pest Documentation

pest. The Elegant Parser

pest is a general purpose parser written in Rust with a focus on accessibility,
correctness, and performance. It uses parsing expression grammars
(or PEG) as input, which are similar in spirit to regular expressions, but
which offer the enhanced expressivity needed to parse complex languages.

Getting started Copy Chapter

The recommended way to start parsing with pest is to read the official book.

Other helpful resources:

• API reference on docs.rs
• play with grammars and share them on our fiddle
• find previous common questions answered or ask questions on GitHub Discussions
• leave feedback, ask questions, or greet us on Gitter or Discord

Example Copy Chapter

The following is an example of a grammar for a list of alphanumeric identifiers
where all identifiers don't start with a digit:

alpha = { 'a'..'z' | 'A'..'Z' }
digit = { '0'..'9' }

ident = { !digit ~ (alpha | digit)+ }

ident_list = _{ ident ~ (" " ~ ident)* }
          // ^
          // ident_list rule is silent which means it produces no tokens

Grammars are saved in separate .pest files which are never mixed with procedural
code. This results in an always up-to-date formalization of a language that is
easy to read and maintain.

Meaningful error reporting Copy Chapter

Based on the grammar definition, the parser also includes automatic error
reporting. For the example above, the input "123" will result in:

thread 'main' panicked at ' --> 1:1
  |
1 | 123
  | ^---
  |
  = unexpected digit', src/main.rs:12

while "ab *" will result in:

thread 'main' panicked at ' --> 1:1
  |
1 | ab *
  |    ^---
  |
  = expected ident', src/main.rs:12

These error messages can be obtained from their default Display implementation,
e.g. panic!("{}", parser_result.unwrap_err()) or println!("{}", e).

Pairs API Copy Chapter

The grammar can be used to derive a Parser implementation automatically.
Parsing returns an iterator of nested token pairs:

use pest_derive::Parser;
use pest::Parser;

#[derive(Parser)]
#[grammar = "ident.pest"]
struct IdentParser;

fn main() {
    let pairs = IdentParser::parse(Rule::ident_list, "a1 b2").unwrap_or_else(|e| panic!("{}", e));

    // Because ident_list is silent, the iterator will contain idents
    for pair in pairs {
        // A pair is a combination of the rule which matched and a span of input
        println!("Rule:    {:?}", pair.as_rule());
        println!("Span:    {:?}", pair.as_span());
        println!("Text:    {}", pair.as_str());

        // A pair can be converted to an iterator of the tokens which make it up:
        for inner_pair in pair.into_inner() {
            match inner_pair.as_rule() {
                Rule::alpha => println!("Letter:  {}", inner_pair.as_str()),
                Rule::digit => println!("Digit:   {}", inner_pair.as_str()),
                _ => unreachable!()
            };
        }
    }
}

This produces the following output:

Rule:    ident
Span:    Span { start: 0, end: 2 }
Text:    a1
Letter:  a
Digit:   1
Rule:    ident
Span:    Span { start: 3, end: 5 }
Text:    b2
Letter:  b
Digit:   2

Defining multiple parsers in a single file Copy Section

The current automatic Parser derivation will produce the Rule enum
which would have name conflicts if one tried to define multiple such structs
that automatically derive Parser. One possible way around it is to put each
parser struct in a separate namespace:

mod a {
    #[derive(Parser)]
    #[grammar = "a.pest"]
    pub struct ParserA;
}
mod b {
    #[derive(Parser)]
    #[grammar = "b.pest"]
    pub struct ParserB;
}

Other features Copy Chapter

• Precedence climbing
• Input handling
• Custom errors
• Runs on stable Rust

Projects using pest Copy Chapter

You can find more projects and ecosystem tools in the awesome-pest repo.

• pest_meta (bootstrapped)
• AshPaper
• brain
• cicada
• comrak
• elastic-rs
• graphql-parser
• handlebars-rust
• hexdino
• Huia
• insta
• jql
• json5-rs
• mt940
• Myoxine
• py_literal
• rouler
• RuSh
• rs_pbrt
• stache
• tera
• ui_gen
• ukhasnet-parser
• ZoKrates
• Vector
• AutoCorrect
• yaml-peg
• qubit
• caith (a dice roller crate)
• Melody
• json5-nodes
• prisma
• ws2markdown (a WordStar to Markdown converter)

Minimum Supported Rust Version (MSRV) Copy Chapter

This library should always compile with default features on Rust 1.83.0.

no_std support Copy Chapter

The pest and pest_derive crates can be built without the Rust standard
library and target embedded environments. To do so, you need to disable
their default features. In your Cargo.toml, you can specify it as follows:

[dependencies]
# ...
pest = { version = "2", default-features = false }
pest_derive = { version = "2", default-features = false }

If you want to build these crates in the pest repository's workspace, you can
pass the --no-default-features flag to cargo and specify these crates using
the --package (-p) flag. For example:

$ cargo build --target thumbv7em-none-eabihf --no-default-features -p pest
$ cargo bootstrap
$ cargo build --target thumbv7em-none-eabihf --no-default-features -p pest_derive

Special thanks Copy Chapter

A special round of applause goes to prof. Marius Minea for his guidance and all
pest contributors, some of which being none other than my friends.