XPath: An Introduction

Stuart Myles

Objectives

• What is XPath?• An introduction to the XPath 1.0 language– XML refresher– XPath basics– What else can you do with XPath 1.0?

• Where to go for more information

XPathXML Path Language

• Path notation with slashes

newsItem/rightsInfo/copyrightHolder

recipe/ingredientList/ingredient

• Like UNIX directory paths or URLS

What is XPath?

• Syntax for defining parts of an XML document– Locate elements or attributes

• Performing operations over data– XPath contains a library of standard functions– Numeric, string, boolean

• A major part of several XML standards– XSLT, XQuery, XML Schema, Schematron

XPath Introduction:XML Refresher

• XML documents contain one or more elements, delimited by start and end tags

<foo></foo>

• Elements can be nested to any depth<foo>

<bar></bar></foo>

XML Attributes and Text Content

• Elements can have attributes<foo lang=“fr”>

<bar id=“theOne” lang=“en”></bar></foo>

• Elements can have text content<foo lang=“fr”>

<bar lang=“en”>theOne</bar></foo>

• Empty elements have no children or text<foo></foo>

• A shorthand for writing empty elements<foo />

XML Namespaces

• Elements can be defined in different namespaces• Namespaces look like URLs• You can use xmlns to declare a default namespace

<newsItem xmlns='http://iptc.org/std/nar/2006-10-01/'>  <itemMeta>   <title>Pope Blesses Astronauts</title> </itemMeta></newsItem>

• newsItem is in the http://iptc.org/std/nar/2006-10-01/ namespace• itemMeta and title are also in the http://iptc.org/std/nar/2006-10-01/ ns

• Child elements inherit from their parents

XML Namespace Prefixes

• You can use xmlns:prefix to declare a namespace and bind it to a prefix<nar:newsItem xmlns:nar='http://iptc.org/std/nar/2006-10-01/'>  <nar:itemMeta>   <nar:title>Pope Blesses Astronauts</nar:title> </nar:itemMeta></nar:newsItem>

• newsItem is in the http://iptc.org/std/nar/2006-10-01/ namespace• itemMeta and title are also in the http://iptc.org/std/nar/2006-10-01/

namespace

• To an XML parser, this document and the previous one are identical

XPath Crash CourseThe Basics: Selecting Elements

• The simplest XPath form:– one or more tag names, separated by slashes (/)newsItem/itemMeta/title <- title under itemMeta

• Use * instead of a tag name to match anythingnewsItem/*/title <- title grandchildren of newsItem

• An empty tag searches all levels of the tree //title Every title element in the doc

newsItem//title Every title under newsItem

XPath: Using Attributes

• Attribute values are indicated by @@rel <- The rel attribute of the current element

• Element and Attribute values are tied by /@link/@rel <- The rel attribute of the link element

• Use [] for conditional selectionslink[@rel] <- link element with a rel attributelink[@rel = “parent”]link[@size &lt; “1000”]link[not(@href)]

XPath and Namespaces

• XPath supports namespaces

nitf:p <- The p element from the nitf namespacexhtml:p <- The p element from the xhtml nsnar:* <- Any element from the nar namespace@atom:* <- Any attribute from the atom ns

Protip: if you can’t figure out why your XPathexpression isn’t matching, check the namespace

What Else Can XPath Do?Numeric, String, Boolean Functions

Publication/FilingMetadata[1]Publication/FilingMetadata[last()]Publication/FilingMetadata[last() - 1]FilingMetadata[position() mod 2 = 0]

FilingMetadata[Category = “q” or Category = “j”]

not(contains(SlugLine, “advisory”))starts-with(FilingOnlineCode, “1”)

And XPath 2.0 adds even more functions, including regular expressions

More XPath Information

List of examples:http://msdn.microsoft.com/en-us/library/ms256086.aspxIntroductory, interactive tutorial:http://www.zvon.org/comp/r/tut-XPath_1.htmlMore advanced tutorial:http://www.ibm.com/developerworks/xml/tutorials/x-xpath/section2.html XPath chapter from XML in a Nutshell:http://oreilly.com/catalog/xmlnut/chapter/ch09.html