public function maybe($value, $subPattern = null)
    {
        return $this->add($this->sanitise($value), $subPattern, '?');
    }
public function maybe($value, $subPattern = null)
    {
        return $this->add($this->sanitise($value), $subPattern, '?');
    }
public function maybe ( $ value , $ subPattern = null ) { return $ this -> add ( $ this -> sanitise ( $ value ) , $ subPattern , '?' ) ; }
Add value or not

@param string  $value Maybe value
@param boolean $subPattern Capture sub patterns?

@return $this
Add value or not