Chapter 6. XPath and XPointer

XML has often been compared to a database because of the way it packages information for easy retrieval. Ignoring the obvious issues of speed and optimization, this isn't a bad analogy. Element names and attributes put handles on data, just as SQL tables use table and field names . Element structure supplies even more information in the form of context (e.g., element A is the child of element B, which follows element C, etc.). With a little knowledge of the markup language, you can locate and extract any piece of information.

This is useful for many reasons. First, you might want to locate specific data from a known location (called a path ) in a particular document. Given a URI and path, you ought to be able to fetch that data automatically. The other benefit is that you can use path information to get really specific about processing a class of documents. Instead of just giving element name or attribute value to configure a stylesheet as with CSS, you could incorporate all kinds of extra contextual details, including data located anywhere in the document. For example, you could specify that items in a list should use a particular kind of bullet given in a metadata section at the beginning of the document.

To express path information in a standard way, the W3C recommends the XML Path Language (also known as XPath). Quickly following on the heels of the XML recommendation, XPath opens up many possibilities for documents and facilitates technologies such as XSLT and DOM. The XML Pointer Language (XPointer) extends XPath into a wider realm, allowing you to locate information in other documents.