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README.md

PEG.js: Parser Generator for JavaScript

http://pegjs.majda.cz/

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

Requirements

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.

Usage

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 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 (which is bundled with PEG.js).

  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 to:

  1. 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).

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

    <script>
      // Replace "arithmeticsParser" with your parser variable
      document.write(arithmeticsParser.parse("2*(3+4)"));
    </script>
    

    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.

Grammar

For detailed description of the grammar see the online documentation.