getTagAndClasses example

    // The preceding constraints (in this case: CKEditor5Element) must be valid.     if ($this->hasViolationsForPrecedingConstraints($constraint)) {
      return;
    }

    $text_editor = $this->createTextEditorObjectFromContext();

    // The single tag for which a style is specified, which we are checking now.     $style_element = HTMLRestrictions::fromString($element);
    assert(count($style_element->getAllowedElements()) === 1);
    [$tag, $classes] = Style::getTagAndClasses($style_element);

    // Ensure the tag is in the range supported by the Style plugin.     $superset = HTMLRestrictions::fromString('<$any-html5-element class>');
    $supported_range = $superset->merge($style_element->extractPlainTagsSubset());
    if (!$style_element->diff($supported_range)->allowsNothing()) {
      $this->context->buildViolation($constraint->nonHtml5TagMessage)
        ->setParameter('@tag', sprintf("<%s>", $tag))
        ->addViolation();
      return;
    }

    

  public function buildConfigurationForm(array $form, FormStateInterface $form_state) {
    $form['styles'] = [
      '#title' => $this->t('Styles'),
      '#type' => 'textarea',
      '#description' => $this->t('A list of classes that will be provided in the "Style" dropdown. Enter one or more classes on each line in the format: element.classA.classB|Label. Example: h1.title|Title. Advanced example: h1.fancy.title|Fancy title.<br />These styles should be available in your theme\'s CSS file.'),
    ];
    if (!empty($this->configuration['styles'])) {
      $as_selectors = '';
      foreach ($this->configuration['styles'] as $style) {
        [$tag, $classes] = self::getTagAndClasses(HTMLRestrictions::fromString($style['element']));
        $as_selectors .= sprintf("%s.%s|%s\n", $tag, implode('.', $classes), $style['label']);
      }
      $form['styles']['#default_value'] = $as_selectors;
    }

    return $form;
  }

  /** * Gets the tag and classes for a parsed style element. * * @param \Drupal\ckeditor5\HTMLRestrictions $style_element * A parsed style element. * * @return array * An array containing two values: * - a HTML tag name * - a list of classes * * @internal */