Question

Flarum uses the powerful s9e TextFormatter library to format posts from plain markup into HTML. You should become familiar with how TextFormatter works before you attempt to extend it.

In Flarum, post content is formatted with a minimal TextFormatter configuration by default. The bundled Markdown and BBCode extensions simply enable the respective plugins on this TextFormatter configuration.

Configuration

You can configure the TextFormatter Configurator instance, as well as run custom logic during parsing and rendering, using the Formatter extender:

use Flarum\Extend;
use Psr\Http\Message\ServerRequestInterface as Request;
use s9e\TextFormatter\Configurator;
use s9e\TextFormatter\Parser;
use s9e\TextFormatter\Renderer;

return [
    (new Extend\Formatter)
        // Add custom text formatter configuration
        ->configure(function (Configurator $config) {
            $config->BBCodes->addFromRepository('B');
        })
        // Modify raw text before it is parsed.
        // This callback should return the modified text.
        ->parse(function (Parser $parser, $context, $text) {
            // custom logic here
            return $newText;
        })
        // Modify the XML to be rendered before rendering.
        // This callback should return the new XML.
        // For example, in the mentions extension, this is used to
        // provide the username and display name of the user being mentioned.
        // Make sure that the last $request argument is nullable (or omitted entirely).
        ->render(function (Renderer $renderer, $context, $xml, Request $request = null) {
            // custom logic here
            return $newXml;
        })
];

With a good understanding of TextFormatter, this will allow you to achieve anything from simple BBCode tag additions to more complex formatting tasks like Flarum's Mentions extension.