Simple, fast, powerful parser toolkit for JavaScript.


nearley ↗️ JS.ORG npm version

nearley is a simple, fast and powerful parsing toolkit. It consists of:
1. [A powerful, modular DSL for describing
2. [An efficient, lightweight Earley
3. [Loads of tools, editor plug-ins, and other

nearley is a streaming parser with support for catching errors
gracefully and providing _all_ parsings for ambiguous grammars. It is
compatible with a variety of lexers (we recommend
moo). It comes with tools for creatingtests,
railroad diagrams and fuzzers from your grammars, and has support for a
variety of editors and platforms. It works in both node and the browser.

Unlike most other parser generators, nearley can handle any grammar you can
define in BNF (and more!). In particular, while most existing JS parsers such
as PEGjs and Jison choke on certain grammars (e.g. [left recursive
ones](, nearley handles them
easily and efficiently by using the [Earley parsing

nearley is used by a wide variety of projects:

- [artificial
- [computational
  classes at universities;
- [compilers for real-world programming
- and nearley itself! The nearley compiler is bootstrapped.

nearley is an npm [staff


Please visit our website to get started! You will find a
tutorial, detailed reference documents, and links to several real-world
examples to get inspired.


Please read this documentbefore working on
nearley. If you are interested in contributing but unsure where to start, take
a look at the issues labeled "up for grabs" on the issue tracker, or message a
maintainer (@kach or @tjvr on Github).

nearley is MIT licensed.

A big thanks to Nathan Dinsmore for teaching me how to Earley, Aria Stewart for
helping structure nearley into a mature module, and Robin Windels for
bootstrapping the grammar. Additionally, Jacob Edelman wrote an experimental
JavaScript parser with nearley and contributed ideas for EBNF support. Joshua
T. Corbin refactored the compiler to be much, much prettier. Bojidar Marinov
implemented postprocessors-in-other-languages. Shachar Itzhaky fixed a subtle
bug with nullables.

Citing nearley

If you are citing nearley in academic work, please use the following BibTeX

  1. ```bibtex
  2. @misc{nearley,
  3.     author = "Kartik Chandra and Tim Radvan",
  4.     title  = "{nearley}: a parsing toolkit for {JavaScript}",
  5.     year   = {2014},
  6.     doi    = {10.5281/zenodo.3897993},
  7.     url    = {}
  8. }
  9. ```