Parsing Expression Grammars in Rust

Documentation | Release Notes

rust-peg is a simple yet flexible parser generator that makes it easy to write robust parsers. 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.

Features

• 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

Example

Parse a comma-separated list of numbers surrounded by brackets into a Vec<u32>:

peg::parser!{
  grammar list_parser() for str {
    rule number() -> u32
      = n:$(['0'..='9']+) {? n.parse().or(Err("u32")) }

    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	parameterized 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

See also

• pegviz is a UI for visualizing rust-peg's trace output to debug parsers.
• There exist several crates to format diagnostic messages on source code snippets in the terminal, including chic, annotate-snippets, codespan-reporting, and codemap-diagnostic.

Development

The rust-peg grammar is written in rust-peg: peg-macros/grammar.rustpeg. To avoid the circular dependency, a precompiled grammar is checked in as peg-macros/grammar.rs. To regenerate this, run cargo xtask bootstrap.

There is a large test suite which uses trybuild to test both functionality (tests/run-pass) and error messages for incorrect grammars (tests/compile-fail). Because rustc error messages change, the compile-fail tests are only run on the minimum supported Rust version to avoid spurious failures.

Use cargo test to run the entire suite, or cargo test -- trybuild trybuild=lifetimes.rs to test just the indicated file. Add --features trace to trace these tests.