Documentation

Table of Contents

Installation

Command Line / Server-side

To use command-line version, install Node.js and npm first. You can then install PEG.js:

$ npm install pegjs

Once installed, you can use the pegjs command to generate your parser from a grammar and use the JavaScript API from Node.js.

Browser

Download the PEG.js library (regular or minified version) and include it in your web page or application using the <script> tag.

Generating a Parser

PEG.js generates parser from a grammar that describes expected input and can specify what the parser returns (using semantic actions on matched parts of the input). Generated parser itself is a JavaScript object with a simple API.

Command Line

To generate a parser from your grammar, use the pegjs command:

$ pegjs arithmetics.pegjs

This writes parser source code into a file with the same name as the grammar file but with “.js” extension. You can also specify the output file explicitly:

$ pegjs arithmetics.pegjs arithmetics-parser.js

If you omit both input and ouptut file, standard input and output are used.

By default, the parser object is assigned to module.exports, which makes the output a Node.js module. You can assign it to another variable by passing a variable name using the -e/--export-var option. This may be helpful if you want to use the parser in browser environment.

JavaScript API

In Node.js, require the PEG.js parser generator module:

var PEG = require("pegjs");

In browser, include the PEG.js library in your web page or application using the <script> tag. The API will be available through the PEG global object.

To generate a parser, call the PEG.buildParser method and pass your grammar as a parameter:

var parser = PEG.buildParser("start = ('a' / 'b')+");

The method will return generated parser object or throw an exception if the grammar is invalid. The exception will contain message property with more details about the error.

To get parser’s source code, call the toSource method on the parser.

Using the Parser

Using the generated parser is simple — just call its parse method and pass an input string as a parameter. The method will return a parse result (the exact value depends on the grammar used to build the parser) or throw an exception if the input is invalid. The exception will contain line, column and message properties with more details about the error.

parser.parse("abba"); // returns ["a", "b", "b", "a"]

parser.parse("abcd"); // throws an exception

You can also start parsing from a specific rule in the grammar. Just pass the rule name to the parse method as a second parameter.

Grammar Syntax and Semantics

The grammar syntax is similar to JavaScript in that it is not line-oriented and ignores whitespace between tokens. You can also use JavaScript-style comments (// ... and /* ... */).

Let's look at example grammar that recognizes simple arithmetic expressions like 2*(3+4). A parser generated from this grammar computes their values.

start
  = additive

additive
  = left:multiplicative "+" right:additive { return left + right; }
  / multiplicative

multiplicative
  = left:primary "*" right:multiplicative { return left * right; }
  / primary

primary
  = integer
  / "(" additive:additive ")" { return additive; }

integer "integer"
  = digits:[0-9]+ { return parseInt(digits.join(""), 10); }

On the top level, the grammar consists of rules (in our example, there are five of them). Each rule has a name (e.g. integer) that identifies the rule, and a parsing expression (e.g. digits:[0-9]+ { return parseInt(digits.join(""), 10); }) that defines a pattern to match against the input text and possibly contains some JavaScript code that determines what happens when the pattern matches successfully. A rule can also contain human-readable name that is used in error messages (in our example, only the integer rule has a human-readable name). The parsing starts at the first rule, which is also called the start rule.

A rule name must be a JavaScript identifier. It is followed by an equality sign (“=”) and a parsing expression. If the rule has a human-readable name, it is written as a JavaScript string between the name and separating equality sign. Rules need to be separated only by whitespace (their beginning is easily recognizable), but a semicolon (“;”) after the parsing expression is allowed.

Rules can be preceded by an initializer — a piece of JavaScript code in curly braces (“{” and “}”). This code is executed before the generated parser starts parsing. All variables and functions defined in the initializer are accessible in rule actions and semantic predicates. Curly braces in the initializer code must be balanced.

The parsing expressions of the rules are used to match the input text to the grammar. There are various types of expressions — matching characters or character classes, indicating optional parts and repetition, etc. Expressions can also contain references to other rules. See detailed description below.

If an expression successfully matches a part of the text when running the generated parser, it produces a match result, which is a JavaScript value. For example:

The match results propagate through the rules when the rule names are used in expressions, up to the start rule. The generated parser returns start rule's match result when parsing is successful.

One special case of parser expression is a parser action — a piece of JavaScript code inside curly braces (“{” and “}”) that takes match results of some of the the preceding expressions and returns a JavaScript value. This value is considered match result of the preceding expression (in other words, the parser action is a match result transformer).

In our arithmetics example, there are many parser actions. Consider the action in expression digits:[0-9]+ { return parseInt(digits.join(""), 10); }. It takes the match result of the expression [0-9]+, which is an array of strings containing digits, as its parameter. It joins the digits together to form a number and converts it to a JavaScript number object.

Parsing Expression Types

There are several types of parsing expressions, some of them containing subexpressions and thus forming a recursive structure:

"literal"
'literal'

Match exact literal string and return it. The string syntax is the same as in JavaScript.

.

Match exactly one character and return it as a string.

[characters]

Match one character from a set and return it as a string. The characters in the list can be escaped in exactly the same way as in JavaScript string. The list of characters can also contain ranges (e.g. [a-z] means “all lowercase letters”). Preceding the characters with ^ inverts the matched set (e.g. [^a-z] means “all character but lowercase letters”).

rule

Match a parsing expression of a rule recursively and return its match result.

( expression )

Match a subexpression and return its match result.

expression *

Match zero or more repetitions of the expression and return their match results in an array. The matching is greedy, i.e. the parser tries to match the expression as many times as possible.

expression +

Match one or more repetitions of the expression and return their match results in an array. The matching is greedy, i.e. the parser tries to match the expression as many times as possible.

expression ?

Try to match the expression. If the match succeeds, return its match result, otherwise return an empty string.

& expression

Try to match the expression. If the match succeeds, just return an empty string and do not advance the parser position, otherwise consider the match failed.

! expression

Try to match the expression and. If the match does not succeed, just return an empty string and do not advance the parser position, otherwise consider the match failed.

& { predicate }

The predicate is a piece of JavaScript code that is executed as if it was inside a function. It should return some JavaScript value using the return statement. If the returned value evaluates to true in boolean context, just return an empty string and do not advance the parser position; otherwise consider the match failed.

The code inside the predicate has access to all variables and functions defined in the initializer at the beginning of the grammar. Curly braces in the predicate code must be balanced.

! { predicate }

The predicate is a piece of JavaScript code that is executed as if it was inside a function. It should return some JavaScript value using the return statement. If the returned value evaluates to false in boolean context, just return an empty string and do not advance the parser position; otherwise consider the match failed.

The code inside the predicate has access to all variables and functions defined in the initializer at the beginning of the grammar. Curly braces in the predicate code must be balanced.

label : expression

Match the expression and remember its match result under given lablel. The label must be a JavaScript identifier.

Labeled expressions are useful together with actions, where saved match results can be accessed by action's JavaScript code.

expression1 expression2 ... expressionn

Match a sequence of expressions and return their match results in an array.

expression { action }

Match the expression. If the match is successful, run the action, otherwise consider the match failed.

The action is a piece of JavaScript code that is executed as if it was inside a function. It gets the match results of labeled expressions in preceding expression as its arguments. The action should return some JavaScript value using the return statement. This value is considered match result of the preceding expression. The action can return null to indicate a match failure.

The code inside the action has access to all variables and functions defined in the initializer at the beginning of the grammar. Curly braces in the action code must be balanced.

expression1 / expression2 / ... / expressionn

Try to match the first expression, if it does not succeed, try the second one, etc. Return the match result of the first successfully matched expression. If no expression matches, consider the match failed.

Compatibility

Both the parser generator and generated parsers should run well in the following environments: