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https://github.com/ohmjs/ohm

A library and language for building parsers, interpreters, compilers, etc.
https://github.com/ohmjs/ohm

compiler grammars javascript parser parsing peg typescript

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A library and language for building parsers, interpreters, compilers, etc.

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README 

        
# [Ohm](https://ohmjs.org/) · [![NPM](https://img.shields.io/npm/v/ohm-js.svg)](https://www.npmjs.com/package/ohm-js) [![Node.js CI](https://github.com/ohmjs/ohm/actions/workflows/node.js.yml/badge.svg)](https://github.com/ohmjs/ohm/actions/workflows/node.js.yml) [![Chat on Discord](https://img.shields.io/badge/chat-on%20discord-7289da.svg?sanitize=true)](https://discord.gg/KwxY5gegRQ)



Ohm is a parsing toolkit consisting of a library and a domain-specific language. You can use it to parse custom file formats or quickly build parsers, interpreters, and compilers for programming languages.



The _Ohm language_ is based on [parsing expression grammars](http://en.wikipedia.org/wiki/Parsing_expression_grammar)

(PEGs), which are a formal way of describing syntax, similar to regular expressions and context-free

grammars. The _Ohm library_ provides a JavaScript interface for creating parsers, interpreters, and

more from the grammars you write.



- **Full support for left-recursive rules** means that you can define left-associative operators in a natural way.

- **Object-oriented grammar extension** makes it easy to extend an existing language with new syntax.

- **Modular semantic actions.** Unlike many similar tools, Ohm completely

  separates grammars from semantic actions. This separation improves modularity and extensibility, and makes both grammars and semantic actions easier to read and understand.

- **Online editor and visualizer.** The [Ohm Editor](https://ohmjs.org/editor/) provides instant feedback and an [interactive visualization](https://dubroy.com/blog/visualizing-packrat-parsing/) that makes the entire execution of the parser visible and tangible. It'll [make you feel like you have superpowers](https://twitter.com/kylestetz/status/1349770893120172036). 💪



Some awesome things people have built using Ohm:



- [Seymour](https://harc.github.io/seymour-live2017/), a live programming environment for the classroom.

- [Shadama](https://tinlizzie.org/~ohshima/shadama2/live2017/), a particle simulation language designed for high-school science.

- [turtle.audio](http://turtle.audio/), an audio environment where simple text commands generate lines that can play music.

- A [browser-based tool](https://www.arthurcarabott.com/konnakkol/) that turns written _Konnakkol_ (a South Indian vocal percussion art) into audio.

- [Wildcard](https://www.geoffreylitt.com/wildcard/), a browser extension that empowers anyone to modify websites to meet their own specific needs, uses Ohm for its spreadsheet formulas.



## Getting Started



The easiest way to get started with Ohm is to use the [interactive editor](https://ohmjs.org/editor/). Alternatively, you can play with one of the following examples on JSFiddle:



- [Basic parsing example](https://jsfiddle.net/pdubroy/p3b1v2xb/)

- [Arithmetic example with semantics](https://jsfiddle.net/pdubroy/15k63qae/)



### Resources



- Tutorial: [Ohm: Parsing Made Easy](https://nextjournal.com/dubroy/ohm-parsing-made-easy)

- The [math example](https://github.com/ohmjs/ohm/tree/main/examples/math/index.html) is extensively commented and is a good way to dive deeper.

- [Examples](https://github.com/ohmjs/ohm/tree/main/examples/)

- [Documentation](doc/README.md)

- For community support and discussion, join us on [Discord](https://discord.gg/KwxY5gegRQ), [GitHub Discussions](https://github.com/ohmjs/ohm/discussions), or the [ohm-discuss mailing list](https://groups.google.com/u/0/g/ohm-discuss).

- For updates, follow [@\_ohmjs on Twitter](https://twitter.com/_ohmjs).



### Installation



#### On a web page



To use Ohm in the browser, just add a single `` tag to your page:



```html

<!-- Development version of Ohm from unpkg.com -->

<script src="https://unpkg.com/ohm-js@17/dist/ohm.js">

```



or



```html



```



This creates a global variable named `ohm`.



#### Node.js



First, install the `ohm-js` package with your package manager:



- [npm](http://npmjs.org): `npm install ohm-js`

- [Yarn](https://yarnpkg.com/): `yarn add ohm-js`

- [pnpm](https://pnpm.io/): `pnpm add ohm-js`



Then, you can use `require` to use Ohm in a script:



```js

const ohm = require('ohm-js');

```



Ohm can also be imported as an ES module:



```js

import * as ohm from 'ohm-js';

```



#### Deno



To use Ohm from [Deno](https://deno.land/):



```js

import * as ohm from 'https://unpkg.com/ohm-js@17';

```



### Basics



#### Defining Grammars



![Instantiating a grammar](https://ohmjs.org/img/docs/instantiating-grammars.png)



To use Ohm, you need a grammar that is written in the Ohm language. The grammar provides a formal

definition of the language or data format that you want to parse. There are a few different ways

you can define an Ohm grammar:



- The simplest option is to define the grammar directly in a JavaScript string and instantiate it

  using `ohm.grammar()`. In most cases, you should use a [template literal with String.raw](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/raw):



  ```js

  const myGrammar = ohm.grammar(String.raw`

    MyGrammar {

      greeting = "Hello" | "Hola"

    }

  `);

  ```



- **In Node.js**, you can define the grammar in a separate file, and read the file's contents and instantiate it using `ohm.grammar(contents)`:



  In `myGrammar.ohm`:



        MyGrammar {

          greeting = "Hello" | "Hola"

        }



  In JavaScript:



  ```js

  const fs = require('fs');

  const ohm = require('ohm-js');

  const contents = fs.readFileSync('myGrammar.ohm', 'utf-8');

  const myGrammar = ohm.grammar(contents);

  ```



For more information, see [Instantiating Grammars](doc/api-reference.md#instantiating-grammars) in the API reference.



#### Using Grammars



![Matching input](https://ohmjs.org/img/docs/matching.png)



Once you've instantiated a grammar object, use the grammar's `match()` method to recognize input:



```js

const userInput = 'Hello';

const m = myGrammar.match(userInput);

if (m.succeeded()) {

  console.log('Greetings, human.');

} else {

  console.log("That's not a greeting!");

}

```



The result is a MatchResult object. You can use the `succeeded()` and `failed()` methods to see whether the input was recognized or not.



For more information, see the [main documentation](doc/README.md).



### Debugging



Ohm has two tools to help you debug grammars: a text trace, and a graphical visualizer.



[![Ohm Visualizer](https://ohmjs.org/img/docs/visualizer-small.png)](https://ohmjs.org/editor)



You can [try the visualizer online](https://ohmjs.org/editor).



To see the text trace for a grammar `g`, just use the [`g.trace()`](doc/api-reference.md#trace)

method instead of `g.match`. It takes the same arguments, but instead of returning a MatchResult

object, it returns a Trace object — calling its `toString` method returns a string describing

all of the decisions the parser made when trying to match the input. For example, here is the

result of `g.trace('ab').toString()` for the grammar `G { start = letter+ }`:



```

ab         ✓ start ⇒  "ab"

ab           ✓ letter+ ⇒  "ab"

ab             ✓ letter ⇒  "a"

ab                 ✓ lower ⇒  "a"

ab                   ✓ Unicode [Ll] character ⇒  "a"

b              ✓ letter ⇒  "b"

b                  ✓ lower ⇒  "b"

b                    ✓ Unicode [Ll] character ⇒  "b"

               ✗ letter

                   ✗ lower

                     ✗ Unicode [Ll] character

                   ✗ upper

                     ✗ Unicode [Lu] character

                   ✗ unicodeLtmo

                     ✗ Unicode [Ltmo] character

           ✓ end ⇒  ""

```



## Publishing Grammars



If you've written an Ohm grammar that you'd like to share with others, see

our [suggestions for publishing grammars](./doc/publishing-grammars.md).



## Contributing to Ohm



Interested in contributing to Ohm? Please read [CONTRIBUTING.md](./CONTRIBUTING.md)

and the [Ohm Contributor Guide](doc/contributing.md).