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PEG.js: Parser Generator for JavaScript

PEG.js is a parser generator for JavaScript based on the parsing expression grammar formalism. It enables you to easily build fast parsers which process complex data or computer languages. You can use it as an underlying tool when writing various data processors, transformers, interpreters, or compilers.


  • Usable from your browser, from JavaScript code, or from a command-line
  • Simple and expressive grammar syntax
  • No separate lexical analysis step — both lexical and syntactical analysis are handled by one tool
  • Handles wide class of grammars (superset of LL(k) and LR(k))
  • Precise and human-friendly error reporting


To build PEG.js, simply run the rake command:

$ rake

Of course, you need to have Rake installed. The command creates PEG.js library in lib/peg.js by processing files in the src directory.


Using PEG.js is easy:

var parser = PEG.buildParser("start = ('a' / 'b')+");
parser.parse("abba"); // returns ["a", "b", "b", "a"]
parser.parse("abcd"); // throws an exception with details about the error

Basically, you need to generate a parser from your grammar and then use it to parse the input.

Generating a Parser

There are three ways how to generate the parser:

  1. Using the online generator
  2. Using the PEG.buildParser function from JavaScript code
  3. Using the command line

The online generator is easiest to use — you just enter your grammar and download the generated parser code. The parser object will be available in a global variable you specify (parser by default).

To generate the parser from JavaScript code, include the lib/compiler.js file and use the PEG.buildParser function. This function accepts a string with a grammar and either returns the built parser object or throws an exception if the grammar is invalid.

To generate the parser from a command line, you need to have Java installed (so that Rhino — which is included in PEG.js — can run). Use the bin/pegjs script on Unix or bin/pegjs.bat batch file on Windows:

$ bin/pegjs arithmeticsParser examples/arithmetics.pegjs

This command will create the parser from the examples/arithmetics.pegjs file and put in into the examples/arithmetics.js file. The parser object will be available in the arithmeticsParser global variable. To learn more about the generator usage, use the --help option.

Using the Generated Parser

To use the generated parser, include the generated file (unless you built the parser straight from the JavaScript code using PEG.buildParser) and use the parse method on the parser object. This method accepts an input string and either returns the parse result (dependent on the actions you specified in the grammar) or throws PEG.grammarParser.SyntaxError exception if the input contains a syntax error. The exception has properties message, line and column, which contain details about the error.

The parser object also has the toSource method that returns its textual representation.


For detailed description of the grammar see the online documentation.


Both the parser generator and generated parsers should run well in the following environments:

  • IE6+
  • Firefox
  • Chrome
  • Safari
  • Opera
  • Rhino