Hello everybody

I want to create a form in Zend framework. I am using the code below for a field:

$this->addElement('text', 'username', array(
    'label'      => 'Username:',
    'required'   => true,
    'filters'    => array('StringTrim'),
    'validators' => array(
        'alnum'
    )
));

This works. But now I also want to add a new validator. In this case StrinLength

$element->addValidator('StringLength', false, array(6, 20));

How can I add this validator in the array I already have? Tnx in advanced