Simple, fast, powerful parser toolkit for JavaScript.
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Simple parsing for node.js.

What is nearley?

nearley uses the Earley parsing algorithm augmented with Joop Leo's optimizations to parse complex data structures easily. nearley is über-fast and really powerful. It can parse literally anything you throw at it.

nearley is used by artificial intelligence and computational linguistics classes at universities, as well as file format parsers, markup languages and complete programming languages. It's an npm staff pick.

Why do I care?

nearley can parse what other JS parse engines cannot, because it uses a different algorithm. The Earley algorithm is general, which means it can handle any grammar you can define in BNF. In fact, the nearley syntax is written in itself (this is called bootstrapping).

PEGjs and Jison are recursive-descent based, and so they will choke on a lot of grammars, in particular left recursive ones.

nearley also has capabilities to catch errors gracefully, and detect ambiguous grammars (grammars that can be parsed in multiple ways).

Installation and Usage

To compile a parser, use the nearleyc command:

npm install -g nearley

Run nearleyc --help for more options.

To use a generated grammar in a node runtime, install nearley as a module:

npm install nearley
var nearley = require("nearley");
var grammar = require("./my-generated-grammar.js");

To use a generated grammar in a browser runtime, include nearley.js (you can hardlink from Github if you want):

<script src=""></script>
<script src="my-generated-grammar.js"></script>

Parser specification

This is a basic overview of nearley syntax and usage. For an advanced styleguide, see this file.

A parser consists of several nonterminals, which are constructions in a language. A nonterminal is made up of a series of either other nonterminals or strings. In nearley, you define a nonterminal by giving its name and its expansions.

Strings are the terminals, which match those string literals (specified as JSON-compatible strings).

The following grammar matches a number, a plus sign, and another number:

expression -> number "+" number

Anything from a # to the end of a line is ignored as a comment:

expression -> number "+" number # sum of two numbers

A nonterminal can have multiple expansions, separated by vertical bars (|):

expression ->
      number "+" number
    | number "-" number
    | number "*" number
    | number "/" number

The parser tries to parse the first nonterminal that you define in a file. However, you can (and should!) introduce more nonterminals as "helpers". In this example, we would have to define the expansion of number.


Each meaning (called a production rule) can have a postprocessing function, that can format the data in a way that you would like:

expression -> number "+" number {%
    function (data, location, reject) {
        return ["sum", data[0], data[2]];

data is an array whose elements match the nonterminals in order. The postprocessor id returns the first token in the match (literally function(data) {data[0];}).

location is the index at which that rule was found. Retaining this information in a syntax tree is useful if you're writing an interpreter and want to give fancy error messages for runtime errors.

If, after examining the data, you want to force the rule to fail anyway, return reject. An example of this is allowing a variable name to be a word that is not a string:

variable -> word {%
    function(data, location, reject) {
        if (KEYWORDS.indexOf(data[0]) === -1) {
            return data[0]; // It's a valid name 
        } else {
            return reject;  // It's a keyword, so reject it

You can write your postprocessors in CoffeeScript by adding @preprocessor coffee to the top of your file. If you would like to support a different postprocessor language, feel free to file a PR!

Epsilon rules

The epsilon rule is the empty rule that matches nothing. The constant null is the epsilon rule, so:

a -> null
    | a "cow"

will match 0 or more cows in a row.


You can use valid RegExp charsets in a rule:

not_a_letter -> [^a-zA-Z]

The . character can be used to represent "any character".


nearley compiles some higher-level constructs into BNF for you. In particular, the *, ?, and + operators from Regexes (or EBNF) are available as shown:

batman -> "na":* "batman" # nananana...nanabatman

You can also use capture groups with parentheses. Its contents can be anything that a rule can have:

banana -> "ba" ("na" {% id %} | "NA" {% id %}):+


You can create "polymorphic" rules through macros:

match3[X] -> $X $X $X
quote[X]  -> "'" $X "'"

main -> match3[quote["Hello?"]]
# matches "'Hello?''Hello?''Hello?'"

Macros are dynamically scoped:

foo[X, Y] -> bar["moo" | "oink" | "baa"] $Y
bar[Z]    -> $X " " $Z # 'remembers' $X from its caller
main -> foo["Cows", "."]
# matches "Cows oink." and "Cows moo."

Macros cannot be recursive (nearleyc will go into an infinite loop trying to expand the macro-loop).

Additional JS

For more intricate postprocessors, or any other functionality you may need, you can include parts of literal JavaScript between production rules by surrounding it with @{% ... %}:

@{% var makeCowWithString = require('./cow.js') %}
cow -> "moo" {% function(d) {makeCowWithString(d[0]); } %}

Note that it doesn't matter where you define these; they all get hoisted to the top of the generated code.


You can include the content of other parser files:

@include "../misc/" # path relative to file being compiled
sum -> number "+" number

There are also some built-in parsers whose contents you can include:

@builtin ""
main -> cow:+

See the builtin/ directory for an index of this library. Contributions are welcome here!

Including a parser imports all of the nonterminals defined in the parser, as well as any JS, macros, and config options defined there.

Custom tokens

Nearley assumes by default that your fundamental unit of parsing, called a token, is a character. That is, you're parsing a list of characters. However, sometimes you want to preprocess your string to turn it into a list of lexical tokens. This means, instead of seeing "1", "2", "3", the nearley might just see a single list item "123". This is called tokenizing, and it can bring you decent performance gains. It also allows you to write cleaner, more maintainable grammars and to prevent ambiguous grammars.

Tokens can be defined in two ways: literal tokens and testable tokens. A literal token matches exactly, while a testable token runs a function to test whether it is a match or not.

var print_tok  = {literal: "print"};
var number_tok = {test: function(x) {return x.constructor === Number; }}

main -> %print_tok %number_tok

Now, instead of parsing the string "print 12", you would parse the array ["print", 12].

You can write your own tokenizer using regular expressions, or choose from several existing tokenizing libraries for node.

(If someone writes a tokenizer plugin for nearley, I would wholeheartedly accept it!)

Using a parser

nearley exposes the following API:

var grammar = require("generated-code.js");
var nearley = require("nearley");

// Create a Parser object from our grammar.
var p = new nearley.Parser(grammar.ParserRules, grammar.ParserStart);

// Parse something
// p.results --> [ ["sum", "1", "1"] ]

The Parser object can be fed data in parts with .feed(data). You can then find an array of parsings with the .results property. If results is empty, then there are no parsings. If results contains multiple values, then that combination is ambiguous.

The incremental feeding design is inspired so that you can parse data from stream-like inputs, or even dynamic readline inputs. For example, to create a Python-style REPL where it continues to prompt you until you have entered a complete block.

p.feed(prompt_user(">>> "));
while (p.results.length < 1) {
    p.feed(prompt_user("... "));

Catching errors

If there are no possible parsings, nearley will throw an error whose offset property is the index of the offending token.

try {
} catch(parseError) {
        "Error at character " + parseError.offset
    ); // "Error at character 2"

Exploring a parser interactively

The global install will provide nearley-test, a simple command-line tool you can use to inspect what a parser is doing. You input a generated grammar.js file, and then give it some input to test the parser against. nearley-test prints out the output if successful, and also gives you the complete parse table used by the algorithm. This is very helpful when you're testing a new parser.

This was previously called bin/nearleythere.js and written by Robin.

The Unparser

The Unparser takes a (compiled) parser and outputs a random string that would be accepted by the parser.

$ nearley-unparse -s number <(nearleyc builtin/

You can use the Unparser to...

  • ...test your parser specification by generating lots of random expressions and making sure all of them are "correct".
  • ...generate random strings from a schema (for example, random email addresses or telephone numbers).
  • ...create fuzzers and combinatorial stress-testers.
  • "Mad-Libs" automatically! (Practical application: automatic grammatically valid loremtext.)

The Unparser outputs as a stream by continuously writing characters to its output pipe. So, if it "goes off the deep end" and generates a huge string, you will still see output scrolling by in real-time.

As far as I know, nearley is the only parser generator with this feature. It is inspired by Roly Fentanes' randexp, which does the same thing with regular expressions.

Automagical Railroad Diagrams

nearley lets you convert your grammars to pretty SVG railroad diagrams that you can include in webpages, documentation, and even papers.

$ nearley-railroad -o grammar.html

Railroad demo

See a bigger example here.

(This feature is powered by railroad-diagrams by tabatkins.)

Still confused?

You can read the calculator example to get a feel for the syntax (see it live here). There are more sample grammars in the /examples directory. For larger examples, we also have experimental parsers for CSV, Lua, and JavaScript.


Clone, hack, PR. Tests live in test/ and can be called with npm test. Make sure you read test/profile.log after tests run, and that nothing has died (parsing is tricky, and small changes can kill efficiency).

If you're looking for something to do, here's a short list of things that would make me happy:

  • Optimize. There are still plenty of optimizations that an enterprising JS-savant could implement.
  • Help build the builtins library by PRing in your favorite primitives.
  • Solutions to issues labeled "up for grabs" on the issue tracker.

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.

Atom users can write nearley grammars with this plugin by Bojidar Marinov.

Sublime Text users can write nearley grammars with this syntax by liam4.

Vim users can use this plugin by Andrés Arana.

Further reading

  • Read my blog post to learn more about the algorithm.
  • Read about Marpa to learn more than you ever thought you wanted to know about parsing.
  • A nearley tutorial written by @gajus.