theme_region_test.module

<?php


/**
 * @file
 * Provides hook implementations for testing purposes.
 */
declare (strict_types=1);

/**
 * Implements hook_preprocess_HOOK() for region templates.
 */
function theme_region_test_preprocess_region(&$variables) {
    if ($variables['region'] == 'sidebar_first') {
        $variables['attributes']['class'][] = 'new_class';
    }
}