pest. Elegant, efficient grammars

[] (https://travis-ci.org/dragostis/pest) [![Coverage Status] (https://coveralls.io/repos/github/dragostis/pest/badge.svg?branch=master)] (https://coveralls.io/github/dragostis/pest?branch=master) [] (https://crates.io/crates/pest)

pest is a PEG parser generator with simplicity and speed in mind.

Usage

You will need Cargo and Rust, follow the installation instructions to get them.

Add the following to Cargo.toml:

toml pest = "0.4"

and in your Rust lib.rs or main.rs:

```rust

[macro_use]

extern crate pest; ```

You can run the calculator example by using cargo run --example calculator.

Documentation

Elegant

pest uses PEG syntax to enable expressive grammar creation.

```rust impl_rdp! { grammar! { expression = _{ paren ~ expression? } paren = { ["("] ~ expression? ~ [")"] } } }

let mut parser = Rdp::new(StringInput::new("(()((())())()")));

assert!(parser.expression()); assert!(parser.end()); ```

Fast

pest generates a fast parser at compile time through the use of macros, without forcing you to use nightly. Yes, it works on stable (1.9.0+).

| Parser generator | Time to parse 272.5 KB of JSON | pest speedup | |------------------|--------------------------------|-------------:| | ANTRL 4 | 153,000 μs (+/- 15,000) | 48.12x | | Bison + Flex | 8,761.9 μs (+/- 697) | 2.76x | | pest | 3,178.9 μs (+/- 40.6) | 1.00x |

Tests have been run on an Intel Q8200, 4GB DDR2, Linux 4.6.2 as follows:

• ANTLR 4 [JSON grammar] (https://github.com/antlr/grammars-v4/blob/master/json/JSON.g4) measured with JMH SingleShotTime
• Bison + Flex JSON grammar with string capturing and printing removed
• pest JSON grammar measured with cargo bench

Features

• simple PEG grammar
• smart error reporting with atomic and silent rules
• precedence climbing rule
• no generation step (one-step compilation)
• fast macro-generated recursive descent parser
• runs on stable Rust
• elegant functional-style Token processing

Comparison

Short comparison with other parsing tools. Please take into consideration that pest is the youngest among them, but is continuously improving.

| | nom | LALRPOP | pest | |----------------------|----------------------|-----------------|--------------------| | type | combinator | generator | generator (macros) | | goals | speed, extensibility | usability | simplicity, speed | | grammar | specialized | LR(1) / LALR(1) | PEG | | steps | 1 | 2 | 1 | | code | separate / mixed | mixed | separate | | extensibility | great | great | little | | great for | binary formats | any text | languages | | error reporting | yes | yes | yes | | GitHub additions | ~10K | ~500K | ~6K |

Roadmap

• Packrat parsing
• parsing parallelization

FAQ

I get recursion errors, what should I do?

You will need to increase the recursion limit of the crate by adding #![recursion_limit = "x"] in your lib.rs or main.rs. Try different values for x until it doesn't error anymore. You can start with a value of 200 and go up or down.