Introduction

XPath is an expression language that is fundamental to XML processing. You can no more master XSLT without mastering XPath than you can master English without learning the alphabet. Several readers of the first edition of XSLT Cookbook took me to task for not covering XPath. This chapter has been added partly to appease them but more so due to the greatly increased power of the latest XPath 2.0 specifications. However, many of these recipes are applicable to XPath 1.0 as well.

In XSLT 1.0, XPath plays three crucial roles. First, it is used within templates for addressing into the document to extract data as it is being transformed. Second, XPath syntax is used as a pattern language in the matching rules for templates. Third, it is used to perform simple math and string manipulations via built-in XPath operators and functions.

XSLT 2.0 retains and strengthens this intimate connection with XPath 2.0 by drawing heavily on the new computational abilities of XPath 2.0. In fact, one can make a reasonable argument that the enhanced capabilities of XSLT 2.0 stem largely from the advances in XPath 2.0. The new XPath 2.0 facilities include sequences, regular expressions, conditional and iterative expressions, and enhanced XML Schema compliant-type system as well as a large number of new built-in functions.

Each recipe in this chapter is a collection of mini-recipes for solving certain classes of XPath problems that often arise while using XSLT. We annotate each XPath expression with the XPath 2.0 commenting convention (: comment :) but users of XPath/XSLT 1.0 should be aware that these comments are not legal syntax. When we are showing the result of an XPath evaluation that is empty, we will write ( ), which happens to be the way one writes a literal empty sequence in XPath 2.0.