parleur.js

A JavaScript library for building text parsers. It is inspired by the parser combinator libraries known from functional programming, such as Parsec.

parleur.js is simple to use and very general. It should be able to handle pretty much any parsing job you throw at it. Unlike parser generators, like the traditional Yacc library, parleur.js is able to parse context dependent formats.

Quick Introduction

I will now walk you through parsing a simple expression using parleur.js. We're going to parse a "variable assignment" like var foo = 42.

First we create a Parleur.Parser object with the text we want to parse:

var parser = new Parleur.Parser("var foo = 42");

We then add calls to parse rules. A parse rule can be very simple or very complex, but always work the same: the look for some pattern at the start of the text, if they succeed they remove the pattern from the text and return it, if they fail they return undefined and put the parser into error state.

Lets just see how it looks:

var parser = new Parleur.Parser("var foo = 42");
parser.string("var");
parser.space();
var name = parser.regex("[a-zA-Z]+");
parser.space();
parser.string("=");
parser.space();
var value = parser.int();
parser.end();

You should notice that the parser.<rule> calls follow our text closely. We should still explain them though:

• parser.string("var") parses the exact string "var" and returns the result.
• parser.space() parses one or more space characters.
• parser.regex("[a-zA-Z]+") parses a match for the pattern [a-zA-Z+] -- the name of our variable.
• parser.int() parses a positive integer.
• parser.end() parses nothing, but checks that there is no more text left to parse.

If everything went well then we should have name = "foo" and value = 42. If one of the rules failed the results will be undefined.

To check that the parsing succeeded we call parser.success(), and we can then use the parsed values to build our own return value:

var parser = new Parleur.Parser("var foo = 42");
parser.string("var");
parser.space();
var name = parser.regex("[a-zA-Z]+");
parser.space();
parser.string("=");
parser.space();
var value = parser.int();
parser.end();

if (parser.success()) {
  return { name: name, value: value }
}

For good measure we should put or parser inside a function:

function parseVariable(text) {
  var parser = new Parleur.Parser(text);
  parser.string("var");
  parser.space();
  var name = parser.regex("[a-zA-Z]+");
  parser.space();
  parser.string("=");
  parser.space();
  var value = parser.int();
  parser.end();

  if (parser.success()) {
    return { name: name, value: value }
  }

  throw parser.errorMessage();
}

If we get to the throw statement it means that the parsing has failed. Calling parser.errorMessage() will return a string telling you exactly why and where the parser failed.

For more examples of how to use parleur.js take a look at the tests in test/ .

Contributing

The API should be pretty stable now, so the best way to contribute is to use parleur.js for something. By using it we will be able to figure if anything is missing or anything should be removed -- or if parleur.js is usable at all.

You can also read through the source code and see if you can spot places where performance or maintainability could be improved.