Polish postcode custom validation for Flexible Checkout Field

poslish_postcode_fcf.php

<?php

/**
 * Validate polish postcode.
 *
 * @param string $field_label .
 * @param string $value .
 */
function wpdesk_is_polish_postcode( $field_label, $value ) {
	if ( ! ( (bool) preg_match( '/^([0-9]{2})-([0-9]{3})$/', $value ) ) ) {
		wc_add_notice( sprintf( '%s nie jest poprawnym kodem pocztowym!', '<strong>' . $field_label . '</strong>' ), 'error' );
	}
}

/**
 * Add custom FCF validation.
 *
 * @param array $custom_validation .
 *
 * @return array
 */
function wpdesk_polish_postcode_custom_validation( $custom_validation ) {
	$custom_validation['is_polish_postcode'] = array(
		'label'    => 'Polski kod pocztowy',
		'callback' => 'wpdesk_is_polish_postcode',
	);
	return $custom_validation;
}
add_filter( 'flexible_checkout_fields_custom_validation', 'wpdesk_polish_postcode_custom_validation' );