PEG.js
======

PEG.js is a simple parser generator for JavaScript that produces fast parsers
with excellent error reporting. You can use it to process complex data or
computer languages and build transformers, interpreters, compilers and other
tools easily.

Features
--------

  * Simple and expressive grammar syntax
  * Integrates both lexical and syntactical analysis
  * Parsers have excellent error reporting out of the box
  * Based on [parsing expression
    grammar](http://en.wikipedia.org/wiki/Parsing_expression_grammar) formalism
    — more powerful than traditional LL(*k*) and LR(*k*) parsers
  * Usable [from your browser](http://pegjs.org/online), from the command line,
    or via JavaScript API

Getting Started
---------------

[Online version](http://pegjs.org/online) is the easiest way to generate a
parser. Just enter your grammar, try parsing few inputs, and download generated
parser code.

Installation
------------

### Node.js

To use the `pegjs` command, install PEG.js globally:

    $ npm install -g pegjs

To use the JavaScript API, install PEG.js locally:

    $ npm install pegjs

If you need both the `pegjs` command and the JavaScript API, install PEG.js both
ways.

### Browser

[Download](http://pegjs.org/#download) the PEG.js library (regular or minified
version) or install it using Bower:

    $ bower install pegjs

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 output 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.

You can tweak the generated parser with several options:

  * `--cache` — makes the parser cache results, avoiding exponential parsing
    time in pathological cases but making the parser slower
  * `--allowed-start-rules` — comma-separated list of rules the parser will be
    allowed to start parsing from (default: the first rule in the grammar)
  * `--plugin` — makes PEG.js use a specified plugin (can be specified multiple
    times)
  * `--extra-options` — additional options (in JSON format) to pass to
    `PEG.buildParser`
  * `--extra-options-file` — file with additional options (in JSON format) to
    pass to `PEG.buildParser`
  * `--trace` — makes the parser trace its progress

### 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 in 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 its source code as a string
(depending on the value of the `output` option — see below). It will throw an
exception if the grammar is invalid. The exception will contain `message`
property with more details about the error.

You can tweak the generated parser by passing a second parameter with an options
object to `PEG.buildParser`. The following options are supported:

  * `cache` — if `true`, makes the parser cache results, avoiding exponential
    parsing time in pathological cases but making the parser slower (default:
    `false`)
  * `allowedStartRules` — rules the parser will be allowed to start parsing from
    (default: the first rule in the grammar)
  * `output` — if set to `"parser"`, the method will return generated parser
    object; if set to `"source"`, it will return parser source code as a string
    (default: `"parser"`)
  * `optimize`— selects between optimizing the generated parser for parsing
    speed (`"speed"`) or code size (`"size"`) (default: `"speed"`)
  * `plugins` — plugins to use

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 `location`, `expected` 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 tweak parser behavior by passing a second parameter with an options
object to the `parse` method. The following options are supported:

  * `startRule` — name of the rule to start parsing from
  * `tracer` — tracer to use

Parsers can also support their own custom options.

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.

The first rule 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. The code inside the
initializer can access the parser object using the `parser` variable and options
passed to the parser using the `options` variable. Curly braces in the
initializer code must be balanced. Let's look at the example grammar from above
using a simple initializer.

    {
      function makeInteger(o) {
        return parseInt(o.join(""), 10);
      }
    }

    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 makeInteger(digits); }

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:

  * An expression matching a literal string produces a JavaScript string
    containing matched text.
  * An expression matching repeated occurrence of some subexpression produces a
    JavaScript array with all the matches.

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*"<br>'*literal*'

Match exact literal string and return it. The string syntax is the same as in
JavaScript. Appending `i` right after the literal makes the match
case-insensitive.

#### .

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”). Appending `i` right after the right
bracket makes the match case-insensitive.

#### *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. Unlike in regular expressions, there is no
backtracking.

#### *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. Unlike in regular expressions, there is no
backtracking.

#### *expression* ?

Try to match the expression. If the match succeeds, return its match result,
otherwise return `null`. Unlike in regular expressions, there is no
backtracking.

#### & *expression*

Try to match the expression. If the match succeeds, just return `undefined` and
do not consume any input, otherwise consider the match failed.

#### ! *expression*

Try to match the expression. If the match does not succeed, just return
`undefined` and do not consume any input, 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 gets the match results of labeled expressions in preceding
expression as its arguments. It should return some JavaScript value using the
`return` statement. If the returned value evaluates to `true` in boolean
context, just return `undefined` and do not consume any input; otherwise
consider the match failed.

The code inside the predicate can access all variables and functions defined in
the initializer at the beginning of the grammar.

The code inside the predicate can also access location information using the
`location` function. It returns an object like this:

    {
      start: { offset: 23, line: 5, column: 6 },
      end:   { offset: 23, line: 5, column: 6 }
    }

The `start` and `end` properties both refer to the current parse position. The
`offset` property contains an offset as a zero-based index and `line` and
`column` properties contain a line and a column as one-based indices.

The code inside the predicate can also access the parser object using the
`parser` variable and options passed to the parser using the `options` variable.

Note that 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 gets the match results of labeled expressions in preceding
expression as its arguments. It should return some JavaScript value using the
`return` statement. If the returned value evaluates to `false` in boolean
context, just return `undefined` and do not consume any input; otherwise
consider the match failed.

The code inside the predicate can access all variables and functions defined in
the initializer at the beginning of the grammar.

The code inside the predicate can also access location information using the
`location` function. It returns an object like this:

    {
      start: { offset: 23, line: 5, column: 6 },
      end:   { offset: 23, line: 5, column: 6 }
    }

The `start` and `end` properties both refer to the current parse position. The
`offset` property contains an offset as a zero-based index and `line` and
`column` properties contain a line and a column as one-based indices.

The code inside the predicate can also access the parser object using the
`parser` variable and options passed to the parser using the `options` variable.

Note that curly braces in the predicate code must be balanced.

#### $ *expression*

Try to match the expression. If the match succeeds, return the matched text
instead of the match result.

#### *label* : *expression*

Match the expression and remember its match result under given label. 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.

#### *expression<sub>1</sub>* *expression<sub>2</sub>* ...  *expression<sub>n</sub>*

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.

To indicate an error, the code inside the action can invoke the `expected`
function, which makes the parser throw an exception. The function takes one
parameter — a description of what was expected at the current position. This
description will be used as part of a message of the thrown exception.

The code inside an action can also invoke the `error` function, which also makes
the parser throw an exception. The function takes one parameter — an error
message. This message will be used by the thrown exception.

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

The code inside the action can also access the text matched by the expression
using the `text` function.


The code inside the action can also access location information using the
`location` function. It returns an object like this:

    {
      start: { offset: 23, line: 5, column: 6 },
      end:   { offset: 25, line: 5, column: 8 }
    }

The `start` property refers to the position at the beginning of the expression,
the `end` property refers to position after the end of the expression. The
`offset` property contains an offset as a zero-based index and `line` and
`column` properties contain a line and a column as one-based indices.

The code inside the action can also access the parser object using the `parser`
variable and options passed to the parser using the `options` variable.

Note that curly braces in the action code must be balanced.

#### *expression<sub>1</sub>* / *expression<sub>2</sub>* / ... / *expression<sub>n</sub>*

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.

Error Messages
--------------

As described above, you can annotate your grammar rules with human-readable
names that will be used in error messages. For example, this production:

    integer "integer"
      = digits:[0-9]+

will produce an error message like:

> Expected integer but "a" found.

when parsing a non-number, referencing the human-readable name "integer."
Without the human-readable name, PEG.js instead uses a description of the
character class that failed to match:

> Expected [0-9] but "a" found.

Aside from the text content of messages, human-readable names also have a
subtler effect on *where* errors are reported. PEG.js prefers shorter matches
for named rules and longer matches for unnamed rules. For example, for this
rule matching a comma-separated list of integers:

    seq
      = integer ("," integer)*

an input like `1,2,a` produces this error message:

> Expected integer but "a" found.

But if we add a human-readable name to the `seq` production:

    seq "list of numbers"
      = integer ("," integer)*

then PEG.js prefers an error message that implies a smaller attempted parse
tree:

> Expected end of input but "," found.

Compatibility
-------------

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

  * Node.js 0.10.0+
  * io.js
  * Internet Explorer 8+
  * Edge
  * Firefox
  * Chrome
  * Safari
  * Opera

Development
-----------

  * [Project website](http://pegjs.org/)
  * [Wiki](https://github.com/pegjs/pegjs/wiki)
  * [Source code](https://github.com/pegjs/pegjs)
  * [Issue tracker](https://github.com/pegjs/pegjs/issues)
  * [Google Group](http://groups.google.com/group/pegjs)
  * [Twitter](http://twitter.com/peg_js)

PEG.js is developed by [David Majda](http://majda.cz/)
([@dmajda](http://twitter.com/dmajda)). The [Bower
package](https://github.com/pegjs/bower) is maintained by [Michel
Krämer](http://www.michel-kraemer.com/)
([@michelkraemer](https://twitter.com/michelkraemer)).

You are welcome to contribute code.  Unless your contribution is really trivial
you should get in touch with me first — this can prevent wasted effort on both
sides. You can send code both as a patch or a GitHub pull request.

Note that PEG.js is still very much work in progress. There are no compatibility
guarantees until version 1.0.