[][src]Crate peg

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.


  • 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


The peg::parser!{} macro encloses a grammar NAME() for INPUT_TYPE { ... } definition containing a set of rules which match components of your language.

Rules are defined with rule NAME(PARAMETERS) -> RETURN_TYPE = PEG_EXPR. The body of the rule, following the =, is a PEG expression, definining how the input is matched to produce a value.

PEG expressions are evaluated at a particular position of the input. When an expression matches, it advances the position and optionally returns a value. The expression syntax and behavior is documented below.

The macro expands to a Rust mod containing a function for each rule marked pub in the grammar. To parse an input sequence, call one of these functions. The call returns a Result<T, ParseError> carrying either the successfully parsed value returned by the rule, or a ParseError containing the failure position and the set of tokens expected there.


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

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

    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]));

Expression Reference


  • "keyword" - Literal: match a literal string.
  • ['0'..='9'] - Pattern: match a single element that matches a Rust match-style pattern. (details)
  • some_rule() - Rule: match a rule defined elsewhere in the grammar and return its result. Arguments in the parentheses are Rust expressions.
  • _ or __ or ___ - Rule (underscore): As a special case, rule names consisting of underscores are invoked without parentheses. These are conventionally used to match whitespace between tokens.
  • (e) - Parentheses: wrap an expression into a group to override normal precedence. Returns the same value as the inner expression. (Use an Action block to set the return value for a sequence).


  • e1 e2 e3 - Sequence: match expressions in sequence (e1 followed by e2 followed by e3), ignoring the return values.
  • a:e1 e2 b:e3 c:e4 { rust } - Action: match e1, e2, e3, e4 in sequence, like above. If they match successfully, run the Rust code in the block and return its return value. The variable names before the colons in the sequence are bound to the results of the corresponding expressions. It is important that the Rust code embedded in the grammar is deterministic and free of side effects, as it may be called multiple times.
  • a:e1 b:e2 c:e3 {? rust } - Conditional action: Like above, but the Rust block returns a Result<T, &str> instead of a value directly. On Ok(v), it matches successfully and returns v. On Err(e), the match of the entire expression fails and it tries alternatives or reports a parse error with the &str e.
  • e1 / e2 / e3 - Ordered choice: try to match e1. If the match succeeds, return its result, otherwise try e2, and so on.


  • expression? - Optional: match zero or one repetitions of expression. Returns an Option.
  • expression* - Repeat: match zero or more repetitions of expression and return the results as a Vec.
  • expression+ - One-or-more: match one or more repetitions of expression and return the results as a Vec.
  • expression*<n,m> - Range repeat: match between n and m repetitions of expression return the results as a Vec. (details)
  • expression ** delim - Delimited repeat: match zero or more repetitions of expression delimited with delim and return the results as a Vec.


  • $(e) - Slice: match the expression e, and return the slice of the input corresponding to the match.
  • &e - Positive lookahead: Match only if e matches at this position, without consuming any characters.
  • !e - Negative lookahead: Match only if e does not match at this position, without consuming any characters.
  • position!() - return a usize representing the current offset into the input without consuming anything.
  • quiet!{ e } - match the expression e, but don't report literals within it as "expected" in error messages.
  • expected!("something") - fail to match, and report the specified string as expected at the current location.
  • precedence!{ ... } - Parse infix, prefix, or postfix expressions by precedence climbing. (details)

Expression details

Pattern expressions

The [pat] syntax expands into a Rust match pattern against the next character (or element) of the input.

To match sets of characters, use Rust's ..= inclusive range pattern syntax and | to match multiple patterns. For example ['a'..='z' | 'A'..='Z'] matches an upper or lower case ASCII alphabet character.

If your input type is a slice of an enum type, a pattern could match an enum variant like [Token::Operator('+')].

[_] matches any single element. As this always matches except at end-of-file, combining it with negative lookahead as ![_] is the idiom for matching EOF in PEG.

Repeat ranges

The repeat operators * and ** can be followed by an optional range specification of the form <n> (exact), <n,> (min), <,m> (max) or <n,m> (range), where n and m are either integers, or a Rust usize expression enclosed in {}.

Precedence climbing

precedence!{ rules... } provides a convenient way to parse infix, prefix, and postfix operators using the precedence climbing algorithm.

pub rule arithmetic() -> i64 = precedence!{
  x:(@) "+" y:@ { x + y }
  x:(@) "-" y:@ { x - y }
  x:(@) "*" y:@ { x * y }
  x:(@) "/" y:@ { x / y }
  x:@ "^" y:(@) { x.pow(y as u32) }
  n:number() { n }
  "(" e:arithmetic() ")" { e }

Each -- introduces a new precedence level that binds more tightly than previous precedence levels. The levels consist of one or more operator rules each followed by a Rust action expression.

The (@) and @ are the operands, and the parentheses indicate associativity. An operator rule beginning and ending with @ is an infix expression. Prefix and postfix rules have one @ at the beginning or end, and atoms do not include @.

Input types

The first line of the grammar declares an input type. This is normally str, but rust-peg handles input types through a series of traits. The library comes with implementations for str, [u8], and [T]. Define the traits below to use your own types as input to peg grammars:

  • Parse is the base trait required for all inputs. The others are only required to use the corresponding expressions.
  • ParseElem implements the [_] pattern operator, with a method returning the next item of the input to match.
  • ParseLiteral implements matching against a "string" literal.
  • ParseSlice implements the $() operator, returning a slice from a span of indexes.

As a more complex example, the body of the peg::parser!{} macro itself is parsed with peg, using a definition of these traits for a type that wraps Rust's TokenTree.

Error reporting

When a match fails, position information is automatically recorded to report a set of "expected" tokens that would have allowed the parser to advance further.

Some rules should never appear in error messages, and can be suppressed with quiet!{e}:

rule whitespace() = quiet!{[' ' | '\n' | '\t']+}

If you want the "expected" set to contain a more helpful string instead of character sets, you can use quiet!{} and expected!() together:

rule identifier()
  = quiet!{[ 'a'..='z' | 'A'..='Z']['a'..='z' | 'A'..='Z' | '0'..='9' ]+}
  / expected!("identifier")


mod ast {
   pub struct Expr;

peg::parser!{grammar doc() for str {
    use self::ast::Expr;

The grammar may begin with a series of use declarations, just like in Rust, which are included in the generated module. Unlike normal mod {} blocks, use super::* is inserted by default, so you don't have to deal with this most of the time.

Rustdoc comments

rustdoc comments with /// before a grammar or pub rule are propagated to the resulting module or function:

/// Parse an array expression.
pub rule array() -> Vec<i32> = "[...]" { vec![] }

As with all procedural macros, non-doc comments are ignored by the lexer and can be used like in any other Rust code.


A rule without parameters can be prefixed with #[cache] if it is likely to be checked repeatedly in the same position. This memoizes the rule result as a function of input position, in the style of a packrat parser.

However, idiomatic code avoids structures that parse the same input repeatedly, so the use of #[cache] is often not a performance win. Simple rules may also be faster to re-match than the additional cost of the hash table lookup and insert.

For example, a complex rule called expr might benefit from caching if used like expr "x" / expr "y" / expr "z", but this could be rewritten to expr ("x" / "y" / "z") which would be even faster.

The precedence!{} syntax is another way to avoid repeatedly matching an expression rule.


If you pass the peg/trace feature to Cargo when building your project, a trace of the rules attempted and matched will be printed to stdout when parsing. For example,

$ cargo run --features peg/trace
[PEG_TRACE] Matched rule type at 8:5
[PEG_TRACE] Attempting to match rule ident at 8:12
[PEG_TRACE] Attempting to match rule letter at 8:12
[PEG_TRACE] Failed to match rule letter at 8:12



Parse error reporting


Utilities for str input



The main macro for creating a PEG parser.



The result type used internally in the parser.



A type that can be used as input to a parser.


A parser input type supporting the [...] syntax.


A parser input type supporting the "literal" syntax.


A parser input type supporting the $() syntax.