Alternate config

module.config.php

<?php
return array(
    'router' => array('routes' => array(
        'api' => array(
            'type' => 'Segment',
            'options' => array(
                'route' => '/api/v1/:controller[/:id]',
                'defaults' => array(
                    '__NAMESPACE__' => 'Api\V1\Controller',
                    'controller'    => 'User', // just an example
                ),
            ),
        ),
    )),

    'controllers' => array(
        'invokables' => array(
            'Api\V1\Controller\User'  => 'Api\Controller\UserController',
            'Api\V1\Controller\Group' => 'Api\Controller\GroupController',
            // etc.
        ),
    ),
);