Documentation
Table of Contents
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 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 -o arithmetics-parser.js arithmetics.pegjs
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:
--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.
--dependency
- Makes the parser require a specified dependency (can be specified multiple times).
--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
. --extra-options-file
- File with additional options (in JSON format) to pass to
peg.generate
. --format
- Format of the generated parser:
amd
,commonjs
,globals
,umd
(default:commonjs
). --optimize
- Selects between optimizing the generated parser for parsing speed
(
speed
) or code size (size
) (default:speed
) --plugin
- Makes PEG.js use a specified plugin (can be specified multiple times).
--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. 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.
To generate a parser, call the peg.generate
method and pass your
grammar as a parameter:
var parser = peg.generate("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.generate
. The following options are
supported:
allowedStartRules
- Rules the parser will be allowed to start parsing from (default: the first rule in the grammar).
cache
- If
true
, makes the parser cache results, avoiding exponential parsing time in pathological cases but making the parser slower (default:false
). dependencies
- Parser dependencies, the value is an object which maps variables used to
access the dependencies in the parser to module IDs used to load them; valid
only when
format
is set to"amd"
,"commonjs"
, or"umd"
(default:{}
). exportVar
- Name of a global variable into which the parser object is assigned to when
no module loader is detected; valid only when
format
is set to"globals"
or"umd"
(default:null
). format
- format of the generated parser (
"amd"
,"bare"
,"commonjs"
,"globals"
, or"umd"
); valid only whenoutput
is set to"source"
(default:"bare"
). optimize
- Selects between optimizing the generated parser for parsing speed
(
"speed"
) or code size ("size"
) (default:"speed"
). 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"
). plugins
- Plugins to use.
trace
- Makes the parser trace its progress (default:
false
).
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.
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 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"
'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”). Appendingi
right after the literal 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 totrue
in boolean context, just returnundefined
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
andend
properties both refer to the current parse position. Theoffset
property contains an offset as a zero-based index andline
andcolumn
properties contain a line and a column as one-based indices.The code inside the predicate can also access 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 tofalse
in boolean context, just returnundefined
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
andend
properties both refer to the current parse position. Theoffset
property contains an offset as a zero-based index andline
andcolumn
properties contain a line and a column as one-based indices.The code inside the predicate can also access 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.
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.To indicate an error, the code inside the action can invoke the
expected
function, which makes the parser throw an exception. The function takes two parameters — a description of what was expected at the current position and optional location information (the default is whatlocation
would return — see below). The 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 two parameters — an error message and optional location information (the default is whatlocation
would return — see below). The 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, theend
property refers to position after the end of the expression. Theoffset
property contains an offset as a zero-based index andline
andcolumn
properties contain a line and a column as one-based indices.The code inside the action can also access options passed to the parser using the
options
variable.Note that 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:
- Node.js 0.10.0+
- Internet Explorer 8+
- Edge
- Firefox
- Chrome
- Safari
- Opera