Learning XML, 2nd Edition

XPath Expressions

Location paths are a subset of a more general concept called XPath expressions. These are statements that can extract useful information from the tree. Instead of just finding nodes, you can count them, add up numeric values, compare strings, and more. They are much like statements in a functional programming language. There are five types, listed here:

Boolean

An expression type with two possible values, true and false.

Node set

A collection of nodes that match an expression’s criteria, usually derived with a location path.

Number

A numeric value, useful for counting nodes and performing simple arithmetic.

String

A fragment of text that may be from the input tree, processed or augmented with generated text.

Result tree fragment

A temporary node tree that has its own root node but cannot be indexed into using location paths.

In XPath, types are determined by context. An operator or function can transform one expression type into another as needed. For this reason, there are well-defined rules to determine what values map to when transformed to another type.

XPath has a rich set of operators and functions for working with each expression type. In the following sections, I will describe these and the rules for switching between types.

Boolean Expressions

Boolean expressions have two values: true or false. As you saw with location step predicates, anything inside the brackets that does not result in a numerical value is forced into a Boolean context. There ...