pegjs

What is pegjs?

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.

What are pegjs's main functionalities?

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

Other packages similar to pegjs

nearley

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.

antlr4

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

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.

README

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