Code-Snippets-Functions/Execute a function on a child site/WooCommerce/restrict-number-being-used-first-last-name-checkout.txt

add_action('woocommerce_after_checkout_validation', function ( $fields, $errors ) {

    if (!preg_match('/^[a-zA-Z]+$/', $fields['billing_first_name'])) {
        $errors->add('billing_first_name', 'First name must contain only alphabetic characters');
    }

    if (!preg_match('/^[a-zA-Z]+$/', $fields['billing_last_name'])) {
        $errors->add('billing_last_name', 'Last name must contain only alphabetic characters');
    }
}, 10, 2);