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PEG.js: Parser Generator for JavaScript

PEG.js is a parser generator for JavaScript based on the parsing expression
grammar [1] formalism. It is designed to be used either from your browser or
from the command line (using Rhino [2] JavaScript interpreter).

Both the parser generator and generated parsers should run well in IE8 and
recent versions of Firefox, Chrome, Safari and Opera, as well as Rhino
JavaScript engine. IE6 and IE7 are not supported.

  Note: IE7 might be supported sometime in the future, IE6 probably not.

To use PEG.js, you need to generate a parser from your grammar and then use the
generated parser in your project.

Generating a Parser
A parser can be generated either online [3] in your browser or using the command
line. Let's look at the second option. You need to follow these steps:

  1. Install Java. This is necessary to run Rhino [2] (which is bundled with

  2. Generate the parser using bin/pegjs script on Unix or bin/pegjs.bat batch
     file on Windows.

For example, to generate a parser from an example grammar in the
examples/arithmetics.pegjs file on Unix, run:

  $ bin/pegjs arithmeticsParser examples/arithmetics.pegjs

This command will create the parser in the examples/arithmetics.js file and will
make it available in the "arithmeticsParser" global variable.

The bin/pegjs command has several options that influence the generator--to learn
more about them, use the --help option.

  Note: In the future, I will probably use Narwhal for the command-line version.

Using the Generated Parser
Let's assume that you want to use the parser in a web page. To do this, you need

  1. Download the minified parser runtime [4] and include it into your page:

       <script src="pegjs-runtime-0.3.min.js"></script>

  2. Include the generated parser into your page:

       <!-- Replace "example/arithmetics.js" with your parser file -->
       <script src="example/arithmetics.js"></script>

     This creates a variable with the parser object in the global scope (you can
     choose name of the variable when generating the parser).

  3. Use the parser, i.e. call the parse method on the parser variable:

         // Replace "arithmeticsParser" with your parser variable

     The "parse" method of the generated parser will return either the result of
     the parsing (dependent on the actions you specified in the grammar) or
     throw PEG.Parser.SyntaxError exception if the input contains a syntax
     error. The exception has properties "message", "line" and "column", which
     contain details about the error.

For detailed description of the grammar see the online documentation [5].

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