WooCommerce registration fileds to Uppercase

I have to transform all the registration fields (Name, Surname, email, State, Address...) to Uppercase.

I mean no CSS transform, like text-transform: uppercase; but transform in PHP before they go to database.

I can't find where I can applicate this PHP function, string strtoupper ( string $string ).

Registration fields are in woocommerce/checkout/form-billing.php file width foreach cycle.

<?php foreach ( $checkout->checkout_fields['billing'] as $key => $field ) : ?>

    <?php woocommerce_form_field( $key, $field, $checkout->get_value( $key ) ); ?>

<?php endforeach; ?>

Can you help me please?

via Chebli Mohamed