XML in a Nutshell, 3rd Edition

XPath Functions

XPath 1.0 defines 27 built-in functions for use in XPath expressions. Various technologies that use XPath, such as XSLT and XPointer, also extend this list with functions they need. XSLT even allows user-defined extension functions.

Every function is evaluated in the context of a particular node, called the context node. The higher-level specification in which XPath is used, such as XSLT or XPointer, decides exactly how this context node is determined. In some cases, the function operates on the context node. In other cases, it operates on the argument, if present, and the context node, if no argument exists. The context node is ignored in other cases.

In the following sections, each function is described with at least one signature in this form:

               return-type function-name(type argument, type argument, ...)

Compared to languages like Java, XPath argument lists are quite loose. Some XPath functions take a variable number of arguments and fill in the arguments that are omitted with default values or the context node.

Furthermore, XPath is weakly typed. If you pass an argument of the wrong type to an XPath function, it generally converts that argument to the appropriate type using the boolean( ), string( ), or number( ) functions, described later. The exceptions to the weak-typing rule are the functions that take a node-set as an argument. Standard XPath 1.0 provides no means of converting anything that isn’t a node-set into a node-set. In some cases, a function can operate ...