Cmaskedtextfield Optional Field

Guys I am trying to implement CMaskedTextField for car license plates

Basically it follows this pattern aa-9*-aa-9999 however for certain states the 3rd placeholder may have just 1 alphabet instead of 2. Like this aa-9*-a-9999

I am trying generalize this by using ‘?’ but it does not seam to work.

$this->widget(‘CMaskedTextField’, array(

	'mask' => 'aa-9*-a?a-9999',

	'placeholder' => '_',

	'completed' => 'function(){alert(1);}',

	'model' => $model,

	'name' => 'registration',


any help??