Introducing react-content-marker v1.0

react-content-marker is a library for React to mark content in a string based on rules. These rules can be simple strings or regular expressions. Let's look at an example.

Say you have a blob of text, and you want to make the numbers in that text more visible, for example by making them bold.

const content = 'The fellowship had 4 Hobbits but only 1 Dwarf.';

Matching numbers can be done with a simple regex: /(\d+)/. If we turn that into a parser:

const parser = {
    rule: /(\d+)/,
    tag: x => <strong>{ x }</strong>,

We can now use that parser to create a content marker, and use it to enhance our content:

import createMarker from 'react-content-marker';
const Marker = createMarker([parser]);
render(<Marker>{ content }</Marker>);

This will show:

The fellowship had 4 Hobbits but only 1 Dwarf.


Advanced usage

Passing parsers

The first thing to note is that you can pass any number of parsers to the createMarker function, and they will all be called in turn. The order of the parsers is very important though, because content that has already been marked will not be parsed again. Let's look at another example.

Say you have a rule that matches content between brackets: /({.*})/, and a rule that matches content between brackets that contain only capital letters: /({[A-W]+})/. Now let's say you are marking this content: I have {CATCOUNT} cats. Whichever rule you passed first will match the content between brackets, and the second rule will not apply. You thus need to make sure that your rules are ordered so that the most important ones come first. Generally, that means you want to have the more specific rules first.

The reason why this happens is that, behind the scene, the matched content is turned into a DOM element, and parsers ignore non-string content. With the previous example, the initial string, I have {CATCOUNT} cats, would be turned into ['I have ', <mark>{CATCOUNT}</mark>, ' cats'] after the first parser is called. The second one then only looks at 'I have ' and ' cats', which do not match.

Using regex

The second important thing to know relates to regex. You might have noticed that I put parentheses in my examples above: they are required for the algorithm to capture content. But that also gives you more flexibility: you can use a regex that matches some content that you do not want to mark. Let's say you want to match only the name of someone who's being greeted, with this rule: /hello (\w+)/i. Applying it to Hello Adrian will only mark the Adrian part of that content.

Sometimes, however, you need to use more complex regex that include several groups of parentheses. When that's the case, by default react-content-marker will mark the content of the last non-null capturing group. In such cases, you can add a matchIndex number to your parser: that index will be used to select the capture group to mark.

Here's a simple example:

const parser = {
    rule: /(hello (world|folks))/i,
    tag: x => <b>{ x }</b>,

Applying this rule to Hello World will show: Hello World. If we want to, instead, make the whole match bold, we'll have to use matchIndex:

const parser = {
    rule: /(hello (world|folks))/i,
    matchIndex: 0,
    tag: x => <b>{ x }</b>,

Now our entire string will correctly be made bold: Hello World.

Advanced example

If you're interested in looking at an advanced usage example of this library, I recommend you check out how we use it in Pontoon, Mozilla's localization platform. We have a long list of parsers there, and they have a lot of edge-cases.

Installation and stuff

react-content-marker is available on npm, so you can easily install it with your favorite javascript package manager:

npm install -D react-content-marker
# or
yarn add react-content-marker

The code is released under the BSD 3-Clause License, and is available on github. If you hit any problems with it, or have a use case that is not covered, please file an issue. And of course, you are always welcome to contribute a patch!

I hope this is useful to someone out there. It has been for me at least, on Pontoon and on several React-based side-projects. I like how flexible it is, and I believe it does more than any other similar tools I could find around the Web.