#lexer #lexical #analyser #generator

nightly rustlex

Lexical analysers generator for Rust, written in Rust

2 unstable releases

Uses old Rust 2015

0.4.0 Mar 23, 2018
0.3.3 May 23, 2016
0.3.2 Aug 31, 2015
0.3.0 Jun 15, 2015
0.2.2 Mar 26, 2015

#1 in #analyser

41 downloads per month
Used in rumblebars

MIT license


RustLex: lexical analysers generator for Rust

Warning: RustLex 0.4.0 and higher works only for the nightly channel of Rust.

If you want to use RustLex with the nightly channel of Rust instead, use version 0.3.4 instead. Note though that syntex is no longer actively maintained and, while this version is provided for compatibility, its use is discouraged.

Build Status

RustLex is a lexical analysers generator, i.e. a program that generate lexical analysers for use in compiler from a description of the language using regular expressions. It is similar to the well-known Lex but is written in Rust and outputs Rust code as the analyser. It differs from Lex by using Rust's new syntax extensions system as the interface for defining lexical analysers. The description of the analyser thus can be directly embedded into a Rust source file, and the generator code will be called by rustc at the macro-expansion phase.

RustLex availability and Rust compatibility

RustLex using syntax extensions, it has to deal with rustc libsyntax. libsyntax is more or less the compiler guts, and it has been explicitely excluded from the Rust 1.0 roadmap. Bottom line is, RustLex inline syntax generation is not usable with Rust stable.

Using Rust nightly, just indicate a dependency to rustlex in your Cargo.toml and add the following lines at the top of your crate:

#[allow(plugin_as_library)] extern crate rustlex;

This will make rustc load the RustLex plugin which contains everything that is needed to generate the code.

Defining a lexer

You can then invoke the rustlex! macro anywhere. The macro will expand into a single lexer structure and implementation describing the lexical analyser.

The rustlex! macro takes as argument the name of the structure and the description of the lexical analyser. The description consists of two parts:

  • definitions of regular expressions
  • definitions of rules

A minimum lexer will look like:

rustlex! SimpleLexer {
    // expression definitions
    let A = 'a';

    // then rules
    A => |lexer:&mut SimpleLexer<R>| Some(TokA ( lexer.yystr() ))

More complex regular expression definition examples can be found in a more complex example. It is worth noting that:

  • characters (standalone or in character class) and strings have to be quoted as in Rust or C (simple quote for character, double quote for strings)
  • an expression definition can be "called" by its identifier in another expression

Using a lexer

The lexer will read characters from a standard Rust Reader and implement a Token iterator.

let inp = BufReader::new("aa".as_bytes());
let mut lexer = SimpleLexer::new(inp);
for tok in lexer {

Advanced lexer features

Token enumeration

By default, rustlex! assumes the existence of a token enumeration named Token in the same module, but this name can be overriden when needed as is the case for the OtherLexer from this example.


As in flex, conditions can be defined to have the lexer switch from one mode to another.

Check out this example.

Arbitrary lexer properties and methods

It is possible to add specific fields to the lexer structure using the property keyword as shown there.

Lexer methods (to be called from action code) can also be defined by a normal impl section.