To parse markdown, front matter, and liquid tags, Librarian uses Parsed, a generic content parser library that supports the format used by DEV / Forem. Parsed was built-in in the first versions of Librarian, but we have moved this implementation to a standalone package (available as librarianphp/parsed on Packagist) so that it can be used within systems other than Librarian.

Parsed comes with a few basic liquid tags already implemented, but it offers support for creating your own custom liquid tags, which is the actual exciting part.

Liquid tags are classes that implement the CustomTagParserInterface. They need to implement a method named parse, which receives the string provided to the liquid tag when called from the markdown file. For instance, this is the full code for the video liquid tag parser class:

<?php
#app/CustomTagParser/VideoTagParser.php

namespace Parsed\CustomTagParser;

use Parsed\CustomTagParserInterface;

class VideoTagParser implements CustomTagParserInterface
{
    public function parse($tag_value, array $params = [])
    {
        return "<video controls>" .
         "<source src=\"$tag_value\" type=\"video/mp4\">" .
         "Your browser does not support the video tag." .
         "</video>";
    }
}

You'll have to include your custom tag parser class within the ContentParser, and this must be done at the application entry point web/index.php, where the app is bootstrapped. For instance, the following piece of code boots the application with $app->librarian->boot() and then proceeds to register the custom tag parser:

# web/index.php
# ...

$app->addService('twig', new TwigServiceProvider());
$app->addService('router', new RouterServiceProvider());
$app->addService('content', new ContentServiceProvider());
$app->addService('librarian', new LibrarianServiceProvider());
$app->addService('devto', new DevtoServiceProvider());

$app->librarian->boot();
$app->content->registerTagParser('video', new VideoTagParser());

# ...

Note: The built-in tag parsers are already registered within ContentParser. These are: video, audio, twitter, youtube and github.

For instance, if you have in your markdown:

 {% video /videos/test.mp4 %}

The tag will be replaced by the following code:

<video controls>
   <source src="/videos/test.mp4" type="video/mp4">
    Your browser does not support the video tag.
</video>