XML is an important practical paradigm in information technology and has a broad range of applications. How to access and retrieve the XML data is crucial to these applications. There are two standard ways for accessing and manipulating XML data, the Simple API for XML (SAX) and the Document Object Model (DOM). However, when an application needs to traverse through XML data, it is not easy to retrieve the required data with these two standard ways. XML data is impossible to be retrieved back and forth by SAX, and the graph-oriented DOM notation is not easy to work with. With such limitation, the W3C supervises the development of three important languages: XPath [3], XQuery and XSLT for exploring and querying XML. Among these three languages, XPath is the key and cornerstone language for the other two. XPath defines expressions for traversing an XML document and specifies the set of nodes (XPath 1.0) or the sequence of nodes (XPath 2.0) in the XML document.