Search by product SKU

/**
 * Search by product SKU
 */
function wpninja_product_search_sku( $join, $query ) {
	if ( ! $query->is_main_query() || is_admin() || ! is_search() || ! is_woocommerce() ) {
		return $join;
	}

	global $wpdb;

	$join .= " LEFT JOIN {$wpdb->postmeta} wpninja_post_meta ON {$wpdb->posts}.ID = wpninja_post_meta.post_id ";

	return $join;
}

add_filter( 'posts_join', 'wpninja_product_search_sku', 10, 2 );

/**
 * Modify the search query with posts_where.
 */
function wpninja_product_search_where( $where, $query ) {
	if ( ! $query->is_main_query() || is_admin() || ! is_search() || ! is_woocommerce() ) {
		return $where;
	}

	global $wpdb;

	$where = preg_replace(
		"/\(\s*{$wpdb->posts}.post_title\s+LIKE\s*(\'[^\']+\')\s*\)/",
		"({$wpdb->posts}.post_title LIKE $1) OR (wpninja_post_meta.meta_key = '_sku' AND wpninja_post_meta.meta_value LIKE $1)", $where );

	return $where;
}

add_filter( 'posts_where', 'wpninja_product_search_where', 10, 2 );