$ npm install -g nearley
- Write your grammar:
# Match a CSS color # http://www.w3.org/TR/css3-color/#colorunits @builtin "whitespace.ne" # `_` means arbitrary amount of whitespace @builtin "number.ne" # `int`, `decimal`, and `percentage` number primitives csscolor -> "#" hexdigit hexdigit hexdigit hexdigit hexdigit hexdigit | "#" hexdigit hexdigit hexdigit | "rgb" _ "(" _ colnum _ "," _ colnum _ "," _ colnum _ ")" | "hsl" _ "(" _ colnum _ "," _ colnum _ "," _ colnum _ ")" | "rgba" _ "(" _ colnum _ "," _ colnum _ "," _ colnum _ "," _ decimal _ ")" | "hsla" _ "(" _ colnum _ "," _ colnum _ "," _ colnum _ "," _ decimal _ ")" hexdigit -> [a-fA-F0-9] colnum -> int | percentage
- Compile your grammar:
$ nearleyc csscolor.ne -o csscolor.js
- Test your grammar:
$ nearley-test -i "#00ff00" csscolor.js Parse results: [ [ '#', [ '0' ], [ '0' ], [ 'f' ], [ 'f' ], [ '0' ], [ '0' ] ] ]
- Turn your grammar into a generator:
$ nearley-unparse -n 3 csscolor.js #Ab21F2 rgb ( -29.889%,7,8172) #a40
- You try it! Type a CSS color here:
…and the parsed output will appear here!
- Create beautiful railroad diagrams to document your grammar
Output looks like:
$ nearley-railroad csscolor.ne -o csscolor.html
See a bigger example here.
- nearley is the first JS parser to use the Earley algorithm (insert your own ‘early bird’ pun here). It also implements Joop Leo's optimizations for right-recursion, making it effectively linear-time for LL(k) grammars.
- nearley lives happily in node, but doesn't mind the browser.
- nearley outputs small files. And its expressive DSL comes with plenty of syntactic sugar to keep your source files short. And sweet.
- nearley's grammar language is powerful and expressive: you can use macros, import from a large builtin library of pre-defined parser-pieces, use a tokenizer for extra performance, and more!
- nearley is built on an idiomatic streaming API. You even have access to partial parses to build predictive user interfaces.
- nearley processes left recursion without
choking. In fact, nearley will parse anything you throw at it
without complaining or going into a
- nearley handles ambiguous grammars gracefully. Ambiguous grammars can be parsed in multiple ways: instead of getting confused, nearley gives you all the parsings (in a deterministic order!).
- nearley allows for debugging with generous error detection. When it catches a parse-time error, nearley tells you exactly what went wrong and where.
- nearley is powerful enough to be bootstrapped. That means nearley uses nearley to compile parts of nearley. nearleyception!
- nearley parsers can be inverted to form
generators which output random strings that
match a grammar. Useful for writing test
cases, fuzzers, and
- You can export nearley parsers as railroad diagrams, which provide easy-to-understand documentation of your grammar.
Projects using nearley
- Artificial Intelligence and NLP: Shrdlite is a programming project in Artificial Intelligence, a course given at the University of Gothenburg and Chalmers University of Technology. It uses nearley for reading instructions in natural language (i.e. English).
- Standard formats: node-dmi is a module that reads iconstate metadata from BYOND DMI files, edtf.js is a parser for Extended Date Time Format, node-krl-parser is a KRL parser for node, bibliography is a BibTeX-to-HTML converter, styled-components converts CSS to React Native.
- Templating and files: uPresent is a markdown-based presentation authoring system, saison is a minimal templating language, and Packdown is a tool to generate human-readable archives of multiple files.
- Mathematics: Solvent is a powerful desktop calculator, and Truth-table is a tool to visualize propositional logic in truth tables.
- Domain-specific languages: Hexant is a cellular automata simulator with a DSL for custom automata, Dicetower is an advanced dice plugin for hubot, deck.zone is a language to create board games, and website-spec is a tool for functional web testing.