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
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.
There are three ways how to generate the parser:
- Using the online generator
- Using the
PEG.buildParser
function from JavaScript code - 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.
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