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How to Use XPath Predicates

While XPath path expressions are excellent for navigating the hierarchy of an XML document, their true power comes from predicates. A predicate is a filter that allows you to select specific nodes from a larger set based on a condition. If an XPath expression is a question you ask an XML document, a predicate is the part that adds "where" or "which."

What is a XPath Predicate?​

A predicate is an expression written inside square brackets []. It is appended to a path expression and acts as a filter on the set of nodes selected by that path. Only the nodes for which the condition inside the brackets is true will be returned.

Syntax: path/expression[condition]

Examples​

Consider the following XML document:

<?xml version="1.0"  encoding="UTF-8"?>

<bookstore>
    <book category="cooking">
        <title lang="en">Everyday Italian</title>
        <author>Giada De Laurentiis</author>
        <price>30.00</price>
    </book>
    <book category="children">
        <title lang="en">Harry Potter</title>
        <author>J. K. Rowling</author>
        <price>29.99</price>
    </book>
    <book category="web">
        <title lang="en">Learning XML</title>
        <author>Tom Nolan</author>
        <price>39.95</price>
    </book>
</bookstore>

Filtering by Position​

This is the simplest type of filter. It allows you to select elements based on their order within a group of siblings. * Note:* XPath positions are 1-based, not 0-based.

Path ExpressionResult
/bookstore/book[1]Selects the first <book> element ("Everyday Italian").
/bookstore/book[last()]Selects the last <book> element ("Learning XML").
/bookstore/book[position()<3]Selects the first two <book> elements ("Everyday Italian" and "Harry Potter").

Filtering by Attribute​

You can filter nodes based on the presence or value of their attributes. To refer to an attribute within a predicate, you must prefix its name with the @ symbol.

Path ExpressionResult
//title[@lang]Selects all <title> elements that have a lang attribute. (In this case, all three titles).
//book[@category='cooking']Selects the <book> element where the category attribute has the exact value 'cooking'. ("Everyday Italian").

Filtering by the Value of a Child Element​

This is one of the most powerful features of predicates. You can filter a set of parent nodes based on the content of their children.

Path ExpressionResult
/bookstore/book[price>35.00]Selects all <book> elements that have a child <price> element with a value greater than 35.00. ("Learning XML").
//book[author='J. K. Rowling']Selects the <book> element whose child <author> has the exact value 'J. K. Rowling'. ("Harry Potter").
/bookstore/book[price>30.00]/titleThis demonstrates chaining. It first selects the book with a price over 30.00 ("Learning XML") and then, from that result, it selects the child <title> element.
warning

In IE 5,6,7,8,9 first node is [0], but according to W3C, it is [1].

To solve this problem in IE, set the SelectionLanguage to XPath.

In JavaScript, you can do in this way: xml.setProperty("SelectionLanguage","XPath");

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