XPath语法

In XPath, there are seven kinds of nodes: element, attribute, text, namespace, processing-instruction, comment, and document (root) nodes.

XPath Terminology

Nodes

In XPath, there are seven kinds of nodes: element, attribute, text, namespace, processing-instruction, comment, and document (root) nodes. XML documents are treated as trees of nodes. The root of the tree is called the document node (or root node).

Look at the following XML document:

<?xml version="1.0" encoding="ISO-8859-1"?>

<book>

<title lang="en">Harry Potter</title>

<author>J K. Rowling</author>

</book>

</bookstore>

Example of nodes in the XML document above:

<bookstore> (document node)

<author>J K. Rowling</author> (element node)

lang="en" (attribute node)

Atomic values

Atomic values are nodes with no children or parent.

Example of atomic values:

J K. Rowling

"en"

Items

Items are atomic values or nodes.

Relationship of Nodes

Parent

Each element and attribute has one parent.

In the following example; the book element is the parent of the title, author, year, and price:

<book>

<title>Harry Potter</title>

<author>J K. Rowling</author>

</book>

Children

Element nodes may have zero, one or more children.

In the following example; the title, author, year, and price elements are all children of the book element:

<book>

<title>Harry Potter</title>

<author>J K. Rowling</author>

</book>

Siblings

Nodes that have the same parent.

In the following example; the title, author, year, and price elements are all siblings:

<book>

<title>Harry Potter</title>

<author>J K. Rowling</author>

</book>

Ancestors

A node's parent, parent's parent, etc.

In the following example; the ancestors of the title element are the book element and the bookstore element:

<book>

<title>Harry Potter</title>

<author>J K. Rowling</author>

</book>

</bookstore>

Descendants

A node's children, children's children, etc.

In the following example; descendants of the bookstore element are the book, title, author, year, and price elements:

<book>

<title>Harry Potter</title>

<author>J K. Rowling</author>

</book>

</bookstore>

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"?>

<book>

<title lang="eng">Harry Potter</title>

</book>

<book>

<title lang="eng">Learning XML</title>

</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 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

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[0]	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

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"?>

<book>

<title lang="eng">Harry Potter</title>

</book>

<book>

<title lang="eng">Learning XML</title>

</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

Let's try to learn some basic XPath syntax by looking at some examples.

The XML Example Document

We will use the following XML document in the examples below.

"books.xml":

<?xml version="1.0" encoding="ISO-8859-1"?>

<title lang="en">Everyday Italian</title>

<author>Giada De Laurentiis</author>

</book>

<title lang="en">Harry Potter</title>

<author>J K. Rowling</author>

</book>

<title lang="en">XQuery Kick Start</title>

<author>James McGovern</author>

<author>Per Bothner</author>

<author>Kurt Cagle</author>

<author>James Linn</author>

<author>Vaidyanathan Nagarajan</author>

</book>

<title lang="en">Learning XML</title>

</book>

</bookstore>

View the "books.xml" file in your browser.

Selecting Nodes

We will use the Microsoft XMLDOM object to load the XML document and the selectNodes() function to select nodes from the XML document:

set xmlDoc=CreateObject("Microsoft.XMLDOM")

xmlDoc.async="false"

xmlDoc.load("books.xml")

xmlDoc.selectNodes(path expression)

Select all book Nodes

The following example selects all the book nodes under the bookstore element:

xmlDoc.selectNodes("/bookstore/book")

If you have IE 5 or higher you can try it yourself.

Select the First book Node

The following example selects only the first book node under the bookstore element:

xmlDoc.selectNodes("/bookstore/book[0]")

If you have IE 5 or higher you can try it yourself

Note: IE5 and later has implemented that [0] should be the first node, but according to the W3C standard it should have been [1]!!

A Workaround!

To solve the [0] and [1] problem in IE5+, you can set the SelectionLanguage to XPath.

The following example selects only the first book node under the bookstore element:

xmlDoc.setProperty "SelectionLanguage", "XPath"

xmlDoc.selectNodes("/bookstore/book[1]")

Try it yourself

Select the prices

The following example selects the text from all the price nodes:

xmlDoc.selectNodes("/bookstore/book/price/text()")

If you have IE 5 or higher you can try it yourself.

Selecting price Nodes with Price>35

The following example selects all the price nodes with a price higher than 35:

xmlDoc.selectNodes("/bookstore/book[price>35]/price")

If you have IE 5 or higher you can try it yourself.

Selecting title Nodes with Price>35

The following example selects all the title nodes with a price higher than 35:

xmlDoc.selectNodes("/bookstore/book[price>35]/title")

If you have IE 5 or higher you can try it yourself.

posted @ 2011-03-08 10:37 jrvin 阅读(249) 评论(0) 编辑收藏举报

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