RewriteRule ^([a-zA-Z0-9_]+)/?([a-zA-Z]+)?$ index.php?module=$1&view=$2 [L,NC]

<script src="assets/app.js"><script>
<?php
    $module = (isset($_REQUEST['module'])?$_REQUEST['module']:'main';
    $view = (isset($_REQUEST['view'])?$_REQUEST['view']:'default';
    include 'modules/'.$module.'/'.$view.'.php';
?>