170 lines
5.5 KiB
Markdown
170 lines
5.5 KiB
Markdown
<p align="center">
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<img src="https://raw.github.com/pest-parser/pest/master/pest-logo.svg?sanitize=true" width="80%"/>
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</p>
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# pest. The Elegant Parser
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[](https://gitter.im/dragostis/pest?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
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[](https://pest-parser.github.io/book)
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[](https://docs.rs/pest)
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[](https://travis-ci.org/pest-parser/pest)
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[](https://codecov.io/gh/pest-parser/pest)
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[](https://app.fuzzit.dev/orgs/pest-parser/dashboard)
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[](https://crates.io/crates/pest)
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[](https://crates.io/crates/pest)
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pest is a general purpose parser written in Rust with a focus on accessibility,
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correctness, and performance. It uses parsing expression grammars
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(or [PEG]) as input, which are similar in spirit to regular expressions, but
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which offer the enhanced expressivity needed to parse complex languages.
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[PEG]: https://en.wikipedia.org/wiki/Parsing_expression_grammar
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## Getting started
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The recommended way to start parsing with pest is to read the official [book].
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Other helpful resources:
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* API reference on [docs.rs]
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* play with grammars and share them on our [fiddle]
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* leave feedback, ask questions, or greet us on [Gitter]
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[book]: https://pest-parser.github.io/book
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[docs.rs]: https://docs.rs/pest
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[fiddle]: https://pest-parser.github.io/#editor
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[Gitter]: https://gitter.im/dragostis/pest
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## Example
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The following is an example of a grammar for a list of alpha-numeric identifiers
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where the first identifier does not start with a digit:
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```rust
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alpha = { 'a'..'z' | 'A'..'Z' }
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digit = { '0'..'9' }
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ident = { (alpha | digit)+ }
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ident_list = _{ !digit ~ ident ~ (" " ~ ident)+ }
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// ^
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// ident_list rule is silent which means it produces no tokens
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```
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Grammars are saved in separate .pest files which are never mixed with procedural
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code. This results in an always up-to-date formalization of a language that is
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easy to read and maintain.
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## Meaningful error reporting
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Based on the grammar definition, the parser also includes automatic error
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reporting. For the example above, the input `"123"` will result in:
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```
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thread 'main' panicked at ' --> 1:1
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1 | 123
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| ^---
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= unexpected digit', src/main.rs:12
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```
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while `"ab *"` will result in:
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```
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thread 'main' panicked at ' --> 1:1
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1 | ab *
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| ^---
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= expected ident', src/main.rs:12
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```
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## Pairs API
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The grammar can be used to derive a `Parser` implementation automatically.
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Parsing returns an iterator of nested token pairs:
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```rust
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extern crate pest;
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#[macro_use]
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extern crate pest_derive;
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use pest::Parser;
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#[derive(Parser)]
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#[grammar = "ident.pest"]
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struct IdentParser;
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fn main() {
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let pairs = IdentParser::parse(Rule::ident_list, "a1 b2").unwrap_or_else(|e| panic!("{}", e));
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// Because ident_list is silent, the iterator will contain idents
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for pair in pairs {
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// A pair is a combination of the rule which matched and a span of input
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println!("Rule: {:?}", pair.as_rule());
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println!("Span: {:?}", pair.as_span());
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println!("Text: {}", pair.as_str());
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// A pair can be converted to an iterator of the tokens which make it up:
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for inner_pair in pair.into_inner() {
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match inner_pair.as_rule() {
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Rule::alpha => println!("Letter: {}", inner_pair.as_str()),
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Rule::digit => println!("Digit: {}", inner_pair.as_str()),
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_ => unreachable!()
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};
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}
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}
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}
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```
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This produces the following output:
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```
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Rule: ident
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Span: Span { start: 0, end: 2 }
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Text: a1
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Letter: a
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Digit: 1
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Rule: ident
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Span: Span { start: 3, end: 5 }
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Text: b2
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Letter: b
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Digit: 2
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```
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## Other features
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* Precedence climbing
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* Input handling
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* Custom errors
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* Runs on stable Rust
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## Projects using pest
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* [pest_meta](https://github.com/pest-parser/pest/blob/master/meta/src/grammar.pest) (bootstrapped)
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* [AshPaper](https://github.com/shnewto/ashpaper)
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* [brain](https://github.com/brain-lang/brain)
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* [Chelone](https://github.com/Aaronepower/chelone)
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* [comrak](https://github.com/kivikakk/comrak)
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* [elastic-rs](https://github.com/cch123/elastic-rs)
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* [graphql-parser](https://github.com/Keats/graphql-parser)
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* [handlebars-rust](https://github.com/sunng87/handlebars-rust)
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* [hexdino](https://github.com/Luz/hexdino)
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* [Huia](https://gitlab.com/jimsy/huia/)
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* [jql](https://github.com/yamafaktory/jql)
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* [json5-rs](https://github.com/callum-oakley/json5-rs)
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* [mt940](https://github.com/svenstaro/mt940-rs)
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* [py_literal](https://github.com/jturner314/py_literal)
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* [rouler](https://github.com/jarcane/rouler)
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* [RuSh](https://github.com/lwandrebeck/RuSh)
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* [rs_pbrt](https://github.com/wahn/rs_pbrt)
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* [stache](https://github.com/dgraham/stache)
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* [tera](https://github.com/Keats/tera)
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* [ui_gen](https://github.com/emoon/ui_gen)
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* [ukhasnet-parser](https://github.com/adamgreig/ukhasnet-parser)
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* [ZoKrates](https://github.com/ZoKrates/ZoKrates)
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## Special thanks
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A special round of applause goes to prof. Marius Minea for his guidance and all
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pest contributors, some of which being none other than my friends.
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