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ooo. .oo.    .ooooo.   .oooo.   oooo d8b  888   .ooooo.  oooo    ooo 
`888P"Y88b  d88' `88b `P  )88b  `888""8P  888  d88' `88b  `88.  .8'  
 888   888  888ooo888  .oP"888   888      888  888ooo888   `88..8'   
 888   888  888    .o d8(  888   888      888  888    .o    `888'    
o888o o888o `Y8bod8P' `Y888""8o d888b    o888o `Y8bod8P'     .8'     
                            -stream streaming parser     .o..P'      


Simple parsing for node.js.


nearley-stream uses the Earley parsing algorithm to parse complex data structures easily.


nearley-stream lets you define grammars in a simple format. Unlike Jison's tokenizer-and-parser approach, I use a single set of definitions. Unlike PEG.js, this parser handles left recursion gracefully and warns you if your grammar is ambiguous (ambiguous grammars are slower and take up more memory).


To compile a parser, use the snearleyc command:

npm install -g nearley-stream

Run snearleyc --help for more options.

Making a Parser

A parser consists of several nonterminals, which are just various constructions. A nonterminal is made up of a series of either nonterminals or strings (enclose strings in "double quotes", and use backslash escaping like in JSON). The following grammar matches a number, a plus sign, and another number:

expression -> number "+" number

The first nonterminal you define is the one that the parser tries to parse.

A nonterminal can have multiple meanings, separated by pipes (|):

expression -> number "+" number   |   number "-" number

Finally, 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) {
        return data[0] + data[2]; // the sum of the two numbers

data is an array whose elements match the nonterminals in order.

To use the generated parser, use:

var Parser = require("parser.js");
var parser = new Parser();
parser.on('result', function (result) {
    // result is 2

var parser2 = new Parser();
parser2.on('result', function (result) {
    // never reached
}).on('error', function (err) {
    // err is an Error("nearley parse error")

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.

The following constants are also defined:

Constant Meaning Regex Equivalent
_char Any character /./
_az Any lowercase letter /[a-z]/
_AZ Any uppercase letter /[A-Z]/
_09 Any digit [0-9]
_s A whitespace character /\s/


A parse error will emit Error("nearley parse error"), which you can catch like this:

parser.on('error', function (err) {
    if (err.message === "nearley parse error") {
        // it was a parse error!