Automating System Administration with Perl, 2nd Edition

Appendix B. The 10-Minute XPath Tutorial

Before we launch into XPath, we need to get three caveats out of the way.

First, in order to understand this appendix, you’ll need to have at least a moderate grip on the subject of XML. Be sure to read Appendix A if you haven’t already.

Second, XPath is a language unto itself. The XPath 1.0 spec consists of 34 relatively dense pages; the XPath 2.0 spec is 118 pages long. This appendix is not going to attempt to do any justice to the richness, expressiveness, and complexity of XPath (especially v2.0). Instead, it is going to focus on the subset of XPath that will be immediately useful to a Perl programmer.

Finally, this appendix will be sticking to XPath 1.0. As of this writing there are no solid Perl modules that I know of that support XPath 2.0.

With all of that aside, let’s get to questions like “What is XPath?” and, perhaps more importantly, “Why should I care?” XPath is a W3C spec for “a language for addressing parts of an XML document.” If you ever have to write code that attempts to select or extract certain parts of an XML document, XPath may make your life a great deal easier. It is a fairly terse but quite powerful language for this task and has a lovely “make it so” quality to it. If you can describe what data you are looking for using the XPath language (and you usually can), the XPath parser can fetch it for you, or allow you to point your program at the right part of the XML document. You can often achieve this with a single line ...