Hi,
Try to edit "includes/post-functions.php" file and go around line 479 ~ 481, you'll see these code :
if($validation_result !== true){
$error_elements[$element_id] = $validation_result;
}
then put these code exactly bellow that line
if ($form_id == 5) {
if (strlen(trim($target_input[$element_name_5])) < 5 && empty($error_elements[$element_id]) ) {
$error_elements[$element_id] = "Zip code must more than 5 Character";
}
}
Those code will work on form id 5, make sure you change the form id with yours
MachForm Support