pest 0.4.1

Elegant, efficient grammars

# pest. Elegant, efficient grammars

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


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

Add the following to Cargo.toml:

pest = "0.4"

and in your Rust or

extern crate pest;

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



pest uses PEG syntax to enable expressive grammar creation.

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

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



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:


  • 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


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



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 or Try different values for x until it doesn't error anymore. You can start with a value of 200 and go up or down.