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XPath uses path expressions to select nodes or node-sets in an XML document. The node is selected by following a path or steps.


The XML Example Document

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>
            


Selecting Nodes

XPath uses path expressions to select nodes in an XML document. The node is selected by following a path or steps. The most useful path expressions are listed below:

Expression Description
nodename Selects all child nodes of the 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

Examples

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.

Examples

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
/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

Examples

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.

Examples

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


The XML Example Document

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>
            


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]

Examples

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 children of the current node
attribute::* Selects all attributes of the current node
child::text() Selects all text child nodes of the current node
child::node() Selects all child nodes 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


An XPath expression returns either a node-set, a string, a Boolean, or a number.


XPath Operators

Below is a list of the operators that can be used in XPath expressions:

Operator Description Example Return value
| Computes two node-sets //book | //cd Returns a node-set with all book and cd elements
+ Addition 6 + 4 10
- Subtraction 6 - 4 2
* Multiplication

6 * 4

24
div Division 8 div 4 2
= Equal price=9.80 true if price is 9.80
false if price is 9.90
!= Not equal price!=9.80 true if price is 9.90
false if price is 9.80
< Less than price<9.80 true if price is 9.00
false if price is 9.80
<= Less than or equal to price<=9.80 true if price is 9.00
false if price is 9.90
> Greater than price>9.80 true if price is 9.90
false if price is 9.80
>= Greater than or equal to price>=9.80 true if price is 9.90
false if price is 9.70
or or price=9.80 or price=9.70 true if price is 9.80
false if price is 9.50
and and price>9.00 and price<9.90 true if price is 9.80
false if price is 8.50
mod Modulus (division remainder) 5 mod 2 1


The following reference library defines the functions required for XPath 2.0, XQuery 1.0 and XSLT 2.0.


Functions Reference

The default prefix for the function namespace is fn:, and the URI is:
http://www.w3.org/2005/02/xpath-functions.

posted on 2006-05-09 14:55 又见炊烟 阅读(...) 评论(...) 编辑 收藏