XPath Tutorial-from w3schools.com - 王朝网络宽屏版

XPath is a set of syntax rules for defining parts of an XML document.

XPath is a major element in the W3C XSLT standard. Without XPath knowledge you will not be able to create XSLT documents.

Start learning XPath!

XPath Introduction

What is XPath?XPath is a syntax for defining parts of an XML document XPath uses paths to define XML elements XPath defines a library of standard functions XPath is a major element in XSLT XPath is not written in XML XPath is a W3C Standard Like Traditional File PathsXPath uses path expressions to identify nodes in an XML document. These path expressions look very much like the expressions you see when you work with a computer file system:

w3schools/xpath/default.asp

XPath ExampleLook at this simple XML document:

<?xml version="1.0" encoding="ISO-8859-1"?><catalog> <cd country="USA"> <title>Empire Burlesque</title> <artist>Bob Dylan</artist> <price>10.90</price> </cd> <cd country="UK"> <title>Hide your heart</title> <artist>Bonnie Tyler</artist> <price>9.90</price> </cd> <cd country="USA"> <title>Greatest Hits</title> <artist>Dolly Parton</artist> <price>9.90</price> </cd></catalog>

The XPath expression below selects the ROOT element catalog:

/catalog

The XPath expression below selects all the cd elements of the catalog element:

/catalog/cd

The XPath expression below selects all the price elements of all the cd elements of the catalog element:

/catalog/cd/price

Note: If the path starts with a slash ( / ) it represents an absolute path to an element!

XPath Defines a Library of Standard FunctionsXPath defines a library of standard functions for working with strings, numbers and Boolean expressions.

The XPath expression below selects all the cd elements that have a price element with a value larger than 10.80:

/catalog/cd[price>10.80]

XPath is Used in XSLTXPath is a major element of the XSLT standard. Without XPath knowledge you will not be able to create XSLT documents.

You can read more about XSLT in our XSLT tutorial.

XPath is a W3C StandardXPath was released as a W3C Recommendation 16. November 1999 as a language for addressing parts of an XML document.

XPath was designed to be used by XSLT, XPointer and other XML parsing software.

You can read more about XML and XSL standards in our W3C tutorial.

XPath Syntax

XML Example DocumentWe will use this simple XML document to describe the XPath syntax:

Locating NodesXML documents can be represented as a tree view of nodes (very similar to the tree view of folders you can see on your computer).

XPath uses a pattern expression to identify nodes in an XML document. An XPath pattern is a slash-separated list of child element names that describe a path through the XML document. The pattern "selects" elements that match the path.

The following XPath expression selects all the price elements of all the cd elements of the catalog element:

/catalog/cd/price

Note: If the path starts with a slash ( / ) it represents an absolute path to an element!

Note: If the path starts with two slashes ( // ) then all elements in the document that fulfill the criteria will be selected (even if they are at different levels in the XML tree)!

The following XPath expression selects all the cd elements in the document:

//cd

Selecting Unknown ElementsWildcards ( * ) can be used to select unknown XML elements.

The following XPath expression selects all the child elements of all the cd elements of the catalog element:

/catalog/cd/*

The following XPath expression selects all the price elements that are grandchild elements of the catalog element:

/catalog/*/price

The following XPath expression selects all price elements which have 2 ancestors:

/*/*/price

The following XPath expression selects all elements in the document:

//*

Selecting BranchesBy using square brackets in an XPath expression you can specify an element further.

The following XPath expression selects the first cd child element of the catalog element:

/catalog/cd[1]

The following XPath expression selects the last cd child element of the catalog element (Note: There is no function named first()):

/catalog/cd[last()]

The following XPath expression selects all the cd elements of the catalog element that have a price element:

/catalog/cd[price]

The following XPath expression selects all the cd elements of the catalog element that have a price element with a value of 10.90:

/catalog/cd[price=10.90]

The following XPath expression selects all the price elements of all the cd elements of the catalog element that have a price element with a value of 10.90:

/catalog/cd[price=10.90]/price

Selecting Several PathsBy using the | operator in an XPath expression you can select several paths.

The following XPath expression selects all the title and artist elements of the cd element of the catalog element:

/catalog/cd/title | /catalog/cd/artist

The following XPath expression selects all the title and artist elements in the document:

//title | //artist

The following XPath expression selects all the title, artist and price elements in the document:

//title | //artist | //price

The following XPath expression selects all the title elements of the cd element of the catalog element, and all the artist elements in the document:

/catalog/cd/title | //artist

Selecting AttributesIn XPath all attributes are specified by the @ prefix.

This XPath expression selects all attributes named country:

//@country

This XPath expression selects all cd elements which have an attribute named country:

//cd[@country]

This XPath expression selects all cd elements which have any attribute:

//cd[@*]

This XPath expression selects all cd elements which have an attribute named country with a value of 'UK':

//cd[@country='UK']

XPath Location Paths

Location Path ExpressionA 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 location steps, each separated by a slash:

An absolute location path:

/step/step/...

A relative location path:

step/step/...

The location steps are evaluated in order one at a time, from left to right. Each step is evaluated against the nodes in the current node-set. If the location path is absolute, the current node-set consists of the root node. If the location path is relative, the current node-set consists of the node where the expression is being used. Location steps consist of:

an axis (specifies the tree relationship between the nodes selected by the location step and the current node) a node test (specifies the node type and expanded-name of the nodes selected by the location step) zero or more predicates (use expressions to further refine the set of nodes selected by the location step) The syntax for a location step is:

axisname::nodetest[predicate]

Example:

child::price[price=9.90]

Axes and Node TestsAn axis defines a node-set relative to the current node. A node test is used to identify a node within an axis. We can perform a node test by name or by type.

AxisName

Description

ancestor

Contains all ancestors (parent, grandparent, etc.) of the current node Note: This axis will always include the root node, unless the current node is the root node

ancestor-or-self

Contains the current node plus all its ancestors (parent, grandparent, etc.)

attribute

Contains all attributes of the current node

child

Contains all children of the current node

descendant

Contains all descendants (children, grandchildren, etc.) of the current node Note: This axis never contains attribute or namespace nodes

descendant-or-self

Contains the current node plus all its descendants (children, grandchildren, etc.)

following

Contains everything in the document after the closing tag of the current node

following-sibling

Contains all siblings after the current node Note: If the current node is an attribute node or namespace node, this axis will be empty

namespace

Contains all namespace nodes of the current node

parent

Contains the parent of the current node

preceding

Contains everything in the document that is before the starting tag of the current node

preceding-sibling

Contains all siblings before the current node Note: If the current node is an attribute node or namespace node, this axis will be empty

self

Contains the current node

ExamplesExample

Result

child::cd

Selects all cd elements that are children of the current node (if the current node has no cd children, it will select an empty node-set)

attribute::src

Selects the src attribute of the current node (if the current node has no src attribute, it will select an empty node-set)

child::*

Selects all child elements of the current node

attribute::*

Selects all attributes of the current node

child::text()

Selects the text node children of the current node

child::node()

Selects all the children of the current node

descendant::cd

Selects all the cd element descendants of the current node

ancestor::cd

Selects all cd ancestors of the current node

ancestor-or-self::cd

Selects all cd ancestors of the current node and, if the current node is a cd element, the current node as well

child::*/child::price

Selects all price grandchildren of the current node

Selects the document root

PredicatesA predicate filters a node-set into a new node-set. A predicate is placed inside square brackets ( [ ] ).

ExamplesExample

Result

child::price[price=9.90]

Selects all price elements that are children of the current node with a price element that equals 9.90

child::cd[position()=1]

Selects the first cd child of the current node

child::cd[position()=last()]

Selects the last cd child of the current node

child::cd[position()=last()-1]

Selects the last but one cd child of the current node

child::cd[position()<6]

Selects the first five cd children of the current node

/descendant::cd[position()=7]

Selects the seventh cd element in the document

child::cd[attribute::type="classic"]

Selects all cd children of the current node that have a type attribute with value classic

Location Path Abbreviated SyntaxAbbreviations can be used when describing a location path.

The most important abbreviation is that child:: can be omitted from a location step.

Abbr

Meaning

Example

none

child::

cd is short for child::cd

attribute::

cd[@type="classic"] is short for

child::cd[attribute::type="classic"]

self::node()

.//cd is short for

self::node()/descendant-or-self::node()/child::cd

parent::node()

../cd is short for

parent::node()/child::cd

/descendant-or-self::node()/

//cd is short for

/descendant-or-self::node()/child::cd

ExamplesExample

Result

Selects all the cd elements that are children of the current node

Selects all child elements of the current node

text()

Selects all text node children of the current node

@src

Selects the src attribute of the current node

Selects all the attributes of the current node

cd[1]

Selects the first cd child of the current node

cd[last()]

Selects the last cd child of the current node

*/cd

Selects all cd grandchildren of the current node

/book/chapter[3]/para[1]

Selects the first para of the third chapter of the book

//cd

Selects all the cd descendants of the document root and thus selects all cd elements in the same document as the current node

Selects the current node

.//cd

Selects the cd element descendants of the current node

Selects the parent of the current node

../@src

Selects the src attribute of the parent of the current node

cd[@type="classic"]

Selects all cd children of the current node that have a type attribute with value classic

cd[@type="classic"][5]

Selects the fifth cd child of the current node that has a type attribute with value classic

cd[5][@type="classic"]

Selects the fifth cd child of the current node if that child has a type attribute with value classic

cd[@type and @country]

Selects all the cd children of the current node that have both a type attribute and a country attribute

XPath Expressions

Numerical ExpressionsNumerical expressions are used to perform arithmetic operations on numbers.

Operator

Description

Example

Result

Addition

6 + 4

Subtraction

6 - 4

Multiplication

6 * 4

div

Division

8 div 4

mod

Modulus (division remainder)

5 mod 2

Note: XPath always converts each operand to a number before performing an arithmetic expression.

Equality ExpressionsEquality expressions are used to test the equality between two values.

Operator

Description

Example

Result

Like (equal)

price=9.80

true (if price is 9.80)

Not like (not equal)

price!=9.80

false

Testing Against a Node-SetIf the test value is tested for equality against a node-set, the result is true if the node-set contains any node with a value that matches the test value.

If the test value is tested for not equal against a node-set, the result is true if the node-set contains any node with a value that is different from the test value.

The result is that the node-set can be equal and not equal at the same time !!!

Relational ExpressionsRelational expressions are used to compare two values.

Operator

Description

Example

Result

Less than

price<9.80

false (if price is 9.80)

Less or equal

price<=9.80

true

Greater than

price>9.80

false

Greater or equal

price>=9.80

true

Note: XPath always converts each operand to a number before performing the evaluation.

Boolean ExpressionsBoolean expressions are used to compare two values.

Operator

Description

Example

Result

price=9.80 or price=9.70

true (if price is 9.80)

and

price<=9.80 and price=9.70

false

XPath Functions

XPath Function LibraryThe XPath function library contains a set of core functions for converting and translating data.

Node Set FunctionsName

Description

Syntax

count()

Returns the number of nodes in a node-set

number=count(node-set)

id()

Selects elements by their unique ID

node-set=id(value)

last()

Returns the position number of the last node in the processed node list

number=last()

local-name()

Returns the local part of a node. A node usually consists of a prefix, a colon, followed by the local name

string=local-name(node)

name()

Returns the name of a node

string=name(node)

namespace-uri()

Returns the namespace URI of a specified node

uri=namespace-uri(node)

position()

Returns the position in the node list of the node that is currently being processed

number=position()

String FunctionsName

Description

Syntax & Example

concat()

Returns the concatenation of all its arguments

string=concat(val1, val2, ..) Example:

concat('The',' ','XML')

Result: 'The XML'

contains()

Returns true if the second string is contained within the first string, otherwise it returns false

bool=contains(val,substr)

Example:

contains('XML','X')

Result: true

normalize-space()

Removes leading and trailing spaces from a string, and replaces all internal sequences of white with one white space

string=normalize-space(string) Example:

normalize-space(' The XML ')

Result: 'The XML'

starts-with()

Returns true if the first string starts with the second string, otherwise it returns false

bool=starts-with(string,substr) Example:

starts-with('XML','X')

Result: true

string()

Converts the value argument to a string

string(value) Example:

string(314)

Result: '314'

string-length()

Returns the number of characters in a string

number=string-length(string) Example:

string-length('Beatles')

Result: 7

substring()

Returns a part of the string in the string argument

string=substring(string,start,length) Example:

substring('Beatles',1,4)

Result: 'Beat'

substring-after()

Returns the part of the string in the string argument that occurs after the substring in the substr argument

string=substring-after(string,substr)

Example:

substring-after('12/10','/')

Result: '10'

substring-before()

Returns the part of the string in the string argument that occurs before the substring in the substr argument

string=substring-before(string,substr)

Example:

substring-before('12/10','/')

Result: '12'

translate()

Performs a character by character replacement. It looks in the value argument for characters contained in string1, and replaces each character for the one in the same position in the string2

string=translate(value,string1,string2) Examples:

translate('12:30','30','45')

Result: '12:45'

translate('12:30','03','54')

Result: '12:45'

translate('12:30','0123','abcd')

Result: 'bc:da'

Number FunctionsName

Description

Syntax & Example

ceiling()

Returns the smallest integer that is not less than the number argument

number=ceiling(number) Example:

ceiling(3.14)

Result: 4

floor()

Returns the largest integer that is not greater than the number argument

number=floor(number)

Example:

floor(3.14)

Result: 3

number()

Converts the value argument to a number

number=number(value)

Example:

number('100')

Result: 100

round()

Rounds the number argument to the nearest integer

integer=round(number) Example:

round(3.14)

Result: 3

sum()

Returns the total value of a set of numeric values in a node-set

number=sum(nodeset) Example:

sum(/cd/price)

Boolean FunctionsName

Description

Syntax & Example

boolean()

Converts the value argument to Boolean and returns true or false

bool=boolean(value)

false()

Returns false

false() Example:

number(false())

Result: 0

lang()

Returns true if the language argument matches the language of the xsl:lang element, otherwise it returns false

bool=lang(language)

not()

Returns true if the condition argument is false, and false if the condition argument is true

bool=not(condition) Example:

not(false())

true()

Returns true

true() Example:

number(true())

Result: 1

XPath Examples

The CD catalogIf you have studied our XML tutorial, you will remember this XML document:

(A fraction of the CD catalog)

<?xml version="1.0" encoding="ISO-8859-1"?><catalog> <cd> <title>Empire Burlesque</title> <artist>Bob Dylan</artist> <country>USA</country> <company>Columbia</company> <price>10.90</price> <year>1985</year> </cd> <cd> <title>Hide your heart</title> <artist>Bonnie Tyler</artist> <country>UK</country> <company>CBS Records</company> <price>9.90</price> <year>1988</year> </cd>....</catalog>

If you have IE 5 or higher you can look at the cdcatalog.xml.

Selecting NodesWe will demonstrate how to select nodes from the XML document by using the selectNodes function in Internet Explorer. This function takes a location path expression as an argument:

xmlobject.selectNodes(XPath expression)

Selecting cd NodesThe following example selects all the cd nodes from the CD catalog:

xmlDoc.selectNodes("/catalog/cd")