Uppercase to Lowercase in URL

This is NOT mod_rewrite rule, it’s WordPress rewrite rule and it proposes to parse variables from URL to wp_query (most of the time).

You can apply your logic during parse_query filter hook., something like this.

add_filter( 'parse_query', function( WP_Query $wp_query ) : WP_Query {
    foreach ( [ 'symbol', 'exchange' ] as $key ) {
        if ( '' !== $wp_query->get( $key ) ) {
            $wp_query->set( $key, strtolower( $wp_query->get( $key ) ) );
        }
    }
    return $wp_query;
});