XPathXPathXML Path XML Path LanguageLanguage

OutlineOutlineXML Path Language (XPath) Data Model Description Node values XPath expressions Relative expressions Simple subset of XPath Predicates Node-Set Functions Full location Steps Axes Node Tests Abbreviated syntax Links to more information

XML Path Language XML Path Language (XPath)(XPath)XPath 1.0 is a W3C

Recommendation (16 November 1999)

used for addressing elements (in XPointer)

used for matching elements (in XSLT and XQuery)

declarative language for specifying paths in trees

syntax somewhat similar to that used for paths in file hierarchies

Data Model - Data Model - example documentexample document

document is viewed as a tree of nodes

e.g. document <Book> <chapter> <heading>The First Chapter</heading> <section>... ...</section> </chapter> <chapter> <heading>The Second Chapter</heading> <section>... ...</section> <section>... ...</section> </chapter></Book>

Data Model - example Data Model - example documentdocument

Data Model Data Model DescriptionDescription

6 types of node: root, element, attribute, text, comment,

processing instruction root of tree is different from (and parent

of) root element of the document (Book in example)

in example root node is red element nodes are yellow text nodes are green

element nodes have associated set of attribute nodes

attribute nodes are not children of element nodes

order of child element nodes is significant

Data Model - Data Model - example documentexample document

More complex document<CD publisher="Deutsche Grammophon" length="PT1H13M37S" > <composer>Johannes Brahms</composer> <performance> <composition>Piano Concerto No. 2</composition> <soloist>Emil Gilels</soloist> <orchestra>Berlin Philharmonic</orchestra> <conductor>Eugen Jochum</conductor> </performance> <performance> <composition>Fantasias Op. 116</composition> <soloist>Emil Gilels</soloist> </performance></CD>

Data Model - example Data Model - example documentdocument

Node valuesNode valueseach attribute and element node has a

value e.g., value of length attribute node is

PT1H13M37S value of an element node is concatenation

of all text node descendants e.g., value of composer element node is

Johannes Brahms e.g., value of second performance element

node is Fantasias Op. 116Emil Gilels

e.g., value of CD element node is Johannes Brahms Piano Concerto No. 2Emil Gilels Berlin PhilharmonicEugen Jochum Fantasias Op. 116Emil Gilels

note : attribute values are not included

XPath expressionsXPath expressionsan XPath expression is either

an absolute expression or a relative expression

an absolute expression starts with '/' is followed by a relative expression and is evaluated starting at the root

node a relative expression is

a sequence of location steps each separated by '/'

example (absolute expression comprising 2 steps): /CD/composer

Relative expressionsRelative expressionsrelative expression is evaluated with

respect to an initial context (set of nodes)

initial context is defined externally (by XPointer or XSLT) <xsl:template match="CD"> <xsl:value-of select="composer"/></xsl:template>

context for composer given by CD each location step

is evaluated with respect to some context

produces a set of nodes which provides the context for the next

location step

Simple subset of Simple subset of XPathXPathsubset uses abbreviated syntax a location step has one of 3 forms:

it is empty, i.e., // element-name predicates @attribute-name predicates

an empty step means search all descendants of the context node

element-name means find all child elements of the context node which have the given name

@attribute-name means find the attribute node of the context node which has the given name

optional predicates (each enclosed in [ and ]) filter nodes found further

Examples – cd.xmlExamples – cd.xml<?xml version="1.0" ?> <CDlist>

<CD> <composer>Johannes Brahms</composer> <soloist>Emil Gilels</soloist> <orchestra>Berlin Philharmonic</orchestra> <conductor>Eugen Jochum</conductor> <date>1972</date> <performance> <composition>Piano Concerto No. 1</composition> </performance>

<publisher>Deutsche Grammophon</publisher> <number>419159-2</number> </CD>

<CD> <soloist>Martha Argerich</soloist> <orchestra>London Symphony Orchestra</orchestra>

<conductor>Claudio Abbado</conductor> <date>1968</date> <performance> <composer>Frederic Chopin</composer>

<composition>Piano Concerto No. 1</composition> </performance><performance> <composer>Franz Liszt</composer>

<composition>Piano Concerto No. 1</composition> <conductor>Antal Dorati</conductor> <date>1984</date>

</performance> <publisher>Deutsche Grammophon</publisher> <number>449719-2</number> </CD></CDlist>

Examples – cd.xmlExamples – cd.xml /CDlist/CD

all child CD elements of the CDlist element that is the child of the root

//composer all composer elements that are descendants of

the root //performance/composer

all composer child elements of performance elements which are descendants of the root

//performance[composer] all performance elements that have a composer

element as a child //CD[performance/date]

all CD elements that have a performance element as a child that has a date element as a child

//performance[conductor][date] all performance elements that have both

conductor and date elements as children

PredicatesPredicatespredicates filter out more nodes from

a node-set S evaluate predicate on each node x in

node-set S with x as the context node the size of S as the context size the position of x in S as the context

position predicate comprises

Boolean expressions: using and, or, not, =, ...

numerical expressions: using +, -, ... node-set expressions: location paths

filtered by predicates node-set functions

Node-Set FunctionsNode-Set Functionslast(): returns context size position(): returns context position count(S): returns number of nodes

in S name(S): returns name of first

node in S id(S): returns nodes who have an

ID-type attribute with a value in S e.g.

position()=2: true if node is 2nd in the context

position()=last(): true if node is last in the context

ExamplesExamplescount(//performance): the number of

performance elements //performance[not(date)]: performance

elements that do not have a date element as a child

all CD elements that have "Deutsche Grammophon" as publisher and have more than 1 performance element as child: //CD [publisher="Deutsche Grammophon" and count(performance) > 1]

or //CD [publisher="Deutsche Grammophon"] [count(performance) > 1]

or //CD [count(performance) > 1] [publisher="Deutsche Grammophon"]

More examplesMore examples//CD/performance[position()=2]

returns the second performance of each CD

//CD/performance[position()=2][date] returns the second performance of each

CD if it has a date (otherwise, returns nothing)

//CD/performance[date and position()=2] returns the same

//CD/performance[date][position()=2] returns the second of those

performance children of each CD that have a date (if any)

Full location StepsFull location Stepsusing full, not abbreviated, syntax a location step has the form

axis :: node-test predicates where axis selects a set of candidate nodes node-test filters candidates based

on node type or name optional predicates

in child::CD[attribute::publisher="Deutsche Grammophon"] child and attribute are axes CD and publisher are node-tests

AxesAxesaxis specifies what nodes, relative to

context node(s), to consider there are 13 axes defined

self: the context node itself parent: the parent of the context node

(note: parent of root is empty) attribute: all attributes of the context node namespace: all namespace nodes of the

context node child, ancestor, descendant (see later) ancestor-or-self: ancestors and the context

node descendant-or-self: descendants and the

context node preceding-sibling, following-sibling,

preceding, following (see later)

Axes: parent, Axes: parent, child, ...child, ...

context node (and self axis) in yellow nodes in parent axis in black nodes in child axis in whitewhite - nodes in preceding sibling axis in green - nodes in following sibling red

Axes: ancestor, Axes: ancestor, descendant, ...descendant, ...

context node C (and self axis) in yellow ancestor (black): elements whose start tag precedes start tag

of C and whose end tag follows end tag of C descendant (white): elements whose start tag follows start

tag of C and whose end tag precedes end tag of C preceding (green): elements whose end tag precedes start

tag of C following (red): elements whose start tag follows end tag of C preceding, following, ancestor, descendant and self together

partition the tree into 5 subtrees

Node TestsNode Testsaxes other than attribute and namespace

include elements, text nodes, comments and processing instructions

principal type of these axes is element node test further restricts nodes

considered by node name

chapter: nodes with name "chapter" *: nodes with any name (of the axis principal

type) by node type

node(): all nodes text(): character data nodes comment(): comment nodes processing-instruction(): processing

instruction nodes

ExamplesExampleschild::*[position()=2]

second child element descendant::node()

all descendant nodes (elements, text nodes, comments or processing instructions)

following-sibling::*[position()=last()] rightmost sibling element

child::section[position()=2]/child::subsection[position()=1] first subsection of the second

section

Abbreviated syntaxAbbreviated syntax

if path starts with //, initial context is the root

Normal syntaxAbbreviation

child:: nothing(child is default axis)

Attribute:: @-/descendant or

-self::node()///

self::node() .parent::node() ..[position()=i] [i]

Examples using Examples using abbreviationsabbreviations*[2]

second child element //*

all descendant elements of the root //text()

all text node descendants of the root section[2]/subsection[1]

first subsection of the second section .//@href

all href attributes in descendants of context node(s)

//section[.//image]/title the titles of sections which contain

images

The family exampleThe family example<?xml version="1.0"?><family> <parent pno="p1" role="mother" spouse="p2"> <name>Janet</name> </parent> <parent pno="p2" role="father" spouse="p1"> <name>John</name> </parent> <child cno="c1" siblings="c2 c3"> <name>Tom</name> </child> <child cno="c2" siblings="c1 c3"> <name>Dick</name> </child> <child cno="c3" siblings="c1 c2"> <name>Harry</name> </child></family>

Location paths on Location paths on familyfamily in the context of the family element, parent[@spouse='p2']/name produces

<name>Janet</name>

the name of the person whose spouse attribute has value 'p2'

parent[id(@spouse)[name='John']]/name produces <name>Janet</name>

the name of the person whose spouse is named John

More location paths More location paths on familyon family in the context of the family element, id(child[name='Dick']/@siblings)/name

produces <name>Tom</name><name>Harry</name>

the names of Dick's siblings

child[id(@siblings)[name='Tom']]/name produces <name>Dick</name><name>Harry</name>

the names of the children who have Tom as a sibling

More InformationMore Informationwww.w3.org/TR/xpath

W3C's Recommendation on XPath

www.vbxml.com/xpathvisualizer/ page for downloading the XPath visualiser

www.w3schools.com/xpath/ XPath tutorial

www.vbxml.com/xsl/tutorials/intro/default.asp XSLT and XPath tutorial