add_action( 'pre_get_posts','search_filter' );
function search_filter( $query ) {
    if ( !is_admin() && $query->is_main_query() ) {
        if ($query->is_search) {
            $query->set( 'post_type', array( 'post' ) );
            $query->set( 'sentence', 1 );
        }
    }
}