Rewrite rules map a pretty URL, like /deals/summer-sale/, to a query that WordPress understands. Use them when you need URLs that do not belong to a normal page, post or taxonomy.

Step 1: add the rule and a query variable

function lotgrafix_rewrite_rules() {
    add_rewrite_rule(
        '^deals/([^/]+)/?$',
        'index.php?lotgrafix_deal=$matches[1]',
        'top'
    );
}
add_action( 'init', 'lotgrafix_rewrite_rules' );

function lotgrafix_query_vars( $vars ) {
    $vars[] = 'lotgrafix_deal';
    return $vars;
}
add_filter( 'query_vars', 'lotgrafix_query_vars' );

Step 2: load your own template

function lotgrafix_deal_template( $template ) {
    $lotgrafix_deal = get_query_var( 'lotgrafix_deal' );

    if ( $lotgrafix_deal ) {
        $custom = get_template_directory() . '/lotgrafix-deal.php';

        if ( file_exists( $custom ) ) {
            return $custom;
        }
    }

    return $template;
}
add_filter( 'template_include', 'lotgrafix_deal_template' );

Step 3: use the value in the template

<?php
$lotgrafix_slug = sanitize_title( get_query_var( 'lotgrafix_deal' ) );
get_header();
?>
<h1><?php echo esc_html( $lotgrafix_slug ); ?></h1>
<?php get_footer(); ?>

Flush the rules once

WordPress caches rewrite rules. After adding a rule, go to Settings, Permalinks and click Save Changes. In a plugin, flush only on activation.

function lotgrafix_activate() {
    lotgrafix_rewrite_rules();
    flush_rewrite_rules();
}
register_activation_hook( __FILE__, 'lotgrafix_activate' );

Warning

Never call flush_rewrite_rules() on init or on every page load. It is an expensive operation that rebuilds the rules and writes to the database.

Rule with several parts

add_rewrite_rule(
    '^reports/([0-9]{4})/([^/]+)/?$',
    'index.php?lotgrafix_year=$matches[1]&lotgrafix_report=$matches[2]',
    'top'
);

Remember to register both lotgrafix_year and lotgrafix_report as query variables.