logos 0.11.0-rc2

Create ridiculously fast Lexers

Create ridiculously fast Lexers.

Logos works by:

  • Resolving all logical branching of token definitions into a state machine.
  • Optimizing complex patterns into Lookup Tables.
  • Avoiding backtracking, unwinding loops, and batching reads to minimize bounds checking.

In practice it means that for most grammars the lexing performance is virtually unaffected by the number of tokens defined in the grammar. Or, in other words, it is really fast.


use logos::Logos;

#[derive(Logos, Debug, PartialEq)]
enum Token {
// Logos requires one token variant to handle errors,
// it can be named anything you wish.

// Tokens can be literal strings, of any length.
#[token = "fast"]

#[token = "."]

// Or regular expressions.
#[regex = "[a-zA-Z]+"]

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.span(), 25..31);
assert_eq!(lex.slice(), "Lexers");

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, Extras};

// 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 {

// 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 type Produces
() Token::Unit
bool Token::Unit or <Token as Logos>::ERROR
Result<(), _> Token::Unit or <Token as Logos>::ERROR
T Token::Value(T)
Option<T> Token::Value(T) or <Token as Logos>::ERROR
Result<T, _> Token::Value(T) or <Token as Logos>::ERROR

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.