A simple Parsing Expression Grammar (PEG) parser generator.

parsing, parser, grammar, peg, parser-generator, parsing-expression-grammars, rust


Parsing Expression Grammars in Rust

Documentation | Release Notes

rust-peg is a simple yet flexible parser generator based on the Parsing Expression Grammar formalism. It provides a Rust macro that builds a recursive descent parser from a concise definition of the grammar.


  • Parse input from &str, &[u8], &[T] or custom types implementing traits
  • Customizable reporting of parse errors
  • Rules can accept arguments to create reusable rule templates
  • Precedence climbing for prefix/postfix/infix expressions
  • Helpful rustc error messages for errors in the grammar definition or the Rust code embedded within it
  • Rule-level tracing to debug grammars


  grammar list_parser() for str {
    rule number() -> u32
      = n:$(['0'..='9']+) { n.parse().unwrap() }
    pub rule list() -> Vec<u32>
      = "[" l:number() ** "," "]" { l }

pub fn main() {
    assert_eq!(list_parser::list("[1,1,2,3,5,8]"), Ok(vec![1, 1, 2, 3, 5, 8]));

See the tests for more examples
Grammar rule syntax reference in rustdoc

Comparison with similar parser generators

crate parser type action code integration input type precedence climbing parameterizd rules streaming input
peg PEG in grammar proc macro (block) &str, &[T], custom Yes Yes No
pest PEG external proc macro (file) &str Yes No No
nom combinators in source library &[u8], custom No Yes Yes
lalrpop LR(1) in grammar build script &str No Yes No