---

name: peggy

prettier-ignore

description: Use when writing PEG grammars with Peggy (formerly PEG.js) - parsing expression grammars, parser generation, and syntax definition

Peggy Parser Generator

Quick Start

// grammar.peggy
Expression = head:Term tail:(_ ("+" / "-") _ Term)* {
  return tail.reduce((result, [, op, , term]) => {
    return op === "+" ? result + term : result - term;
  }, head);
}

Term = Integer / "(" _ expr:Expression _ ")" { return expr; }

Integer = digits:[0-9]+ { return parseInt(digits.join(""), 10); }

_ = [ \t\n\r]*

import * as peggy from 'peggy';
import fs from 'fs';

const grammar = fs.readFileSync('grammar.peggy', 'utf-8');
const parser = peggy.generate(grammar);
const result = parser.parse('2 + 3 * 4'); // 14

Core Syntax

Pattern	Meaning
"literal"	Match exact string
[a-z]	Character class
rule1 / rule2	Ordered choice (try rule1 first)
rule*	Zero or more
rule+	One or more
rule?	Optional
&rule	Positive lookahead
!rule	Negative lookahead
label:rule	Capture as variable
{ code }	Action (return value)

Rule Definitions

// Named rule
Identifier = [a-zA-Z_][a-zA-Z0-9_]* { return text(); }

// With semantic action
Number = digits:[0-9]+ { return parseInt(digits.join(""), 10); }

// Choice with labels
BinaryOp = left:Term op:("+" / "-") right:Term {
  return { type: "binary", op, left, right };
}

Built-in Functions

• text() - Matched text as string
• location() - Start/end positions
• expected(desc) - Throw expected error
• error(message) - Throw custom error

CLI Usage

npx peggy grammar.peggy -o parser.js
npx peggy --format es grammar.peggy  # ES module output

Tips

• Order choices from most to least specific
• Use !. for "not end of input"
• Whitespace is significant; define _ rule for optional whitespace
• Use @ prefix for pluck: @value:Rule returns just the labeled value