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 command-line version, install Node.js and npm first. You can then install PEG.js:
$ npm install pegjs
Once installed, you can use the pegjs command to generate your parser from a grammar and use the JavaScript API from Node.js.
Download the PEG.js library (regular or minified version) and include it in your web page or application using the <script> tag.
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.
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 through 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 throw an exception if the grammar is invalid. The exception will contain message property with more details about the error.
To get parser’s source code, call the toSource method on 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 line, column 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 also start parsing from a specific rule in the grammar. Just pass the rule name to the parse method as a second parameter.
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. 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:
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.
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”).
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 and. 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 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 has access to all variables and functions defined in the initializer at the beginning of the grammar. 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 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 has access to all variables and functions defined in the initializer at the beginning of the grammar. Curly braces in the predicate code must be balanced.
Match the expression and remember its match result under given lablel. 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 has access to all variables and functions defined in the initializer at the beginning of the grammar. 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). 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 patch or 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 316,631 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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