Crate pest

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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
  • play with grammars and share them on our fiddle
  • leave feedback, ask questions, or greet us on Gitter


The core of pest is the trait Parser, which provides an interface to the parsing functionality.

The accompanying crate pest_derive can automatically generate a Parser from a PEG grammar. Using pest_derive is highly encouraged, but it is also possible to implement Parser manually if required.

.pest files

Grammar definitions reside in custom .pest files located in the crate src directory. Parsers are automatically generated from these files using #[derive(Parser)] and a special #[grammar = "..."] attribute on a dummy struct.

#[grammar = "path/to/my_grammar.pest"] // relative to src
struct MyParser;

The syntax of .pest files is documented in the pest_derive crate.

Inline grammars

Grammars can also be inlined by using the #[grammar_inline = "..."] attribute.


Types for different kinds of parsing failures.
Types and iterators for parser output.
Constructs useful in prefix, postfix, and infix operator parsing with the Pratt parsing method.
Constructs useful in infix operator parsing with the precedence climbing method.


Testing tool that compares produced errors.
Testing tool that compares produced tokens.


Line iterator for Spans, created by Span::lines().
Line iterator for Spans, created by Span::lines_span().
The complete state of a Parser.
A cursor position in a &str which provides useful methods to manually parse that string.
A span over a &str. It is created from either two Positions or from a Pair.


The current atomicity of a ParserState.
The current lookahead status of a ParserState.
Match direction for the stack. Used in PEEK[a..b]/stack_match_peek_slice.
A token generated by a Parser.


A trait with a single method that parses strings.
A trait which parser rules must implement.


Sets the maximum call limit for the parser state to prevent stack overflows or excessive execution times in some grammars. If set, the calls are tracked as a running total over all non-terminal rules that can nest closures (which are passed to transform the parser state).
Creates a ParserState from a &str, supplying it to a closure f.

Type Definitions

Type alias to simplify specifying the return value of chained closures.