XPath in Overview

XPath was created by the World Wide Web Consortium, W3C (www.w3.org), the same group that created XML and standardized HTML. W3C has three main levels for their specifications:

• Working drafts, which are works in progress and still subject to much change

• Candidate recommendations, which are the step before a specification becomes a recommendation

• Recommendations, which are the final versions of specifications

XPath 1.0 is in recommendation status, and XPath 2.0 is in working draft status as of this writing. You can find the official XPath 1.0 Web page at http://www.w3.org/TR/xpath. Here's what W3C has to say about XPath 1.0:

XPath is the result of an effort to provide a common syntax and semantics for functionality shared between XSL Transformations (XSLT) and XPointer. The primary purpose of XPath is to address parts of an XML document. In support of this primary purpose, it also provides basic facilities for manipulation of strings, numbers and booleans. XPath uses a compact, non-XML syntax to facilitate use of XPath within URIs and XML attribute values. XPath operates on the abstract, logical structure of an XML document, rather than its surface syntax. XPath gets its name from its use of a path notation as in URLs for navigating through the hierarchical structure of an XML document.

In addition to its use for addressing, XPath is also designed so that it has a natural subset that can be used for matching (testing whether or not a node matches a pattern); this use of XPath is described in [the XSLT specification].

The whole XPath story began with XSLT. XSLT itself was the result of trying to fill a long-standing needletting people format and work with the data in general markup documents. W3C's efforts here stretch back quite a way, back to its original Document Style Semantics and Specification Language (DSSSL) specification. DSSSL was originally intended to work with Standardized General Markup Language (SGML), the ancestor of both HTML and XML, and to let you style SGML documents for presentation. However, SGML never became very popular, although its descendants like HTML and XML did.

In the XML branch of the family, W3C concentrated on its Extensible Stylesheet Language (XSL) to let users format XML in a general way, much as a word processor might. However, XSL by itself was so complex that it never really caught on eithertoday, software giants like Microsoft use Cascading Style Sheets (CSS) to format XML (Microsoft has said publicly that it considers the future of XSL in question).

However, one part of XSL did actually catch onExtensible Stylesheet Language Transformations (XSLT). XSLT doesn't worry so much about formatting XML as giving you easy access to the data in an XML document and transforming it into another kind of documentHTML is the most common, where you display the data in an XML document in HTML format. But you can also use XSLT to transform XML documents into other XML documents that might have different organization (reorganizing an XML database's records from storing purchase data by customer to using records organized by product ID, for example) or additional data (such as computing the average score for each of a set of student's records). You can also use XSLT to transform XML into plain text, or even rich text format (RTF) files or JavaScript filesin fact, just about any kind of files you want.

In this way, XSLT presents an extraordinarily powerful way of handling your XML data without resorting to a lot of programming involving XML parsers and Java or C++. All you have to do, as we've already seen, is to create an XSLT stylesheet and let an XSLT processor (like the one in Internet Explorer) do the rest. For that reason, XSLT became popular, and eventually split off from the XSL specification. And when W3C started creating XSLT 1.0, they quickly realized that giving people access to the data in XML documents using simple path expressions was such a great idea that XPath 1.0 was born.

XPath 1.0

XPath 1.0 is the version that's most supported today, and we're going to spend the first half of the book on it. XPath 1.0 was central to XSLT 1.0in fact, the two specifications became W3C recommendations on the same day (November 16, 1999). Originally a part of the XSLT specification, XPath broke out and became its own specification when W3C realized that many XML specifications needed a way to access the parts of XML documents.

As its own specification, however, XPath 1.0 could be used by many XML-related specifications, not confined to use with XSLT alone (if you've worked with XML, you know that many of the specifications are related ). That's how XPath was intended to be usedin a host language or specification of some sort , not simply on its own (although we'll see some tools later in this chapter that do use XPath by itself).

Today, XPath is essential to other XML-related specifications, such as XPointer, which lets you select and refer to fragments of XML documents as well as XQuery, which is all about addressing the data, treating that data as you would a standard database. We'll take a look at those two specifications and how XPath works with them, in this book.

XPath 2.0

The new version of XPath is version 2.0, and you can see the W3C specification for it at http://www.w3.org/TR/xpath20/. There's not a lot of software support for XPath 2.0 yet. This new version is considerably more involved than XPath 1.0and that growth is an indication of the importance of XPath. We'll be covering XPath 2.0 in the second half of this book.

XPath 2.0 is tied not just to XSLT (which is now also in version 2.0), but the new XQuery specification (XQuery is all about treating XML documents as you would treat a database file)the XSLT and XQuery groups at W3C are jointly responsible for XPath 2.0, and W3C says that XPath 2.0 and XQuery 1.0 are "generated from a common source." (In fact, XQuery 1.0 is an extension of XPath 2.0.)

That gives us the historical backgroundnow it's time to get some of the basic details about XPath itself, giving us the foundation we'll need in the coming chapters.