PEG.js is a simple parser generator for JavaScript that produces fast parsers with excellent error reporting. It allows you to define a grammar in a simple syntax and then generates a parser for that grammar.
Grammar Definition
This feature allows you to define a grammar using PEG.js syntax. The example demonstrates a simple arithmetic expression parser that can handle addition and multiplication.
const peg = require('pegjs');
const grammar = `
start = expression
expression = term ('+' term)*
term = factor ('*' factor)*
factor = number / '(' expression ')'
number = [0-9]+ { return parseInt(text(), 10); }
`;
const parser = peg.generate(grammar);
const result = parser.parse('2*(3+4)');
console.log(result); // Outputs: 14
Custom Error Messages
PEG.js provides excellent error reporting. This example shows how PEG.js can generate custom error messages when the input does not conform to the defined grammar.
const peg = require('pegjs');
const grammar = `
start = expression
expression = term ('+' term)*
term = factor ('*' factor)*
factor = number / '(' expression ')'
number = [0-9]+ { return parseInt(text(), 10); }
`;
const parser = peg.generate(grammar, { output: 'source', format: 'commonjs' });
try {
parser.parse('2*(3+)');
} catch (e) {
console.log(e.message); // Outputs: Expected "(", number, or whitespace but ")" found.
}
Semantic Actions
Semantic actions allow you to specify what should happen when a rule matches. This example demonstrates how to add two numbers together when the '+' operator is encountered.
const peg = require('pegjs');
const grammar = `
start = expression
expression = left:term '+' right:term { return left + right; }
term = number
number = [0-9]+ { return parseInt(text(), 10); }
`;
const parser = peg.generate(grammar);
const result = parser.parse('3+4');
console.log(result); // Outputs: 7
Nearley is a powerful and flexible parser generator for JavaScript. It supports a wider range of grammars compared to PEG.js and can handle more complex parsing tasks. Nearley uses Earley parsing algorithm which is more powerful but can be slower than PEG.js for certain grammars.
ANTLR (Another Tool for Language Recognition) is a powerful parser generator that can be used to read, process, execute, or translate structured text or binary files. It is more feature-rich and supports multiple target languages, but it has a steeper learning curve compared to PEG.js.
Jison is a parser generator that converts a context-free grammar into a JavaScript parser. It is similar to PEG.js but uses a different parsing algorithm (LALR(1)). Jison is more suitable for traditional compiler construction tasks.
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.
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) or install it using Bower:
$ bower install pegjs
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 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 progressIn 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 useUsing 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, 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 fromtracer — tracer to useParsers 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.
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:
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. Unlike in regular expressions, there is no backtracking.
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.
Try to match the expression. If the match succeeds, return its match result,
otherwise return null. Unlike in regular expressions, there is no
backtracking.
Try to match the expression. If the match succeeds, just return undefined 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
undefined 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 undefined 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 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.
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 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 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.
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.
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 string 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.
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:
PEG.js is developed by David Majda (@dmajda). The Bower package is maintained by Michel Krämer (@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.
FAQs
Parser generator for JavaScript
The npm package pegjs receives a total of 286,369 weekly downloads. As such, pegjs popularity was classified as popular.
We found that pegjs demonstrated a not healthy version release cadence and project activity because the last version was released a year ago. It has 1 open source maintainer collaborating on the project.
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