Mandatory reference number field

gistfile1.txt

// Validate before insert/update for the 'property' post type
add_filter('wp_insert_post_data', function ($data, $postarr) {
    if (($data['post_type'] ?? '') !== 'property') {
        return $data;
    }

    // Try to read from POST (classic) or meta_input (REST/Gutenberg)
    $value = isset($_POST['_reference_number'])
        ? trim(wp_unslash($_POST['_reference_number']))
        : '';

    if ($value === '' && isset($postarr['meta_input']['_reference_number'])) {
        $value = trim((string) $postarr['meta_input']['_reference_number']);
    }

    $is_valid = (bool) preg_match('/^[A-Za-z0-9\-]{4,8}$/', $value);

    if ($data['post_status'] === 'publish' && !$is_valid) {
        //$data['post_status'] = 'draft'; // block publish
        add_filter('redirect_post_location', function ($location) {
            return add_query_arg('reference_error', 1, $location);
        });
    }

    return $data;
}, 10, 2);

// Show an error message after redirect
add_action('admin_notices', function () {
    if (isset($_GET['reference_error'])) {
        echo '<div class="notice notice-error"><p><strong>Reference Number</strong> is required and must match <code>[A-Za-z0-9\-]{4,8}</code>.</p></div>';
    }
});

add_action('admin_enqueue_scripts', function($hook){
    if (!in_array($hook, ['post.php','post-new.php'], true)) return;

    $screen = get_current_screen();
    if (!$screen || $screen->post_type !== 'property') return;

    wp_add_inline_script('jquery-core', "
    jQuery(function($){
        var form = $('#post');
        form.on('submit', function(e){
            var field = $('#_reference_number');
            if (!field.length) return; // nothing to validate

            var val = $.trim(field.val() || '');
            var ok = /^[A-Za-z0-9\\-]{4,8}$/.test(val);

            if (!ok){
                e.preventDefault();
                alert('Reference Number is required and must match [A-Za-z0-9-]{4,8}.');
                field.focus();
            }
        });
    });
    ");
});