LinkExternalProtocolsConstraint.php

<?php

namespace Drupal\link\Plugin\Validation\Constraint;

use Drupal\Core\StringTranslation\TranslatableMarkup;
use Drupal\Core\Validation\Attribute\Constraint;
use Symfony\Component\Validator\Constraint as SymfonyConstraint;

/**
 * Defines a protocol validation constraint for links to external URLs.
 */
class LinkExternalProtocolsConstraint extends SymfonyConstraint {
  
  /**
   * The error message.
   *
   * @var string
   */
  public $message = "The path '@uri' is invalid.";

}