custom_validation.module

<?php

/**
 * @file
 * Contains custom_validation.module..
 */

use Drupal\Core\Routing\RouteMatchInterface;

/**
 * Implements hook_help().
 */
function custom_validation_help($route_name, RouteMatchInterface $route_match) {
  switch ($route_name) {
    // Main module help for the custom_validation module.
    case 'help.page.custom_validation':
      $output = '';
      $output .= '<h3>' . t('About') . '</h3>';
      $output .= '<p>' . t('Just trying out some custom validation of entities') . '</p>';
      return $output;

    default:
  }
}


/**
 * Implements hook_entity_type_alter().
 */
function custom_validation_entity_type_alter(array &$entity_types) {
  $node = $entity_types['node'];
  $node->addConstraint('PreventAnon', []);
}


/**
 * Implements hook_entity_bundle_field_info_alter().
 */
function custom_validation_entity_bundle_field_info_alter(&$fields, \Drupal\Core\Entity\EntityTypeInterface $entity_type, $bundle) {
  if ($entity_type->id() == 'node' && $bundle == 'article') {
    $fields['field_shirt_size']->addConstraint('ShirtSize', ['sizes' => ['M','L','XL']]);
    $fields['field_zip_code']->addConstraint('ZipCode', ['iso' => 'IN']);
  }
}