kach/nearley

Health v2 · maintenance only

Adoption not part of the score

3742 stars · 235 forks observed · 2026-08-28

What it is AI-extracted, prompt v1, taxonomy v1, 2026-08-29, confidence not recorded

nearley is a JavaScript parser toolkit built on the Earley parsing algorithm, consisting of a BNF-based grammar DSL compiler and a lightweight streaming parser runtime. It handles any context-free grammar including left-recursive and ambiguous ones, works in Node.js and browsers, and ships with tooling for testing, unparser-based fuzzing, and railroad diagram generation.

Use cases

• parse a custom DSL or configuration file format in JavaScript
• build a compiler frontend for a programming language
• parse file formats that PEG.js or Jison choke on, like left-recursive grammars
• handle ambiguous grammars and get all possible parse trees
• write a streaming parser for a REPL that consumes input incrementally
• generate railroad diagrams to document a grammar
• generate random valid strings from a grammar for fuzz testing
• teach or study parsing algorithms and computational linguistics

When to choose

• you need to parse arbitrary or ambiguous context-free grammars, including left recursion
• you want a streaming parser with graceful error handling in Node or the browser
• you need all possible parsings for an ambiguous grammar
• you want tooling like fuzzers, railroad diagrams, and grammar tests out of the box
• you're building a DSL, markup language, or small language compiler in JavaScript

When to avoid

• you need maximum parsing speed on simple LL(k) grammars where hand-written or PEG parsers are faster
• you want automatic AST generation with minimal boilerplate rather than writing postprocessors
• you're parsing a well-known format like JSON where a dedicated parser is simpler
• you need parser combinators or a runtime grammar-construction API instead of a compiled grammar file

Facets

library · maturity stable

parser compiler developer-tools parsers programming-languages developer-tools browser cross-platform earley-algorithm parser-generator grammar-dsl streaming-parser ambiguous-grammars lexer-integration railroad-diagrams fuzzing scannerless-parsing natural-language-processing nodejs javascript

Member repositories

For agents

markdown · JSON · MCP: product_card(name="kach/nearley")