You may refer to hook with example that will guide you how to apply validation.
Or
You can send me problem URL with test data you want to validate and I'll try to fix your issue.
*** Please make a FULL BACKUP of your database and website.***
I would also eventually need to request temporary access (WP-Admin and FTP) to your site. Preferably to a test site where the problem has been replicated if possible in order to be of better help and check if some configurations might need to be changed.
I would additionally need your permission to de- and re-activate Plugins and the Theme, and to change configurations on the site. This is also a reason the backup is really important. If you agree to this, please use the form fields I have enabled below to provide temporary access details (wp-admin and FTP).
I have set the next reply to private which means only you and I have access to it.
I've added following code to your custom.php file:
add_filter('cred_form_validate','my_validation',10,2);
function my_validation($field_data, $form_data)
{
//field data are field values and errors
list($fields,$errors)=$field_data;
//validate if specific form
if ($form_data['id']==283)
{
//check my_field value
if (strlen(trim($fields['wpcf-id-number']['value'])) != 13 )
{
//set error message for my_field
$errors['wpcf-id-number']='Please enter exact 13 digits';
}
}
//return result
return array($fields,$errors);
}
Now the field is validated for exact 13 characters.
I hope above solution will help you to resolve your issue.