Use of path expressions is a common feature in most XML query languages, and many evaluation methods for path expression queries have been proposed recently. However, there are few researches on the issue of optimizing regular path expression queries. In this paper, two kinds of path expression optimization principles are proposed, named path shortening and path complementing, respectively. The path shortening principle reduces the querying cost by shortening the path expressions with the knowledge of XML schema. While the path complementing principle substitutes the user queries with the equivalent lower-cost path expressions. The experimental results show that these two techniques can largely improve the performance of path expression query processing.