Or with Wordpress Rewrite Rules in functions.php :

function feed_dir_rewrite( $wp_rewrite ) {
$feed_rules = array( 'search/(.+)' => 'index.php?s=' . $wp_rewrite->preg_index(1));
$wp_rewrite->rules = $feed_rules + $wp_rewrite->rules;
}
add_filter( 'generate_rewrite_rules', 'feed_dir_rewrite' );