Wordpress custom rewrite example

functions.php

/**
 * Custom rewrite rule
 * 1) conseils-pratiques/trucs-et-astuces clienteles
 *
 * Note: using page_id param seems important (no luck with pagename param)
 *
 * @param $wp_rewrite
 */
function kw_custom_rewrite( $wp_rewrite ) {
    $feed_rules = array(
        '^page-slug/sub-page-slug/([^/]+)/?'  =>  'index.php?page_id=15&some_new_parameter=' . $wp_rewrite->preg_index(1),
    );
    $wp_rewrite->rules = $feed_rules + $wp_rewrite->rules;
}
function kw_custom_wp_querystring() {
    add_rewrite_tag('%some_new_parameter%','([^&]+)');
}
add_filter( 'generate_rewrite_rules', 'kw_custom_rewrite' );
add_action( 'init', 'kw_custom_wp_querystring');