Flux checkout - custom validatio

function.php

add_filter('woocommerce_checkout_fields', 'add_custom_checkout_field');
function add_custom_checkout_field($fields) {
    $fields['billing']['billing_custom_field'] = array(
        'type'        => 'text',
        'label'       => __('BTW-nummer'),
        'required'    => false, // has to be false, otherwise it's always checked dispite of country selected!
        'class'       => array('form-row-wide', 'tax-number-class'),
        'label_class' => array('tax-number-label'),
        'clear'       => true,
    );
    return $fields;
}


function flux_checkout_custom_inline_error_message( $messages ) {
	$value = $_POST['fields']['billing_custom_field']['value'] ?? false;
    $is_present = isset( $_POST['fields']['billing_custom_field']['value'] ) ? true : false;
	$country = $_POST['fields']['billing_country']['value'] ?? false;

	if ( $country !== 'BE' ) {
		$messages['billing_custom_field']['message'] = 'Please enter the value.';
		$messages['billing_custom_field']['isCustom'] = false;
		return $messages;
	}

	if ( $is_present && ! $value ) {
		$messages['billing_custom_field']['message'] = 'Please enter the value.';
		$messages['billing_custom_field']['isCustom'] = true;
		return $messages;
	}

	if ( ! $value ) {
		return $messages;
	}

    if ($value == '') {
        return $messages;
    }

	if ( preg_match( '/^\d{10}$/', $value ) ) {
		return $messages;
	}

	$messages['billing_custom_field']['message'] = 'BTW-nummer moet exact 10 cijfers bevatten.';
	$messages['billing_custom_field']['isCustom'] = true;

	return $messages;
} 
add_filter( 'flux_checkout_check_for_inline_errors', 'flux_checkout_custom_inline_error_message' );


// Add JavaScript to show/hide BTW-nummer field based on country selection
add_action('wp_footer', 'hide_btw_nummer_for_non_belgium');
function hide_btw_nummer_for_non_belgium() {
    if (!is_checkout()) return;
    ?>
    <script type="text/javascript">
        jQuery(document).ready(function($) {
            // Function to check if Belgium is selected and show/hide the BTW field
            function checkCountry() {
                var country = $('#billing_country').val();
                if (country === 'BE') {
                    $('.tax-number-class').show();
                } else {
                    $('.tax-number-class').hide();
                    $('#billing_custom_field').val(''); // Clear the field when hidden
					$('#billing_custom_field').closest('.form-row').removeClass('woocommerce-invalid validate-required')
                }
            }

            // Run on page load
            checkCountry();

            // Run when country is changed
            $(document).on('change', '#billing_country', function() {
                checkCountry();
            });
        });
    </script>
    <?php
}