DataTransformer for Markdown Syntax and custom Markdown input field

MarkDownTransformer.php

<?php
namespace Common\ContentBundle\DataTransformer;

use Symfony\Component\Form\DataTransformerInterface;

/**
 * Class MarkdownTransformer
 * @package Common\ContentBundle\DataTransformer
 */
class MarkdownTransformer implements DataTransformerInterface
{

    /**
     * Transforms the HTML string back to markdown.
     *
     * @param string $val
     * @return string
     */
    public function transform($val)
    {
        // strong to double asterisks:
        $val = preg_replace("/\<strong\>(.+?)\<\/strong\>/i", "**$1**", $val);

        // italic to double underscores:
        $val = preg_replace("/\<em\>(.+?)\<\/em\>/i", "__$1__", $val);

        return $val;
    }

    /**
     * Transforms the markdown string to html.

     * @param  string $val
     * @return string
     */
    public function reverseTransform($val)
    {
        // double asterisks to strong:
        $val = preg_replace("/\*\*(.+?)\*\*/i", "<strong>$1</strong>", $val);

        // double underscores to italic:
        $val = preg_replace("/\_\_(.+?)\_\_/i", "<em>$1</em>", $val);

        return $val;
    }
}

MarkDownType.php

<?php
namespace Common\Form;

use Common\ContentBundle\DataTransformer\MarkdownTransformer;
use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\Extension\Core\Type\TextType;
use Symfony\Component\OptionsResolver\OptionsResolver;
use Symfony\Component\Form\FormBuilderInterface;

/**
 * Class MarkDownType
 * @package Common\Form
 */
class MarkDownType extends AbstractType
{

    /**
     * @param FormBuilderInterface $builder
     * @param array                $options
     */
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $transformer = new MarkdownTransformer();
        $builder->addModelTransformer($transformer);
    }

    /**
     * @param OptionsResolver $resolver
     */
    public function configureOptions(OptionsResolver $resolver)
    {
        $resolver->setDefaults(array(
            'attr' => array(
                'help' => 'To bold words, surround them with double asterisks. e.g.: This is <strong>**bold**</strong> now. To emphasize words surround the with double underscores.',
            ),
        ));
    }

    /**
     * @return mixed
     */
    public function getParent()
    {
        return TextType::class;
    }
}