search only custom post types in wordpress

custom_post_type search

add_filter( 'pre_get_posts', 'cpt_search' );
/**
 * tgm_cpt_search function.
 *
 * This function modifies the main WordPress query to include an array of post types instead of the default 'post' post type.
 *
 * @param mixed $query The original query
 * @return $query The amended query
 *
 */
function cpt_search( $query ) {
    if ( $query->is_search )
        $query->set( 'post_type', array( 'post_type1', 'post_type2', 'post_type3') );
    return $query;
};