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/prims.ne) -6.22E94
You can use the Unparser to…
- …test your parser specification by generating lots of random expressions and making sure all of them are “correct”.
- …generate random strings from a schema (for example, random email addresses or telephone numbers).
- …create fuzzers and combinatorial stress-testers.
- …play “Mad-Libs” automatically! (Practical application: automatic grammatically valid loremtext.)
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 regex.ne -o grammar.html
See a bigger example here.
(This feature is powered by
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.
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.
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.