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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 is designed to be used either from your browser or from the command line (using [Rhino]( JavaScript interpreter).
Both the parser generator and generated parsers should run well in IE6+ and recent versions of Firefox, Chrome, Safari and Opera, as well as Rhino JavaScript engine.
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&mdash;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:
// 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](