kach/nearley

Nearley is a JavaScript library for building parsers. A parser is a program that reads text written in a specific format and turns it into structured data your code can work with. If you have ever needed to interpret a custom file format, a mini programming language, or a structured command syntax, a parser is what you would build, and nearley is a toolkit that makes that job much easier. The library is built around the Earley parsing algorithm, which is a technique known for being able to handle a very wide range of grammar definitions, including some patterns that trip up other popular JavaScript parser tools. Most other parsers in the JavaScript ecosystem have restrictions on the kinds of grammars they accept, such as struggling with left-recursive rules (where a grammar rule refers back to itself on the left side). Nearley handles these without issue. You describe your language using a purpose-built grammar syntax, write those rules in a .ne file, and nearley compiles that into a parser you can use in Node.js or in the browser. The library also includes a set of supporting tools: you can generate railroad diagrams (visual flowcharts of your grammar), create fuzz testers to throw random inputs at your parser, and write grammar-aware tests. It integrates with several editors and works with external lexers, with moo being the recommended option. Nearley supports streaming input, meaning it can parse data that arrives incrementally rather than all at once. It also handles ambiguous grammars, cases where a single input can be interpreted in more than one valid way, by returning all possible parsings. The README notes real-world uses including university AI and linguistics courses, file format parsers, markup language compilers, and compilers for real programming languages. The nearley compiler itself is built with nearley.