Parser generator for JavaScript

  • peggy

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Peggy is a simple parser generator for JavaScript that produces fast parsers with excellent error reporting. You can use it to process complex data or computer languages and build transformers, interpreters, compilers and other tools easily.
Peggy is the successor of PEG.js.

Migrating from PEG.js

Peggy version 1.x.x is API compatible with the most recent PEG.js release. Follow these steps to upgrade:
  1. Uninstall pegjs (and @types/pegjs if you're using the DefinitelyTyped type definitions - we now include type definitions as part of peggy itself).
  2. Replace all require("pegjs") or import ... from "pegjs" with require("peggy") or import ... from "peggy" as appropriate.
  3. Any scripts that use the pegjs cli should now use peggy instead.
  4. That's it!


  • Simple and expressive grammar syntax
  • Integrates both lexical and syntactical analysis
  • Parsers have excellent error reporting out of the box
  • Based on parsing expression
grammar formalism — more powerful than traditional LL(k) and LR(k) parsers or via JavaScript API

Getting Started

Online version is the easiest way to generate a parser. Just enter your grammar, try parsing few inputs, and download generated parser code.


Full documentation is available at


To get started, check out peggy, install the dependencies, and run build:
git clone
cd peggy
npm install
(cd docs && npm install)
(cd web-test && npm install)
npm run build
npm run test:web

Please see the Contribution Guidelines for details on how to contribute code.


Peggy was originally developed by David Majda (@dmajda). It is currently maintained by Joe Hildebrand (@hildjj).
You are welcome to contribute code. Unless your contribution is really trivial you should get in touch with us first — this can prevent wasted effort on both sides.