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add_filter( 'posts_search', 'db_filter_authors_search' );
function db_filter_authors_search( $posts_search ) {
 
	// Don't modify the query at all if we're not on the search template
	// or if the LIKE is empty
	if ( !is_search() || empty( $posts_search ) )
		return $posts_search;
 
	global $wpdb;
	// Get all of the users of the blog and see if the search query matches either
	// the display name or the user login
	add_filter( 'pre_user_query', 'db_filter_user_query' );
	$search = sanitize_text_field( get_query_var( 's' ) );
	$args = array(
		'count_total' => false,
		'search' => sprintf( '*%s*', $search ),
		'search_fields' => array(
			'display_name',
			'user_login',
		),
		'fields' => 'ID',
	);
	$matching_users = get_users( $args );
	remove_filter( 'pre_user_query', 'db_filter_user_query' );
	// Don't modify the query if there aren't any matching users
	if ( empty( $matching_users ) )
		return $posts_search;
	// Take a slightly different approach than core where we want all of the posts from these authors
	$posts_search = str_replace( ')))', ")) OR ( {$wpdb->posts}.post_author IN (" . implode( ',', array_map( 'absint', $matching_users ) ) . ")))", $posts_search );
	return $posts_search;
}
/**
 * Modify get_users() to search display_name instead of user_nicename
 */
function db_filter_user_query( &$user_query ) {
 
	if ( is_object( $user_query ) )
		$user_query->query_where = str_replace( "user_nicename LIKE", "display_name LIKE", $user_query->query_where );
	return $user_query;
}