FormatActivationKey.php

<?php
/**
 * @copyright Lupimedia ltd 2013
 */

namespace Contentled\Filter;

use Zend\Filter\Exception;
use Zend\Filter\FilterInterface;

class FormatActivationKey implements FilterInterface
{
    /**
     * Returns the result of filtering $value
     *
     * @param  mixed $value
     * @throws Exception\RuntimeException If filtering $value is impossible
     * @return mixed
     */
    public function filter($value)
    {
        $value = str_replace('-', '', $value);
        return trim(chunk_split($value, 4, '-'), '-');
    }
} 

FormatActivationKeyFactory.php

<?php
/**
 * @copyright Lupimedia ltd 2013
 */

namespace Contentled\Filter;


use Zend\ServiceManager\FactoryInterface;
use Zend\ServiceManager\ServiceLocatorInterface;

class FormatActivationKeyFactory implements FactoryInterface
{
    /**
     * Create service
     *
     * @param ServiceLocatorInterface $serviceLocator
     * @return mixed
     */
    public function createService(ServiceLocatorInterface $serviceLocator)
    {
        return new FormatActivationKey(); // you could inject dependencies here
    }

} 

module.php

<?php
/**
 * @copyright Lupimedia ltd 2013
 */

namespace Contentled;

class Module
{
    /**
     * @return array
     */
    public function getServiceConfig()
    {
        return [
            'factories' => [
                'FormatActivationKey' => 'Contentled\Filter\FormatActivationKeyFactory',
                'Form\ConfirmRegistration' => function($sm) {
                    $form = new Form\ConfirmRegistration('confirm-registration');

                    $plugins = $sm->get('FilterManager');
                    $chain = new FilterChain();
                    $chain->setPluginManager($plugins);
                    $form->getFormFactory()->getInputFilterFactory()->setDefaultFilterChain($chain);
                    return $form;
                },
            ]
        ];
    }
}