wp-add-page-filter-rewrite-rule.php

<?php

add_action('init', 'hwk_add_rewrite_rule');
function hwk_add_rewrite_rule(){
    add_rewrite_tag('%page_filter%', '([^&]+)');
    add_rewrite_rule('^contact/([^/]*)/?', 'index.php?pagename=contact&page_filter=$matches[1]', 'top');
}

// example.com/contact/commercial
// page-contact.php:
// get_query_var('page_filter');
// Result: 'commercial';