To add custom validation on the WooCommerce checkout page for the first and last names to contain only letters, you can use the `woocommerce_after_checkout_validation` filter hook. This hook allows you to perform custom validation on the submitted checkout data before processing the order. Here’s how you can add the custom validation: 1. Open your theme’s `functions.php` file or a custom plugin file. 2. Add the following code to implement the validation: function custom_validate_checkout_names($fields, $errors) { // Validate First Name if (isset($_POST[‘billing_first_name’]) && !preg_match(‘/^[A-Za-z]+$/’, $_POST[‘billing_first_name’])) { $errors->add(‘billing_first_name’, __(‘First name must contain only letters.’, ‘text-domain’)); } // Validate Last Name if (isset($_POST[‘billing_last_name’]) && !preg_match(‘/^[A-Za-z]+$/’, $_POST[‘billing_last_name’])) { $errors->add(‘billing_last_name’, __(‘Last name must contain only letters.’, ‘text-domain’)); } return $fields; } add_filter(‘woocommerce_after_checkout_validation’, ‘custom_validate_checkout_names’, 10, 2);   In this code, we use the `woocommerce_after_checkout_validation` filter hook to add our custom validation function `custom_validate_checkout_names`. The `custom_validate_checkout_names` function checks the submitted first and last names for containing only