Top Ten Tips to Using XPath and XPointer
by John E. Simpson | Pages: 1, 2, 3, 4

Figure 3 shows the default expression altered to select all attributes. (Note that entire attributes are selected -- their names, the equal signs, and their values.)

Figure 3: XPath Visualiser, selecting all attributes.

Finally, as you can see in Figure 4, you can use any conformant XPath 1.0 expression to select document content. The expression in Figure 4 says to select the text nodes representing the names of all relics whose prices begin with the digit 3.

Figure 4: XPath Visualiser, selecting the names of relics with particular kinds of prices.

I encourage you to download XPath Visualiser and dig into it. Don't forget to read the documentation and perhaps participate in the discussion group at the site; both will help you understand the specific system requirements for installing and running the package -- as well as its few limitations.

6. Explore EXSLT.

This tip is not, strictly speaking, XPath-related. But in the context of XSLT stylesheets, it can significantly enhance your ability to locate and process XML content.

EXSLT -- an acronym for Extensions to XSLT -- is an unofficial but well-organized community effort to fill some of the gaps in the XSLT 1.0 Recommendation. These gaps include, for example, the ability to process dates in various ways and the ability to transform source trees into multiple result-tree documents in a single pass. There are also quite a few commonly needed mathematical operations which the XPath numeric functions and operators don't address: given a node-set whose members all have numeric values, it's frequently necessary to select the element(s) with the largest or smallest values.

While EXSLT extension functions and elements have no official standing, support for them is built into numerous XSLT processors. To use the extensions, you need to know the name of the EXSLT module in question, for example "math" for the numeric processing features. Then include a namespace declaration in your stylesheet, as follows:

   xmlns:math="http://www.exslt.org/math"

Of course, you have to replace "math" with the appropriate module name, if you're using a different one. Then just use the function or extension element as you would any other. For instance, you might use an XPath expression such as:

   (//product)[price = math:min(price)]

to locate the product with the lowest price.

Detailed instructions for using EXSLT extensions, including links to the community mailing list, can be found at the EXSLT site.

7. Fail-safe your XPointers.

In a perfect world, or at least a perfect Web, hyperlinks would never fail. Targeted resources would never move or disappear, Web servers and the network itself would never encounter any interruptions, and -- most importantly -- document authors would never make any errors (typographical or cognitive) in identifying resources in the first place.

The world and the Web being what they are, though, broken links are a fact of life. Luckily, the W3C Working Group (WG) responsible for the XPointer specs has taken this into consideration; they've provided us with a means to make XPointer-based hyperlinks a little less fragile. This solution involves "chaining" XPointers together, one after another. The XPointers are evaluated left to right; the first one which does not fail becomes, for all practical purposes, the XPointer. Consider an XML document like the following:

   <villain> 
      <name>Blofeld</name> 
      <film>Thunderball</film> 
   </villain>

An XLink into this document might attempt to locate a film element only if the villain's name is Goldfinger, using an XPointer:

    xlink:href="xpointer(//film[../name = 'Goldfinger'])"

While XLink-, let alone XPointer-aware applications are not exactly thick on the ground at the moment, we can guess that such an application would return the equivalent of a classic HTTP 404 "Document not found" error, given the above document. We could make the link more robust by "chaining" a fallback:

   xlink:href="xpointer(//film[../name = 'Goldfinger']) 
xpointer(//film[1])"

Note: code above is a single line, split across two for formatting reasons.

This instructs the application that if it can't locate the villain Goldfinger's film, it should try to locate the first film element in the document, whatever it is.

Note that this works only with scheme-type XPointers (such as the xpointer()-schemed XPointers above). If you're using shorthand XPointers, you're limited to the familiar "get it right on the first try" form.

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