XPath Predicates

XPath (XML Path Language) is a query language used to select nodes from an XML document. One of the key features of XPath is predicates, which allow you to filter nodes based on specific conditions.In this chapter, we will explore what XPath predicates are, how they work, and provide examples from basic to advanced levels. By the end, you will be able to use XPath predicates effectively in XML documents.

What is an XPath Predicate?

A predicate in XPath is an expression that appears inside square brackets ([]) and is used to filter nodes from a set of nodes. It allows you to narrow down a selection by adding conditions or constraints.

Syntax of an XPath Predicate

				
					/path/to/element[condition]

Basic Example

Let’s consider a simple XML file:

				
					<library>
  <book>
    <title>Harry Potter</title>
    <author>J.K. Rowling</author>
  </book>
  <book>
    <title>The Hobbit</title>
    <author>J.R.R. Tolkien</author>
  </book>
  <book>
    <title>The Alchemist</title>
    <author>Paulo Coelho</author>
  </book>
</library>

Basic XPath Predicate Usage

If you want to select the second <book> element in the <library>, you can use a predicate.

				
					/library/book[2]

Explanation

/library/book selects all <book> elements.
[2] specifies the second <book>.

Output

				
					<book>
  <title>The Hobbit</title>
  <author>J.R.R. Tolkien</author>
</book>

Predicates Based on Attribute Values

If an element contains an attribute, you can filter nodes based on that attribute’s value. Consider the following XML structure:

				
					<library>
  <book category="fiction">
    <title>The Hobbit</title>
    <author>J.R.R. Tolkien</author>
  </book>
  <book category="non-fiction">
    <title>Becoming</title>
    <author>Michelle Obama</author>
  </book>
</library>

Example 1: Select books in the “fiction” category

				
					/library/book[@category='fiction']

Explanation

/library/book selects all <book> elements.
[@category='fiction'] filters books where the category attribute is fiction.

Output

				
					<book category="fiction">
  <title>The Hobbit</title>
  <author>J.R.R. Tolkien</author>
</book>

Predicates with Functions

You can combine predicates with XPath functions like contains(), starts-with(), and last() to build more complex queries.

Example 1: Using `contains()` function

If you want to find all books whose title contains the word “The”:

				
					/library/book[contains(title, 'The')]

Explanation

contains(title, 'The') checks if the <title> element contains the string “The”.

Output

				
					<book category="fiction">
  <title>The Hobbit</title>
  <author>J.R.R. Tolkien</author>
</book>
<book category="non-fiction">
  <title>Becoming</title>
  <author>Michelle Obama</author>
</book>

Example 2: Using `last()` function

To select the last book in the library:

				
					/library/book[last()]

Explanation

last() returns the last element in the node set.

Output

				
					<book category="non-fiction">
  <title>Becoming</title>
  <author>Michelle Obama</author>
</book>

Advanced Predicate Use Cases

Example 1: Multiple Conditions in a Predicate

You can use multiple conditions in a single predicate with logical operators like and and or.

				
					/library/book[@category='fiction' and contains(title, 'Hobbit')]

Explanation

[@category='fiction' and contains(title, 'Hobbit')] ensures that the book is in the fiction category and its title contains the word “Hobbit”.

Output

				
					<book category="fiction">
  <title>The Hobbit</title>
  <author>J.R.R. Tolkien</author>
</book>

Example 2: Using Numeric Comparison

Select books with titles longer than 10 characters:

				
					/library/book[string-length(title) > 10]

Explanation

string-length(title) gets the length of the title string.
The predicate filters books whose title length is greater than 10 characters.

Output

				
					<book category="fiction">
  <title>The Hobbit</title>
  <author>J.R.R. Tolkien</author>
</book>
<book category="non-fiction">
  <title>Becoming</title>
  <author>Michelle Obama</author>
</book>