/**
* Set Sub Group into bracket syntax
*
* @param $sub
*
* @return $this
*/
public function setSub($sub)
{
$this->sub = null;
if (Validate::bracket($sub)) {
$this->sub = $sub;
} elseif (is_string($sub)) {
$this->sub = "[{$sub}]";
}
return $this;
}
