Tuesday, 7 June 2011

XPath


XPath is a language for finding information in an XML document

What is XPath?

  • XPath is a syntax for defining parts of an XML document
  • XPath uses path expressions to navigate in XML documents
  • XPath contains a library of standard functions
  • XPath is a major element in XSLT
  • XPath is a W3C recommendation

XSLT uses XPath to find information in an XML document. XPath is used to navigate through elements and attributes in XML documents. We will use the following XML document in the examples below.
<?xml version="1.0" encoding="ISO-8859-1"?>

<bookstore>

<book>
  <title lang="eng">Harry Potter</title>
  <price>29.99</price>
</book>

<book>
  <title lang="eng">Learning XML</title>
  <price>39.95</price>
</book>

</bookstore>

 

Select Nodes

Expression
Description
nodename
Selects all child nodes of the named node
/
Selects from the root node
//
Selects nodes in the document from the current node that match the selection no matter where they are
.
Selects the current node
..
Selects the parent of the current node
@
Selects attributes

In the table below we have listed some path expressions and the result of the expressions:
Path Expression
Result
bookstore
Selects all the child nodes of the bookstore element
/bookstore
Selects the root element bookstore
Note: If the path starts with a slash ( / ) it always represents an absolute path to an element!
bookstore/book
Selects all book elements that are children of bookstore
//book
Selects all book elements no matter where they are in the document
bookstore//book
Selects all book elements that are descendant of the bookstore element, no matter where they are under the bookstore element
//@lang
Selects all attributes that are named lang

 

Predicates

Predicates are used to find a specific node or a node that contains a specific value.
Predicates are always embedded in square brackets.
In the table below we have listed some path expressions with predicates and the result of the expressions:
Path Expression
Result
/bookstore/book[1]
Selects the first book element that is the child of the bookstore element.
Note: IE5 and later has implemented that [0] should be the first node, but according to the W3C standard it should have been [1]!!
/bookstore/book[last()]
Selects the last book element that is the child of the bookstore element
/bookstore/book[last()-1]
Selects the last but one book element that is the child of the bookstore element
/bookstore/book[position()<3]
Selects the first two book elements that are children of the bookstore element
//title[@lang]
Selects all the title elements that have an attribute named lang
//title[@lang='eng']
Selects all the title elements that have an attribute named lang with a value of 'eng'
/bookstore/book[price>35.00]
Selects all the book elements of the bookstore element that have a price element with a value greater than 35.00
/bookstore/book[price>35.00]/title
Selects all the title elements of the book elements of the bookstore element that have a price element with a value greater than 35.00

 

Selecting Unknown Nodes

XPath wildcards can be used to select unknown XML elements.
Wildcard
Description
*
Matches any element node
@*
Matches any attribute node
node()
Matches any node of any kind

In the table below we have listed some path expressions and the result of the expressions:
Path Expression
Result
/bookstore/*
Selects all the child nodes of the bookstore element
//*
Selects all elements in the document
//title[@*]
Selects all title elements which have any attribute
Selecting Several Paths
By using the | operator in an XPath expression you can select several paths.
In the table below we have listed some path expressions and the result of the expressions:
Path Expression
Result
//book/title | //book/price
Selects all the title AND price elements of all book elements
//title | //price
Selects all the title AND price elements in the document
/bookstore/book/title | //price
Selects all the title elements of the book element of the bookstore element AND all the price elements in the document


XPath Axes
An axis defines a node-set relative to the current node.
AxisName
Result
ancestor
Selects all ancestors (parent, grandparent, etc.) of the current node
ancestor-or-self
Selects all ancestors (parent, grandparent, etc.) of the current node and the current node itself
attribute
Selects all attributes of the current node
child
Selects all children of the current node
descendant
Selects all descendants (children, grandchildren, etc.) of the current node
descendant-or-self
Selects all descendants (children, grandchildren, etc.) of the current node and the current node itself
following
Selects everything in the document after the closing tag of the current node
following-sibling
Selects all siblings after the current node
namespace
Selects all namespace nodes of the current node
parent
Selects the parent of the current node
preceding
Selects everything in the document that is before the start tag of the current node
preceding-sibling
Selects all siblings before the current node
self
Selects the current node


Location Path Expression
A location path can be absolute or relative.
An absolute location path starts with a slash (/) and a relative location path does not. In both cases the location path consists of one or more steps, each separated by a slash:
An absolute location path:

/step/step/...

A relative location path:

step/step/...
Each step is evaluated against the nodes in the current node-set.
A step consists of:
  • an axis (defines the tree-relationship between the selected nodes and the current node)
  • a node-test (identifies a node within an axis)
  • zero or more predicates (to further refine the selected node-set)
The syntax for a location step is:
                        axisname::nodetest[predicate]
Ex:
Example
Result
child::book
Selects all book nodes that are children of the current node
attribute::lang
Selects the lang attribute of the current node
child::*
Selects all element children of the current node
attribute::*
Selects all attributes of the current node
child::text()
Selects all text node children of the current node
child::node()
Selects all children of the current node
descendant::book
Selects all book descendants of the current node
ancestor::book
Selects all book ancestors of the current node
ancestor-or-self::book
Selects all book ancestors of the current node - and the current as well if it is a book node
child::*/child::price
Selects all price grandchildren of the current node

1 comment:

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