[][src]Crate logos

Logos logo


Create ridiculously fast Lexers.

Logos has two goals:

  • To make it easy to create a Lexer, so you can focus on more complex problems.
  • To make the generated Lexer faster than anything you'd write by hand.

To achieve those, Logos:


use logos::Logos;

#[derive(Logos, Debug, PartialEq)]
enum Token {
    // Tokens can be literal strings, of any length.


    // Or regular expressions.

    // Logos requires one token variant to handle errors,
    // it can be named anything you wish.
    // We can also use this variant to define whitespace,
    // or any other matches we wish to skip.
    #[regex(r"[ \t\n\f]+", logos::skip)]

fn main() {
    let mut lex = Token::lexer("Create ridiculously fast Lexers.");

    assert_eq!(lex.next(), Some(Token::Text));
    assert_eq!(lex.span(), 0..6);
    assert_eq!(lex.slice(), "Create");

    assert_eq!(lex.next(), Some(Token::Text));
    assert_eq!(lex.span(), 7..19);
    assert_eq!(lex.slice(), "ridiculously");

    assert_eq!(lex.next(), Some(Token::Fast));
    assert_eq!(lex.span(), 20..24);
    assert_eq!(lex.slice(), "fast");

    assert_eq!(lex.next(), Some(Token::Text));
    assert_eq!(lex.slice(), "Lexers");
    assert_eq!(lex.span(), 25..31);

    assert_eq!(lex.next(), Some(Token::Period));
    assert_eq!(lex.span(), 31..32);
    assert_eq!(lex.slice(), ".");

    assert_eq!(lex.next(), None);


Logos can also call arbitrary functions whenever a pattern is matched, which can be used to put data into a variant:

use logos::{Logos, Lexer};

// Note: callbacks can return `Option` or `Result`
fn kilo(lex: &mut Lexer<Token>) -> Option<u64> {
    let slice = lex.slice();
    let n: u64 = slice[..slice.len() - 1].parse().ok()?; // skip 'k'
    Some(n * 1_000)

fn mega(lex: &mut Lexer<Token>) -> Option<u64> {
    let slice = lex.slice();
    let n: u64 = slice[..slice.len() - 1].parse().ok()?; // skip 'm'
    Some(n * 1_000_000)

#[derive(Logos, Debug, PartialEq)]
enum Token {
    #[regex(r"[ \t\n\f]+", logos::skip)]

    // Callbacks can use closure syntax, or refer
    // to a function defined elsewhere.
    // Each pattern can have it's own callback.
    #[regex("[0-9]+", |lex| lex.slice().parse())]
    #[regex("[0-9]+k", kilo)]
    #[regex("[0-9]+m", mega)]

fn main() {
    let mut lex = Token::lexer("5 42k 75m");

    assert_eq!(lex.next(), Some(Token::Number(5)));
    assert_eq!(lex.slice(), "5");

    assert_eq!(lex.next(), Some(Token::Number(42_000)));
    assert_eq!(lex.slice(), "42k");

    assert_eq!(lex.next(), Some(Token::Number(75_000_000)));
    assert_eq!(lex.slice(), "75m");

    assert_eq!(lex.next(), None);

Logos can handle callbacks with following return types:

Return typeProduces
boolToken::Unit or <Token as Logos>::ERROR
Result<(), _>Token::Unit or <Token as Logos>::ERROR
Option<T>Token::Value(T) or <Token as Logos>::ERROR
Result<T, _>Token::Value(T) or <Token as Logos>::ERROR
Skipskips matched input
Filter<T>Token::Value(T) or skips matched input

Callbacks can be also used to do perform more specialized lexing in place where regular expressions are too limiting. For specifics look at Lexer::remainder and Lexer::bump.

Token disambiguation

Rule of thumb is:

  • Longer beats shorter.
  • Specific beats generic.

If any two definitions could match the same input, like fast and [a-zA-Z]+ in the example above, it's the longer and more specific definition of Token::Fast that will be the result.

This is done by comparing numeric priority attached to each definition. Every consecutive, non-repeating single byte adds 2 to the priority, while every range or regex class adds 1. Loops or optional blocks are ignored, while alternations count the shortest alternative:

  • [a-zA-Z]+ has a priority of 1 (lowest possible), because at minimum it can match a single byte to a class.
  • foobar has a priority of 12.
  • (foo|hello)(bar)? has a priority of 6, foo being it's shortest possible match.


pub use crate::source::Source;



This module contains a bunch of traits necessary for processing byte strings.



Macro for creating lookup tables where index matches the token variant as usize.



Lexer is the main struct of the crate that allows you to read through a Source and produce tokens for enums implementing the Logos trait.


Type that can be returned from a callback, informing the Lexer, to skip current token match. See also logos::skip.


Iterator that pairs tokens with their position in the source.



Type that can be returned from a callback, either producing a field for a token, or skipping it.



Trait implemented for an enum representing all tokens. You should never have to implement it manually, use the #[derive(Logos)] attribute on your enum.



Predefined callback that will inform the Lexer to skip a definition.

Type Definitions


Byte range in the source.

Derive Macros