The README.md file in the root of the repository had become too large, and as a result the file became hard to maintain.
This commit extracts all the documentation and moves it to separate but managable files within the docs directory, a new folder also located in the root of the repository.
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.
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.
> PEG.js is still very much work in progress. There are no compatibility guarantees until version 1.0
Table of Contents
-----------------
- [Features](#features)
- [Getting Started](#getting-Started)
- [Installation](#installation)
* [Node.js](#nodejs)
* [Browser](#browser)
* [Latest](#latest)
- [Generating a Parser](#generating-a-parser)
* [Command Line](#command-line)
* [JavaScript API](#javascript-api)
- [Using the Parser](#using-the-parser)
- [Grammar Syntax and Semantics](#grammar-syntax-and-semantics)
If you omit both input and output file, standard input and output are used.
By default, the generated parser is in the Node.js module format. You can
override this using the `--format` option.
You can tweak the generated parser with several options:
* `-a`, `--allowed-start-rules` — comma-separated list of rules the parser will be allowed to start parsing from (default: the first rule in the grammar)
* `--cache` — makes the parser cache results, avoiding exponential parsing time in pathological cases but making the parser slower
* `-d`, `--dependency` — makes the parser require a specified dependency (can be specified multiple times)
* `-e`, `--export-var` — name of a global variable into which the parser object is assigned to when no module loader is detected
* `--extra-options` — additional options (in JSON format) to pass to `peg.generate`
* `-c`, `--config`, `--extra-options-file` — file with additional options (in JSON or JavaScript) to pass to `peg.generate`
* `-f`, `--format` — format of the generated parser: `amd`, `bare`, `commonjs`, `es`, `globals`, `umd` (default: `commonjs`)
* `-O`, `--optimize` — selects between optimizing the generated parser for parsing speed (`speed`) or code size (`size`) (default: `speed`)
* `-p`, `--plugin` — makes PEG.js use a specified plugin (can be specified multiple times)
* `--trace` — makes the parser trace its progress
**NOTE:** On the command line, unless it's a repeatable option, any option on the right side will take priority over either the same option mentioned
before or it's counter part:
- `pegjs -f es -f bare` will set `options.format` to `bare`
- `pegjs --no-trace --trace` will set `options.trace` to `true`
- `pegjs -a start,Rule -a Rule,Template` will set `options.allowedStartRules` to `[ "start", "Rule", "Template" ]`
### JavaScript API
In Node.js, require the PEG.js parser generator module:
```js
var peg = require("pegjs");
```
In browser, include the PEG.js library in your web page or application using the
`<script>` tag. If PEG.js detects an AMD loader, it will define itself as a
module, otherwise the API will be available in the `peg` global object.
After this there are 3 methods that you will mainly use:
```js
var grammar = "start = ('a' / 'b')+";
var ast, parser;
ast = peg.parser.parse(grammar);
parser = peg.compiler.compile(ast);
parser = peg.generate(grammar);
```
The most common is `peg.generate`, which is the API alternative to the
[PEG.js CLI shown above](#command-line). More about each method is
explained below, along with their respective options.
#### peg.parser.parse(input[, options])
You can simply parse a grammar and get it's AST by using the parser:
```js
var ast = peg.parser.parse("start = ('a' / 'b')+");
```
The following option's are used by the PEG.js parser:
* `extractComments` - If `true`, the parser will collect all comments in the grammar (default: `false`).
* `reservedWords` - An array of words that the parser wont allow as labels for rules (default: [ES5](http://es5.github.io/#x7.6.1)).
When `extractComments` is set to `true`, the parser will collect all comments
in the grammar and return them on the `comments` property (as an object map)
on the `grammar` AST node (the AST directly returned by the parser). Each
comment property has it's offset as the property key, while the property value
has the following structure:
```js
{
text: 'all text between /* or */, or // and end of line',
multiline: true|false,
location: location()
}
```
When set to `false`, the `comments` property will be set to `null`.
For more information about `location()`, see the helper's description in
This method will take a parsed grammar (the PEG.js AST), set default options,
execute each pass (passing the ast to each one), then return the result, which
depends on the value of the `output` option.
```js
var ast = peg.parser.parse("start = ('a' / 'b')+");
var parser = peg.compiler.compile(ast);
```
You can tweak the generated parser by passing a third parameter with an options
object. The following options are supported:
Option | default value | description
--- | --- | ---
allowedStartRules | first rule in the grammar | rules the generated parser is allowed to start parsing from
cache | `false` | makes the generated parser cache results, avoiding exponential parsing time in pathological cases but making the parser slower
dependencies | `{}` | parser dependencies, the value is an object which maps variables used to access the dependencies to module IDs used to load them; valid only when `format` is set to `"amd"`, `"commonjs"`, `"es"`, or `"umd"`
exportVar | `null` | name of an optional global variable into which the generated parser object is assigned to when no module loader is detected; valid only when `format` is set to `"globals"` or `"umd"`
format | `"bare"` | format of the generated parser (`"amd"`, `"bare"`, `"commonjs"`, `"es"`, `"globals"`, or `"umd"`); valid only when `output` is set to `"source"`
header | `null` | this option is only handled if it's an array or a string:
optimize | `"speed"` | selects between optimizing the generated parser for parsing speed (`"speed"`) or code size (`"size"`)
output | `"parser"` | if set to `"parser"`, the method will return generated parser object; if set to `"source"`, it will return generated parser source code as a string
trace | `false` | makes the generated parser trace its progress
The `header` options behavior will change depending on the option type:
* `[ string1, string2, ... ]` will add each element (all expected to be strings) as a separate line comment
* `string` will simply append the string (e.g. `"/* eslint-disable */"`) after the `Generated by ...` comment
#### peg.generate(grammar[, options])
Will generate a parser from the given grammar (the _input_ passed to `peg.parser.parse`):
```js
var parser = peg.generate("start = ('a' / 'b')+");
```
This method will return a generated parser object or its source code as a string
(depending on the value of the `output` option - see above). It will throw an
exception if the grammar is invalid. The exception will contain the usual `message`
property with more details about the error, along with a `location` property to
track the location of the error. You can easily tell if the exception was thrown
by PEG.js because it will always have the `name` property set to `GrammarError`.
You can tweak the generated parser by passing a second parameter with an options
object to `peg.generate`. The following options are supported:
* All options that you pass to `peg.compiler.compile`
* `parser` - an optional object with option's passed to the PEG.js parser (`peg.parser.parse`)
* `plugins` - plugins to use _(executed by `peg.generate`)_
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 generate the parser) or throw an exception
if the input is invalid. The exception will contain `location`, `expected`,
`found`, and `message` properties with more details about the error.
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.
### Action Execution Environment
Actions and predicates have these variables and functions
available to them.
* All variables and functions defined in the initializer at the
beginning of the grammar are available.
* Labels from preceding expressions are available as local
variables, which will have the match result of the labelled
expressions.
A label is only available after its labelled expression is
matched:
```pegjs
rule = A:('a' B:'b' { /* B is available, A is not */ } )
```
A label in a sub-expression is only valid within the
sub-expression:
```pegjs
rule = A:'a' (B: 'b') (C: 'b' { /* A and C are available, B is not */ })
```
* `options` is a variable that contains the parser options.
* `error(message, where)` will report an error and throw an
exception. `where` is optional; the default is the value of
`location()`.
* `expected(message, where)` is similar to `error`, but reports
> Expected _message_ but "_other_" found.
* `location()` returns an object like this:
```js
{
start: { offset: 23, line: 5, column: 6 },
end: { offset: 25, line: 5, column: 8 }
}
```
For actions, `start` refers to the position at the beginning of
the preceding expression, and `end` refers to the position
after the end of the preceding expression.
For predicates, `start` and `end` are the same, the location
where the predicate is evaluated.
`offset` is a 0-based character index within the source text.
`line` and `column` are 1-based indices.
Note that `line` and `column` are somewhat expensive to
compute, so if you need location frequently, you might want to
use `offset()` or `range()` instead.
* `offset()` returns the start offset.
* `range()` returns an array containing the start and end
offsets, such as `[23, 25]`.
* `text()` returns the source text between `start` and `end`
(which will be "" for predicates).
### Balanced Braces
Code fragments such as actions and predicates must have balanced curly braces,
because pegjs doesn't parse the contents. It only looks at balanced braces to
find the end of the code fragment.
If your code fragment needs an unbalanced brace in a string literal, you
can balance it in a comment. For example:
```pegjs
brace = [{}] {
return text() === "{" ? 1 : -1; // } for balance
}
```
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 to match
named rules completely or not at all, but not partially. Unnamed rules,
on the other hand, can produce an error in the middle of their
subexpressions.
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 4+
* Internet Explorer 9+
* Edge
* Firefox
* Chrome
* Safari
* Opera
Development
-----------
PEG.js is currently maintained by [Futago-za Ryuu](https://github.com/futagoza). Since it's [inception](https://www.google.com/search?q=inception+meaning) in 2010, PEG.js was maintained by [David Majda](https://majda.cz/) ([@dmajda](http://twitter.com/dmajda)), until [May 2017](https://github.com/pegjs/pegjs/issues/503).
The [Bower package](https://github.com/pegjs/bower) is maintained by [Michel Krämer](http://www.michel-kraemer.com/) ([@michelkraemer](https://twitter.com/michelkraemer)).
* 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](https://pegjs.org/online), from the command line, or via JavaScript API
You are welcome to contribute code using [GitHub pull requests](https://github.com/pegjs/pegjs/pulls). Unless your contribution is really trivial you should get in touch with me first (preferably by creating a new issue on the [issue tracker](https://github.com/pegjs/pegjs/issues)) - this can prevent wasted effort on both sides.
## documentation
> Before submitting a pull request, please make sure you've checked out the [Contribution Guidelines](https://github.com/pegjs/pegjs/blob/master/CONTRIBUTING.md).
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 line comments (`// ...`) and block comments (`/* ... */`).
Let's look at an example grammar that recognizes simple arithmetic expressions like `2*(3+4)`. A parser generated from this grammar computes their values.
```pegjs
start
= additive
additive
= left:multiplicative "+" right:additive { return left + right; }
/ multiplicative
multiplicative
= left:primary "*" right:multiplicative { return left * right; }
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 a *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 options passed to the parser using the `options` variable. Curly braces in the initializer code must be [balanced](./balanced-braces.md).
Let's look at the example grammar from above using a simple initializer:
```pegjs
{
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. For a more detailed description, check out the [Parsing Expression Types](./parsing-expression-types.md) document.
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 a match result of the preceding expression (in other words, the parser action is a match result transformer).
In our arithmetic's 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.
Actions and predicates have these variables and functions available to them.
* All variables and functions defined in the initializer at the beginning of the grammar are available.
* Labels from preceding expressions are available as local variables, which will have the match result of the labelled expressions.
A label is only available after its labelled expression is matched:
```pegjs
rule = A:('a' B:'b' { /* B is available, A is not */ } )
```
A label in a sub-expression is only valid within the sub-expression:
```pegjs
rule = A:'a' (B: 'b') (C: 'b' { /* A and C are available, B is not */ })
```
* `options` is a variable that contains the parser options.
* `error(message, where)` will report an error and throw an exception. `where` is optional; the default is the value of `location()`.
* `expected(message, where)` is similar to `error`, but reports: Expected _message_ but "_other_" found
* `location()` returns an object like this:
```js
{
start: { offset: 23, line: 5, column: 6 },
end: { offset: 25, line: 5, column: 8 }
}
```
For actions, `start` refers to the position at the beginning of the preceding expression, and `end` refers to the position after the end of the preceding expression.
For predicates, `start` and `end` are the same, the location where the predicate is evaluated.
`offset` is a 0-based character index within the source text.
`line` and `column` are 1-based indices.
Note that `line` and `column` are somewhat expensive to compute, so if you need location frequently, you might want to use `offset()` or `range()` instead.
* `offset()` returns the start offset.
* `range()` returns an array containing the start and end offsets, such as `[23, 25]`.
* `text()` returns the source text between `start` and `end` (which will be "" for predicates).
Code fragments such as actions and predicates must have balanced curly braces, because PEG.js doesn't parse the contents. It only looks at balanced braces to find the end of the code fragment.
If your code fragment needs an unbalanced brace in a string literal, you can balance it in a comment. For example:
Appending `i` right after either [a literal](./parsing-expression-types.md#literalliteral) or a [a character set](./parsing-expression-types.md#characters) makes the match case-insensitive. The rules shown in the following example all produce the same result:
When parsing input with a generated parser, there are times when the parser will throw an error (on failure or when you call [expected()](./action-execution-environment.md) from one of the action blocks) with a message that contains a comma separated list of expressions that where expected or found. This is usually fine, but sometimes it is preferred to have a more human-readable name.
You can annotate your grammar rules with human-readable names that will be used in the error messages. For example, this:
```pegjs
integer "integer" = $[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 expression 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 to match named rules completely or not at all, but not partially. Unnamed rules, on the other hand, can produce an error in the middle of their subexpressions.
For example, for this rule matching a comma-separated list of integers:
```pegjs
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:
```pegjs
seq "list of numbers"
= integer ("," integer)*
```
then PEG.js prefers an error message that implies a smaller attempted parse tree:
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.
#### . *(dot character)*
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 they are escaped in JavaScript strings. The list of characters can also contain ranges (e.g. `[a-f]` means “any character between (and including) _a_ and _f_, 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](./backtracking.md).
#### *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](./backtracking.md).
#### *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](./backtracking.md).
#### & *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 (the expression was matched, when it shouldn't have).
#### & { *predicate* }
This is a positive assertion. No input is consumed.
The predicate should be JavaScript code, and it's executed as a function. Curly braces in the predicate must be [balanced](./balanced-braces.md).
The predicate should `return` a boolean value. If the result is _truthy_, the match result is `undefined`, otherwise the match is considered failed.
The predicate has access to all variables and functions in the [Action Execution Environment](./action-execution-environment.md).
#### ! { *predicate* }
This is a negative assertion. No input is consumed.
The predicate should be JavaScript code, and it's executed as a function. Curly braces in the predicate must be [balanced](./balanced-braces.md).
The predicate should `return` a boolean value. If the result is _falsy_, the match result is `undefined`, otherwise the match is considered failed.
The predicate has access to all variables and functions in the [Action Execution Environment](./action-execution-environment.md).
#### $ *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.
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.
PEG.js generates a parser from grammar that describes expected input and can specify what the parser returns (using semantic actions on matched parts of the input). The generated parser itself is a JavaScript object with a simple API.
### Command Line
To generate a parser from your grammar, use the `pegjs` command:
```console
$ 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:
If you omit both input and output files, the standard input and output are used (REPL mode).
By default, the generated parser is in the Node.js module format. You can override this using the `--format` option.
You can tweak the generated parser with several options:
* `-a`, `--allowed-start-rules` — comma-separated list of rules the parser will be allowed to start parsing from (default: the first rule in the grammar)
* `--cache` — makes the parser cache results, avoiding exponential parsing time in pathological cases but making the parser slower
* `-d`, `--dependency` — makes the parser require a specified dependency (can be specified multiple times)
* `-e`, `--export-var` — name of a global variable into which the parser object is assigned to when no module loader is detected
* `--extra-options` — additional options (in JSON format) to pass to `peg.generate`
* `-c`, `--config`, `--extra-options-file` — file with additional options (in JSON or JavaScript) to pass to `peg.generate`
* `-f`, `--format` — format of the generated parser: `amd`, `bare`, `commonjs`, `es`, `globals`, `umd` (default: `commonjs`)
* `-O`, `--optimize` — selects between optimizing the generated parser for parsing speed (`speed`) or code size (`size`) (default: `speed`)
* `-p`, `--plugin` — makes PEG.js use a specified plugin (can be specified multiple times)
* `--trace` — makes the parser trace its progress
**NOTE:** On the command line, unless it's a repeatable option, any option on the right side will take priority over either the same option mentioned before or it's counter part:
- `pegjs -f es -f bare` will set `options.format` to `bare`
- `pegjs --no-trace --trace` will set `options.trace` to `true`
- `pegjs -a start,Rule -a Rule,Template` will set `options.allowedStartRules` to `[ "start", "Rule", "Template" ]`
### Module
To generate a parser using the module, take a look at the section about the method `peg.generate()` described in the [JavaScript API documentation](./javascript-api.md). This method is the API alternative to the command line tool as it does everything for it, the command line tool just takes care of validating and handling input, output and options.
The [online editor](https://pegjs.org/online) is the easiest way to generate a parser. Just enter your grammar, try parsing some input, and download the generated parser code.
If you want a more flexible way to generate a parser, check out the [installation guide](./installation.md).
To use the `pegjs` command, install PEG.js globally:
```console
$ npm install -g pegjs
```
To use the JavaScript API, install PEG.js locally:
```console
$ npm install pegjs
```
If you need both the `pegjs` command and the JavaScript API, install PEG.js both ways.
If you plan to use the `pegjs` command only in your `package.json`, it is suggested you install it locally only, as all locally installed packages that contain command line tools are available to it.
### Browser
[Download](https://pegjs.org/#download) the PEG.js library (regular or minified version) or install it using Bower:
```console
$ bower install pegjs
```
### Latest
To use the latest features, fixes and changes of PEG.js, install the packaged dev release:
```console
$ npm install pegjs@dev
```
Alternatively, you can directly install from the repository (larger then the packaged dev release):
In browsers, include the PEG.js library in your web page or application using the `<script>` tag. If PEG.js detects an AMD loader, it will define itself as a module, otherwise the API will be available in the `peg` global object.
After this there are 3 methods and 1 class that you will mainly use:
```js
var grammar = "start = ('a' / 'b')+";
var passes = [ /* check, transform and generate passes */ ];
var parser, session, ast;
ast = peg.parser.parse(grammar);
session = new peg.Session( { grammar, passes } );
parser = peg.compiler.compile(ast, session);
parser = peg.generate(grammar);
```
The most common method you will use is `peg.generate()`, which is the API alternative to the [PEG.js CLI](./generating-a-parser.md). More about each method is explained below, along with their respective options.
#### peg.parser.parse(input[, options])
You can simply parse a grammar and get it's AST by using the parser:
```js
var ast = peg.parser.parse("start = ('a' / 'b')+");
```
The following option's are used by the PEG.js parser:
* `extractComments` - If `true`, the parser will collect all comments in the grammar (default: `false`).
* `reservedWords` - An array of words that the parser wont allow as labels for rules (default: [ES5](http://es5.github.io/#x7.6.1)).
When `extractComments` is set to `true`, the parser will collect all comments in the grammar and return them on the `comments` property (as an object map) on the `grammar` AST node (the AST directly returned by the parser). Each comment property has it's offset as the property key, while the property value has the following structure:
```js
{
text: 'all text between /* or */, or // and end of line',
multiline: true|false,
location: location()
}
```
When set to `false`, the `comments` property will be set to `null`.
For more information about `location()`, as well more methods you can use in JavaScript actions within your grammar, see the helper's description in [Action Execution Environment](../grammar/action-execution-environment.md).
#### new peg.compiler.Session([options])
An instance of this class holds helpers (methods, objects with methods, constants, etc) that are shared between multiple calls to `peg.compiler.compile()`.
> This approach is used rather then a traditional compiler class because it provides the plugin developer with an easier way to replace certain components rather then creating their own version of PEG.js to override one or more internal methods.
The following option's are used by the Session API, but are currently all optional:
* `grammar` - The input that was parsed (source for the PEG.js AST passed to the compiler and the current passes).
* `opcodes` - An `enum` like hashmap (plain object) that is used by the bytecode and parser generators.
* `parser` - A pre-generated PEG.js grammar parser that should return an instance of the PEG.js AST's Grammar class. Can be replaced to add additional syntax features, or allow an alternative syntax for the grammar.
* `passes` - An object with each property being an array of methods that will check or alter the AST passed to them.
* `visitor` - An object that should contain the `ASTVisitor` class for the PEG.js AST, as well as the `build()` helper.
* `vm` - An object that should contain `runInContext()`, a wrapper for `eval` based on Node's `vm.runInContext()` method.
* `warn` - A method called only when PEG.js encounters an error that doesn't stop the parser from being generated.
* `error` - A method that will be called when PEG.js encounters an error that will stop the parser from being generated.
This class will also return some helper methods:
* `parse( input, options )` - an alias for `this.parser.parse( input, options )`
* `buildVisitor( functions )` - an alias for `this.visitor.build( functions )`
This method takes a parsed grammar (the PEG.js AST), sets default options, executes each pass currently within the session (passing the _ast_, _session_ and _options_ arguments to each one), then returns the result, which depends on the value of the `output` option.
```js
var grammar = "start = ('a' / 'b')+";
var ast = peg.parser.parse(grammar);
var passes = peg.util.convertPasses( peg.compiler.passes );
var session = new peg.Session( { grammar, passes } );
var parser = peg.compiler.compile(ast, session);
```
You can tweak the generated parser by passing a third parameter with an options object. The following options are supported:
Option | default value | description
--- | --- | ---
allowedStartRules | first rule | rules the generated parser is allowed to start parsing from
cache | `false` | makes the generated parser cache results, avoiding exponential parsing time in pathological cases but making the parser slower
context | `{}` | contains a map of variables used by `peg.util.vm.runInContext()` when the `output` option is set to `"parser"`
dependencies | `{}` | parser dependencies, the value is an object which maps variables used to access the dependencies to module IDs used to load them; valid only when `format` is set to `"amd"`, `"commonjs"`, `"es"`, or `"umd"`
exportVar | `null` | name of an optional global variable into which the generated parser object is assigned to when no module loader is detected; valid only when `format` is set to `"globals"` or `"umd"`
format | `"bare"` | format of the generated parser (`"amd"`, `"bare"`, `"commonjs"`, `"es"`, `"globals"`, or `"umd"`); valid only when `output` is set to `"source"`
header | `null` | adds additional comments or content after the `Generated by ...` comment; this option is only handled if it's an array or a string:
optimize | `"speed"` | selects between optimizing the generated parser for parsing speed (`"speed"`) or code size (`"size"`)
output | `"parser"` | if set to `"parser"`, the method will return generated parser object; if set to `"source"`, it will return generated parser source code as a string
trace | `false` | makes the generated parser trace its progress
The `header` options behavior will change depending on the option type:
* `[ string1, string2, ... ]` will add each element (all expected to be strings) as a separate line comment
* `string` will simply append the string (e.g. `"/* eslint-disable */"`) after the `Generated by ...` comment
#### peg.generate(grammar[, options])
Will generate a parser from the given grammar (the _input_ passed to `peg.parser.parse()`):
```js
var parser = peg.generate("start = ('a' / 'b')+");
```
This method will return a generated parser object or its source code as a string (depending on the value of the `output` option - see above). It will throw an exception if the grammar is invalid. The exception will contain the usual `message` property with more details about the error, along with a `location` property to track the location of the error. You can easily tell if the exception was thrown by PEG.js because it will always have the `name` property set to `GrammarError`.
You can tweak the generated parser by passing a second parameter with an options object to `peg.generate()`. The following options are supported:
* All options that you pass to `peg.compiler.compile()`
* `parser` - an optional object with option's passed to the PEG.js parser (`peg.parser.parse()`)
* `plugins` - plugins to use _(their `use()` method is executed by `peg.generate()`)_
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 generate the parser) or throw an exception if the input is invalid. The exception will contain `location`, `expected`, `found`, and `message` properties with more details about the error.
PEG.js is currently maintained by [Futago-za Ryuu](https://github.com/futagoza) ([@futagozaryuu](https://twitter.com/futagozaryuu)).
Since it's [inception](https://www.google.com/search?q=inception+meaning) in 2010, PEG.js was maintained by [David Majda](https://majda.cz/) ([@dmajda](http://twitter.com/dmajda)), until [May 2017](https://github.com/pegjs/pegjs/issues/503).
The [Bower package](https://github.com/pegjs/bower) is maintained by [Michel Krämer](http://www.michel-kraemer.com/) ([@michelkraemer](https://twitter.com/michelkraemer)).
### Contribution
You are welcome to contribute code using [GitHub pull requests](https://github.com/pegjs/pegjs/pulls). Unless your contribution is really trivial you should get in touch with me first (preferably by creating a new issue on the [issue tracker](https://github.com/pegjs/pegjs/issues)) - this can prevent wasted effort on both sides.
> Before submitting a pull request, please make sure you've checked out the [Contribution Guidelines](https://github.com/pegjs/pegjs/blob/master/.github/CONTRIBUTING.md).
1. Create a fork of https://github.com/pegjs/pegjs
2. Clone your fork, and optionally create a new branch
3. Run the command `npm install` from the root of your clone
4. Add and commit your changes
5. Validate your changes:
- Lint the JavaScript changes (command line only, run `gulp lint` or `npm run lint`)
- Run tests to ensure nothing's broken: [see separate documentation](https://github.com/pegjs/pegjs/blob/master/test/README.md)
6. If validation fails: reverse your commit, fix the problem and then add/commit again
7. Push the commits from your clone to the fork
8. From your fork, start a new pull request
It's also a good idea to check out the [gulpfile.js](https://github.com/pegjs/pegjs/blob/master/gulpfile.js) that defines various tasks that are commented with a description of each task.
To see the list of contributors check out the [repository's contributors page](https://github.com/pegjs/pegjs/graphs/contributors).
Futago-za Ryuu
7 years ago