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.
- 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 formalism — more powerful than traditional LL(k) and LR(k) parsers
- Usable from your browser, from the command line, or via JavaScript API
Online version is the easiest way to generate a parser. Just enter your grammar, try parsing few inputs, and download generated parser code.
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.
Download the PEG.js library (regular or minified version).
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.
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.
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)
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— iftrue, 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")
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 offset, line, column,
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. Only one option is currently supported:
startRule— name of the rule to start parsing from
Parsers can also support their own custom options.
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. 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.
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 part of the input.
- 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.
There are several types of parsing expressions, some of them containing subexpressions and thus forming a recursive structure:
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.
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.
Match a parsing expression of a rule recursively and return its match result.
Match a subexpression and return its match result.
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.
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.
Try to match the expression. If the match succeeds, return its match result, otherwise return an empty string.
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.
Try to match the expression. If the match does not succeed, just return an empty string and do not advance the parser position, otherwise consider the match failed.
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 an empty string and do not advance the parser position;
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 the current parse position using
the offset function. It returns a zero-based character index into the input
string. The code can also access the current line and column using the line
and column functions. Both return one-based indexes.
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.
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 an empty string and do not advance the parser position;
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 the current parse position using
the offset function. It returns a zero-based character index into the input
string. The code can also access the current line and column using the line
and column functions. Both return one-based indexes.
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.
Try to match the expression. If the match succeeds, return the matched string instead of the match result.
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.
Match a sequence of expressions and return their match results in an array.
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 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 string matched by the expression
using the text function.
The code inside the action can also access the parse position at the beginning
of the action's expression using the offset function. It returns a zero-based
character index into the input string. The code can also access the line and
column at the beginning of the action's expression using the line and column
functions. Both return one-based indexes.
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.
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.
Both the parser generator and generated parsers should run well in the following environments:
- Node.js 0.6.6+
- IE 8+
- Firefox
- Chrome
- Safari
- Opera
PEG.js is developed by David Majda (@dmajda). 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.