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Getting Started

The easiest way to get started with Ohm is to use the interactive editor. Alternatively, you can play with one of the following examples on JSFiddle:



On a web page

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

<!-- Development version of Ohm from -->
<script src=""></script>


<!-- Minified version, for faster page loads -->
<script src=""></script>

This creates a global variable named ohm.


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

  • npm: npm install ohm-js
  • Yarn: yarn add ohm-js
  • pnpm: pnpm add ohm-js

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

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

As of v16.2.0, Ohm can also be imported as an ES module:

import ohm from 'ohm-js';


To use Ohm from Deno:

import ohm from '';


Defining Grammars

Instantiating a grammar

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:

    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:

    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 in the API reference.

Using Grammars

Matching input

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

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.


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

Ohm Visualizer

You can try the visualizer online.

To see the text trace for a grammar g, just use the g.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 ⇒ ""