Prevent Flux Stepper from advancing to the next step if EU VAT number is invalid.

functions.php

<?php
/**
 * Prevent Flux Stepper from advancing to the next step if EU VAT number is invalid.
 */
add_filter( 'flux_checkout_check_for_inline_errors', 'flux_validate_eu_vat_number', 10, 1 );
function flux_validate_eu_vat_number( $messages ) {
    // Check if fields were sent in the AJAX request
    if ( ! isset( $_POST['fields'] ) || ! is_array( $_POST['fields'] ) ) {
        return $messages;
    }
    // Retrieve the fields posted during the 'Next step' AJAX request
    $fields = wc_clean( wp_unslash( $_POST['fields'] ) );
    // Ensure our target field is present in the current step payload
    if ( isset( $fields['woocommerce_eu_vat_number'] ) ) {
        $vat_value = $fields['woocommerce_eu_vat_number']['value'];
        // Remove spaces, dashes, and dots (matching your JS: .replace(/[\s\-\.]/g, '') )
        $vat_cleaned = preg_replace( '/[\s\-\.]/', '', $vat_value );
        // EU VAT Regex pattern
        $pattern = '/^(AT|BE|BG|CY|CZ|DE|DK|EE|EL|ES|FI|FR|HR|HU|IE|IT|LT|LU|LV|MT|NL|PL|PT|RO|SE|SI|SK|XI)[A-Z0-9]{2,13}$/i';
        // Check if the value is empty or fails the regex check
        if ( empty( $vat_value ) || ! preg_match( $pattern, $vat_cleaned ) ) {
            // Guarantee an array structure exists for this field
            if ( ! isset( $messages['woocommerce_eu_vat_number'] ) || ! is_array( $messages['woocommerce_eu_vat_number'] ) ) {
                $messages['woocommerce_eu_vat_number'] = array();
            }
            
            // Set the custom error flag & message to block the stepper
            $messages['woocommerce_eu_vat_number']['message']  = __( 'You must enter a valid EU VAT number.', 'woocommerce' );
            $messages['woocommerce_eu_vat_number']['isCustom'] = true; 
        }
    }
    
    return $messages;
}