is_match(r#" "Aristote" "Platon" "#), true);

// build a regex that operates on &[u8]
let r = regex!("(byte)?string$"B);
assert_eq!(r.is_match(b"bytestring"), true);

// there's no problem using the multiline definition syntax
let r = regex!(r#"(?x)
assert_eq!(r.find("This is lazy_regex-2.2!").unwrap().as_str(), "lazy_regex-2.2");
// (look at the regex_captures! macro to easily extract the groups)
// this line doesn't compile because the regex is invalid:
let r = regex!("(unclosed");

Supported regex flags: i, m, s, x, U.

See regex::RegexBuilder.

Test a match

use lazy_regex::regex_is_match;

let b = regex_is_match!("[ab]+", "car");
assert_eq!(b, true);

doc: regex_is_match!

Extract a value

use lazy_regex::regex_find;

let f_word = regex_find!(r#"\bf\w+\b"#, "The fox jumps.");
assert_eq!(f_word, Some("fox"));
let f_word = regex_find!(r#"\bf\w+\b"#B, b"The forest is silent.");
assert_eq!(f_word, Some(b"forest" as &[u8]));

doc: regex_find!


use lazy_regex::regex_captures;

let (_, letter) = regex_captures!("([a-z])[0-9]+"i, "form A42").unwrap();
assert_eq!(letter, "A");

let (whole, name, version) = regex_captures!(
    r#"(\w+)-([0-9.]+)"#, // a literal regex
    "This is lazy_regex-2.0!", // any expression
assert_eq!(whole, "lazy_regex-2.0");
assert_eq!(name, "lazy_regex");
assert_eq!(version, "2.0");

There’s no limit to the size of the tuple. It’s checked at compile time to ensure you have the right number of capturing groups.

You receive "" for optional groups with no value.

doc: regex_captures!

Replace with captured groups

The regex_replace! and regex_replace_all! macros bring once compilation and compilation time checks to the replace and replace_all functions.

Replacing with a closure

use lazy_regex::regex_replace_all;

let text = "Foo8 fuu3";
let text = regex_replace_all!(
    |_, name, digit| format!("F<{}>{}", name, digit),
assert_eq!(text, "F<oo>8 F<uu>3");

The number of arguments given to the closure is checked at compilation time to match the number of groups in the regular expression.

If it doesn’t match you get, at compilation time, a clear error message.

Replacing with another kind of Replacer

use lazy_regex::regex_replace_all;
let text = "UwU";
let output = regex_replace_all!("U", text, "O");
assert_eq!(&output, "OwO");

Shared lazy static

When a regular expression is used in several functions, you sometimes don’t want to repeat it but have a shared static instance.

The regex! macro, while being backed by a lazy static regex, returns a reference.

If you want to have a shared lazy static regex, use the lazy_regex! macro:

use lazy_regex::*;

pub static GLOBAL_REX: Lazy<Regex> = lazy_regex!("^ab+$"i);

Like for the other macros, the regex is static, checked at compile time, and lazily built at first use.

doc: lazy_regex!


  • Return an instance of once_cell::sync::Lazy<regex::Regex> or once_cell::sync::Lazy<regex::bytes::Regex> that you can use in a public static declaration.
  • Return a lazy static Regex checked at compilation time and built at first use.
  • Extract captured groups as a tuple of &str.
  • Extract the leftmost match of the regex in the second argument, as a &str, or a &[u8] if the B flag is set.
  • Test whether an expression matches a lazy static regular expression (the regex is checked at compile time)
  • Replaces the leftmost match in the second argument with the value returned by the closure given as third argument.
  • Replaces all non-overlapping matches in the second argument with the value returned by the closure given as third argument.


  • A compiled regular expression for matching arbitrary bytes.
  • A configurable builder for a regular expression.
  • Captures represents a group of captured strings for a single match.
  • A value which is initialized on the first access.
  • A compiled regular expression for matching Unicode strings.
  • A configurable builder for a regular expression.