Filter Class

<?php
/**
 * @file
 * Contains \Drupal\my_module\Plugin\Filter\MyFilter.
 */
namespace Drupal\my_module\Plugin\Filter;
use Drupalilter\FilterProcessResult;
use Drupalilter\Plugin\FilterBase;
/**
 * Provides a filter that <your filter description here>
 *
 * @Filter(
 *   id = "my_module_my_filter_id",
 *   title = @Translation("My Filter Short Description."),
 *   description = @Translation("My Filter Long Description"),
 *   type = Drupalilter\Plugin\FilterInterface::TYPE_TRANSFORM_IRREVERSIBLE,
 * )
 */
class MyFilter extends FilterBase {
  /**
   * {@inheritdoc}
   * Filter tips callback. Displays help with using this filter 
   */
  public function tips($long = FALSE) {
    if ($long) {
      return $this->t("My filter's full description of what it does and how to use it");
    } else {
      return $this->t('One line description of my filter');
    }
  }
  /**
   * {@inheritdoc}
   * Filter process callback.
   */
  public function process($text, $langcode) {
    /* Do something here with $text, which is the node content
     * you are transforming.  The following is a skeleton for doing
     * shortcode style search and replace */
    if (stristr(strtoupper($text), '[SHORTCODE') !== FALSE) {
      $text = preg_replace_callback("/\[SHORTCODE\|([^\]]+)\]/i",
 
        function ($results) {
          /* Do something here with $results, which contains everything
           * from inside the square brackets after SHORTCODE, and put
           * the results in $newText */
          return $newText;
        }
       }
      , $text);
    }
    return $text;
  }
}

Notes:

• The @Filter section in the comments at the top of the file is very important and must be formatted and configured correctly.  Otherwise, your file won't be loaded.
• The filename must match the classname (e.g. "MyFilter.php" for the example above) and be placed in "src/Plugin/Filter/" relative to your module's root directory.