pest

The Elegant Parser

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rust-peg

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Parsing Expression Grammar (PEG) parser generator for Rust

regex

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An implementation of regular expressions for Rust. This implementation uses finite automata and guarantees linear time matching on all inputs.

Quick Overview

Pest is a fast and easy-to-use parser written in Rust. It uses Parsing Expression Grammars (PEGs) to define syntax, allowing for expressive and flexible grammar definitions. Pest aims to simplify the process of writing parsers while maintaining high performance.

Pros

Easy to learn and use, with a simple syntax for defining grammars
Excellent performance due to its Rust implementation
Generates helpful error messages for debugging
Supports both runtime and compile-time grammar checking

Cons

Limited to PEG parsing, which may not be suitable for all parsing needs
Requires understanding of PEG concepts, which can be challenging for beginners
Less flexible than hand-written parsers for complex scenarios
Documentation could be more comprehensive for advanced use cases

Code Examples

Defining a simple grammar:

use pest_derive::Parser;

#[derive(Parser)]
#[grammar = "grammar.pest"]
pub struct MyParser;

Parsing input using the defined grammar:

use pest::Parser;

let pairs = MyParser::parse(Rule::main, "Hello, world!")
    .expect("unsuccessful parse");

for pair in pairs {
    // Process the parsed data
    println!("Rule: {:?}, Span: {:?}", pair.as_rule(), pair.as_span());
}

Extracting data from parsed results:

let inner = pair.into_inner().next().unwrap();
let value = inner.as_str();
println!("Extracted value: {}", value);

Getting Started

Add Pest to your Cargo.toml:

[dependencies]
pest = "2.5"
pest_derive = "2.5"

Create a grammar file (e.g., grammar.pest) and define your rules:

main = { SOI ~ greeting ~ "," ~ name ~ "!" ~ EOI }
greeting = { "Hello" | "Hi" }
name = { ASCII_ALPHA+ }

Use the grammar in your Rust code:

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

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

fn main() {
    let pairs = MyParser::parse(Rule::main, "Hello, World!")
        .expect("unsuccessful parse");
    // Process the parsed data
}

Competitor Comparisons

lalrpop

3,004

LR(1) parser generator for Rust

Pros of LALRPOP

Generates faster parsers for complex grammars
Supports left-recursive rules, allowing for more natural grammar definitions
Provides better error messages and recovery mechanisms

Cons of LALRPOP

Steeper learning curve, especially for those unfamiliar with LR parsing
Less flexible for handling ambiguous grammars
Requires more boilerplate code for parser setup

Code Comparison

LALRPOP grammar example:

Term: i32 = {
    <n:Num> => n,
    "(" <t:Term> ")" => t,
};

Num: i32 = r"[0-9]+" => i32::from_str(<>).unwrap();

Pest grammar example:

term = { num | "(" ~ term ~ ")" }
num = { ASCII_DIGIT+ }

LALRPOP offers more control over the parsing process and type annotations, while Pest provides a more concise and intuitive grammar definition. LALRPOP is better suited for complex, performance-critical parsers, whereas Pest excels in simplicity and ease of use for simpler grammars.

nom

9,335

Rust parser combinator framework

Pros of nom

More flexible and powerful, allowing for complex parsing scenarios
Better performance for certain types of parsing tasks
Extensive ecosystem with many pre-built parsers and combinators

Cons of nom

Steeper learning curve due to its more complex API
Can be more verbose for simple parsing tasks
Requires more manual error handling and reporting

Code Comparison

nom example:

use nom::{
  IResult,
  bytes::complete::tag,
  sequence::tuple
};

fn parser(input: &str) -> IResult<&str, (&str, &str)> {
  tuple((tag("Hello"), tag(" world!")))(input)
}

pest example:

use pest::Parser;

#[derive(Parser)]
#[grammar = "hello.pest"]
struct HelloParser;

// In hello.pest:
// greeting = { "Hello" ~ " world!" }

Both nom and pest are popular parsing libraries for Rust, each with its own strengths. nom offers more flexibility and power for complex parsing scenarios, while pest provides a more user-friendly approach for simpler grammars. The choice between them often depends on the specific requirements of the parsing task at hand.

rust-peg

1,443

Parsing Expression Grammar (PEG) parser generator for Rust

Pros of rust-peg

More mature and established project with a longer history
Supports left-recursive grammars, allowing for more expressive parsing rules
Generates faster parsers for certain grammar types

Cons of rust-peg

Less actively maintained, with fewer recent updates
More complex syntax for defining grammars
Limited documentation and examples compared to Pest

Code Comparison

Pest grammar example:

number = { ASCII_DIGIT+ }
operation = { "+" | "-" | "*" | "/" }
expression = { number ~ (operation ~ number)* }

rust-peg grammar example:

number -> u32
    = n:$(['0'..='9']+) { n.parse().unwrap() }

operation -> char
    = ['+' | '-' | '*' | '/']

expression -> Vec<u32>
    = n:number op:operation e:expression { vec![n, e[0]] }
    / n:number { vec![n] }

Both Pest and rust-peg are parsing expression grammar (PEG) libraries for Rust, offering different approaches to grammar definition and parsing. Pest focuses on simplicity and ease of use, while rust-peg provides more advanced features at the cost of complexity. The choice between them depends on the specific requirements of your parsing project and personal preferences.

regex

3,465

An implementation of regular expressions for Rust. This implementation uses finite automata and guarantees linear time matching on all inputs.

Pros of regex

More efficient for simple pattern matching tasks
Widely recognized syntax, familiar to developers from other languages
Extensive documentation and community support

Cons of regex

Limited expressiveness for complex parsing tasks
Can become difficult to read and maintain for intricate patterns
Less flexibility in handling nested structures or context-sensitive grammars

Code Comparison

regex:

use regex::Regex;

let re = Regex::new(r"^\d{4}-\d{2}-\d{2}$").unwrap();
let is_date = re.is_match("2023-05-15");

pest:

use pest::Parser;

#[derive(Parser)]
#[grammar = "date.pest"]
struct DateParser;

let pairs = DateParser::parse(Rule::date, "2023-05-15").unwrap();

Summary

regex is better suited for simple pattern matching tasks and benefits from widespread familiarity. pest excels in more complex parsing scenarios, offering greater expressiveness and maintainability for intricate grammars. The choice between the two depends on the specific requirements of your project, with regex being more appropriate for straightforward pattern matching and pest for more sophisticated parsing needs.

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README

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

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

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

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

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

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

Precedence climbing
Input handling
Custom errors
Runs on stable Rust

Projects using pest

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

Minimum Supported Rust Version (MSRV)

This library should always compile with default features on Rust 1.61.0.

no_std support

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

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.

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