nearley ships with a host of tools to help you develop, test, and maintain your grammars easily.

nearley-test: Exploring a parser interactively

A global install of nearley provides nearley-test, a simple command-line tool you can use to inspect what a parser is doing. You input a generated grammar.js file, and then give it some input to test the parser against. nearley-test prints out the output if successful, and optionally pretty-prints the internal parse table used by the algorithm. This is helpful to test a new parser.

nearley-unparse: The Unparser

The Unparser takes a (compiled) parser and outputs a random string that would be accepted by the parser.

$ nearley-unparse -s number <(nearleyc builtin/

You can use the Unparser to…

The Unparser outputs as a stream by continuously writing characters to its output pipe. So, if it “goes off the deep end” and generates a huge string, you will still see output scrolling by in real-time.

To limit the size of the output, you can specify a bound on the depth with the -d flag. This switches the Unparser to a different algorithm. A larger depth bound corresponds to larger generated strings.

As far as I know, nearley is the only parser generator with this feature. It is inspired by Roly Fentanes’ randexp, which does the same thing with regular expressions.

nearley-railroad: Automagical Railroad Diagrams

nearley lets you convert your grammars to pretty SVG railroad diagrams that you can include in webpages, documentation, and even papers.

$ nearley-railroad -o grammar.html

Railroad demo

See a bigger example here.

(This feature is powered by railroad-diagrams by tabatkins.)

Other Tools

This section lists nearley tooling created by other developers. These tools are not distributed with nearley, so if you have problems, please contact the respective author for support instead of opening an issue with nearley.

Atom users can write nearley grammars with this plugin by Bojidar Marinov.

TextMate and Sublime Text users can use this language by yours truly.

Sublime Text users can write nearley grammars with this syntax by liam4.

Vim users can use this plugin by Tim (based on this older plugin by Andrés Arana).

Visual Studio Code users can use this extension by Pouya Kary.

Python users can convert nearley grammars to Python using lark by Erez.

Node users can programmatically access the unparser using nearley-there by Scott Tolksdorf.

Browser users can use nearley-playground by Guillermo Webster to explore nearley interactively in the browser. There is also a Mac app by Pouya Kary.

Webpack users can use nearley-loader by Andrés Arana to load grammars directly.

Gulp users can use gulp-nearley by Joseph Junker to compile grammars with a gulpfile.