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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).


  • Usable from your browser 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 (strict superset of LL(k) and LR(k))
  • Precise and human-friendly error reporting


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

To generate a parser, you can use the online generator or the command line. For the second option, you need to have Java installed. Than you can generate the parser using 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 generated parser will be available in the arithmeticsParser global variable. The generator has several useful options—to learn more about them, use --help.

Using the Generated Parser

To use the parser in a web page, follow the following example:

<!DOCTYPE html>
  <title>Arithmetics Parser Example</title>
  <script src="examples/arithmetics.js"></script> <!-- Include the parser. -->
    function calculate(expression) {
      /* Use the parser to compute a value of an arithmetic expression. */
      var result;
      try {
        result = arithmeticsParser.parse(document.getElementById("expression").value);
      } catch (e) {
        result = e.message;
      document.getElementById("result").innerText = result;
  <input type="text" id="expression" value="2*(3+4)">
  <input type="button" id="calculate" value="Calculate" onclick="calculate();">
  <div id="result"></div>

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.


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.