XPath tutorial

This article or chapter is incomplete and its contents need further attention.Some information may be missing or may be wrong, spelling and grammar may have to be improved, use your judgment!Web technology tutorials seriesIntermediatePage created byDaniel K. Schneider, 25 November 2007Lastmodified byDaniel K. Schneider, 11 September 2015Contents[]1 IntroductionXPath is a language for addressing parts of an XML document. Basic understanding of XPath is needed forXSLT andXQuery programming. In this piece, we shall show how to create XSLT stylesheets that use some moderately complex XPath sTip: Read the free O"Reilly book:http://commons.oreilly.com/wiki/index.php/XPath_and_XPointerLearning ObjectivesUnderstand how to use XPath expressions, in order use XSLT and XQuery more effectivelyLearn some XSLT programming constructions (conditions and loops)Being able to cope with most XML to HTML transformationsPrerequisitesEditing XML tutorial (being able to use a simple DTD).XSLT Tutorial - Basics, introductory XSLT (xsl:template, xsl:apply-templates and xsl:value-of)Next stepsXSLT to generate SVG tutorialXQuery tutorial - basicsMaterialsWe use the same XML document in most examples. You can download the file or look at a wiki page:xpath-jungle.xmlXPath tutorial - basics/XML example codeDirectory:http://tecfa.unige.ch/guides/xml/examples/xpath/DisclaimerThis is an unfinished, not very nice, introductoryXPath tutorial. Cut/paste from slides with a few fixes. It needs more work, since right now it"s more like a list of XPath features ... -Daniel K. SchneiderThere may be typos (sorry) and mistakes (sorry again)2 Introduction to XML Path Language2.1 Definition and historyXPath is a language for addressing parts of an XML document, In support of this primary purpose, it also provides basic facilities for manipulation of strings, numbers and booleans.Its 2.0 editition was described as 「XPath 2.0 is an expression language that allows the processing of values conforming to the data model defined inXQuery/XPath Data Model (XDM). The data model provides a tree representation of XML documents as well as atomic values such as integers, strings, and booleans, and sequences that may contain both references to nodes in an XML document and atomic values. The result of an XPath expression may be a selection of nodes from the input documents, or an atomic value, or more generally, any sequence allowed by the data model. The name of the language derives from its most distinctive feature, the path expression, which provides a means of hierarchic addressing of the nodes in an XML tree.」 (XML Path Language (XPath) 2.0,W3C Recommendation 23 January 2007, retrieved 16:38, 9 February 2010 (UTC).XPath uses a compact non-XML syntax (to facilitate use of XPath within URIs and XML attribute values).XPath gets its name from its use of a path notation as in URLs for navigating through the hierarchical structure of an XML document.XPath was defined at the same time as XSLT (nov 1999)Initally, it was developped to support XSLT and XPointer (XML Pointer Language used for XLink, XInclude, etc.). Today it is also used by XQuery and other applications. Many programming languages support an XPath library, e.g. PHP5.In plain English, XPath allows retrieving parts of an XML document. XPath expressions can be as simple as the name of an XML element or so complicated that only experts can understand it ....SpecificationsXPath 1.0 (nov 1999) (http://www.w3.org/TR/xpath)XPath 2.0 (Jan 2007) (http://www.w3.org/TR/xpath20/)XPath 2.0 Functions and Operatorshttp://www.w3.org/TR/xquery-operators/XQuery 1.0 and XPath 2.0 Data Model (XDM)http://www.w3.org/TR/xpath-datamodel/XPath 1.0 is used by XSLT 1.0, i.e. the XSLT processor included in virtually every web browser as of Jan 2010 and since the early 2000"s in IE/Mozilla.XPath 2.0 is a superset of XPath 1.0 and is used by XSLT 2.0,XQuery and other specifications.Reference manualsTransforming XML with XSLT (includes a useful overview of XSLT/XPath functions)2.2 XSLT, XQuery and XPathEach time a given XSLT or XQuery instruction needs to address (refers to) parts of an XML document, we use XPath expressions. XPath expressions also can contain functions, simple math and boolean expressions.WithinXSLT, XPath expressions are typicially used in match, select and test attributes:

Xpath expressions in an XSLT templateBelow is an XQuery example taken from theXQuery tutorial - basicsfor $t in fn:doc("catalog09.xml")//c3msbricklet $n := count($t//c3mssoft)where ($n > 1)order by $nreturn <result> {$t/title/text()} owns {$n} bricks </result>2.3 PlaytimeMost XML editors will display the XPath of a selected element. In addition, you can search with XPath expressions, i.e. test an expression that you then would use in your XSLT or XQuery code.Below is a screenshot of the XML Exchanger editor. As you can see we entered the //participant expression in the search box. The result is a so-called node-set that is displayed in the XPath pane at the bottom.

Xpath seach in the Exchanger editor3 The XPath Syntax and the document model3.1 Xpath SyntaxXPath expressions can be quite simple or very complex. An Xpath expression may include a location path (i.e. "where to look for"), node tests (i.e. identifying a node) and predicates(i.e. additional tests).There are two notations for location paths.(1) abbreviatedThis simple notation allows to locate itself, children, parents, attributes and combinations of these. Going up or down is called an axis. The abbreviated form has limited axis.para means "all "para" child elements of the current context(2) unabbreviatedUnabbreviated location path allows to search in more axis then just parents, children, and siblings.child::para" is identical to para above.The abbreviated location path look a bit like file paths. E.g. the following expression:/section/titlemeans: find all title nodes below section nodesSyntax overview of the primary (relatively simple) XPath expressionsThe picture below is not entirely correct, i.e. the "green" elements are part of the so called node-test

The result of an Xpath expression can be various data types, e.g. sets of nodes, a single node, a number, etc. Most often the result, is a set of nodes.The formal specification of an XML Pathis very complex, i.e. has about 39 clauses and is very difficult to understandSome expressions shown here are beyond the scope of this tutorial, don"t panic!3.2 The document model of XPathXPath sees an XML document as a tree structureEach information (XML elements, attributes, text, etc.) is called a node. This is fairly similar to the W3C DOM model an XML or XSLT processor would use.Nodes that XPath can seeroot node ATTENTION: The root is not necessarily the XML root element. E.g. processing instructions like a stylesheet declaration are also nodes.Elements and attributesSpecial nodes like comments, processing instructions, namespace declarations.Nodes XPath can"t seeXPath looks at the final document, therefore it can"t see entities and document type declarations....The XML contextWhat a given XPath expression means, is always defined by a given XML context, i.e. the current node in the XML tree a processor is looking at.4 Using simple location pathBelow, we present a few expressions for locating nodes using the simple abbreviated syntax. As we said before, location paths can be horribly complex, but simple location path look a bit like file path that you would use in HTML links or in an operating system like Unix, Windows or MacOS.4.1 List of simple location pathDocument root node - returns the document root (which is not necessarily the XML root!)/Direct child elementXML_element_nameDirect child of the root node/XML_element_nameChild of a childXML_element_name/XML_element_nameDescendant of the root//XML_element_nameDescendant of a nodeXML_element_name//XML_element_nameParent of a node../A far cousin of a node../../XML_element_name/XML_element_name/XML_element_name4.2 Example - Extracting titles from an XML file with XSLTLet us recall that we use the same XML document in most examples. You can download the file or look at a wiki page:xpath-jungle.xmlXPath tutorial - basics/XML example code<?xml version="1.0"?><project> <title>The Xpath project</title> <participants> <participant> <FirstName>Daniel</FirstName> <qualification>8</qualification> <description>Daniel will be the tutor</description> <FoodPref picture="dolores_001.jpg">Sea Food</FoodPref> </participant> <participant> <FirstName>Jonathan</FirstName> <qualification>5</qualification> <FoodPref picture="dolores_002.jpg">Asian</FoodPref> </participant> <participant> <FirstName>Bernadette</FirstName> <qualification>8</qualification> <description>Bernadette is an arts major</description> </participant> <participant> <FirstName>Nathalie</FirstName> <qualification>2</qualification> </participant> </participants> <problems> <problem> <title>Initial problem</title> <description>We have to learn something about Location Path</description> <difficulty level="5">This problem should not be too hard</difficulty> </problem> <solutions> <item val="low">Buy a XSLT book</item> <item val="low">Find an XSLT website</item> <item val="high">Register for a XSLT course and do exercices</item> </solutions> <problem> <title>Next problem</title> <description>We have to learn something about predicates</description> <difficulty level="6">This problem is a bit more difficult</difficulty> </problem> <solutions> <item val="low">Buy a XSLT book</item> <item val="medium">Read the specification and do some exercises</item> <item val="high">Register for a XPath course and do exercices</item> </solutions> </problems></project>TaskWe would like to get a simple list of problem titlesSolutionXSLT template (file:xpath-jungle-1.xsl<?xml version="1.0" encoding="utf-8"?><xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0"> <xsl:output method="html"/> <xsl:template match="/project"> <html> <body bgcolor="#FFFFFF"> <h1><xsl:value-of select="title" /></h1> Here are the titles of our problems: <ul> <xsl:apply-templates select="problems/problem" /> </ul> </body> </html> </xsl:template><xsl:template match="problems/problem"> <li><xsl:value-of select="title" /></li></xsl:template></xsl:stylesheet>(1) XSLT template for the root elementThe XPath of the "match" means: apply the template to the project element node, which is a direct child of the root nodeThe execution context of this template is therefore the element "project"The xsl:apply-templates expression will now tell the processor to find a rule for the problems/problem descendant.(2) XSLT template for the problems/problem elementThis second rule will be triggered by the first rule, because problems/problem is indeed a location that can be found in project element. The processor then can extract the value of the title element.Alternatively we could have written this rule as:<xsl:template match="problems/problem/title"> <li><xsl:apply-templates/></li></xsl:template>or<xsl:template match="problems/problem/title"> <li><xsl:value-of select="."/></li></xsl:template>(3) Result HTML<html> <body bgcolor="#FFFFFF"> <h1>The Xpath project</h1> Here are the titles of our problems: <ul> ""<li>Initial problem</li>"" ""<li>Next problem</li>"" </ul> </body> </html>Live example:xpath-jungle-1.xml4.3 Attribute Location PathsOf course, XPath also also to locate attributes. We shall show the principle, using a few examples.(1) To find an attribute of a child element in the current context use:@attribute_nameExample:@val(2) Find attributes of an element in a longer location path starting from root/element_name/element_name/@attribute_nameExample:/project/problems/solutions/item/@val(3) Find attributes in the whole document: //@attribute_nameAs you can see you can combine element location with attribute identification.4.4 Example - Create an html img link from an attributeXML fragmentSame as aboveTaskDisplay a list of First Names plus their food preferencesXSLT (Filexpath-jungle-2.xsl<?xml version="1.0" encoding="utf-8"?><xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0"> <xsl:output method="html"/> <xsl:template match="/"> <html> <body bgcolor="#FFFFFF"> <h1>What do we know about our participants?</h1> Here are some food preferences: <ul> <xsl:apply-templates select=".//participant" /> </ul> </body> </html> </xsl:template><xsl:template match="participant"> <li><xsl:value-of select="FirstName"/> <xsl:apply-templates select="FoodPref"/> </li></xsl:template><xsl:template match="FoodPref"> prefers <xsl:value-of select="."/>. <img src="{@picture}"/> <br clear="all"/></xsl:template></xsl:stylesheet>The second rule will display names of participants and launch a template for FoodPrefNote: Not all participants have a FoodPref element. If it is absent it will just be ignored.The third rule (FoodPref) displays the text (contents) of FoodPref and then makes an HTML img tagParts of the result<h1>What do we know about our participants?</h1> Here are some food preferences: <ul> <li>Daniel prefers Sea Food. <img src="dolores_001.jpg"><br clear="all"></li> <li>Jonathan prefers Asian. <img src="dolores_002.jpg"><br clear="all"></li> <li>Bernadette</li> <li>Nathalie</li> </ul>Live example:xpath-jungle-2.xml4.5 Location wildcardsSometimes (but not often!), it is useful to work with wildcardsYou have to understand that only one rule will be applied per element. Rules with wildcards have less priority and btw. this is why "your rules" are applied before the system default rules.Find all child nodes of type XML element*Find all child nodes (including comments, etc.)node()Find all element attributes@*Find all text nodestext()Combine locationsuse the "|" operator, an example is just below.Example: XSLT includes two built-in default rules. They rely on using these wildcards.This rule applies to the document root and all other elements<xsl:template match="*|/"> <xsl:apply-templates/> </xsl:template>Text and attribute values are just copied<xsl:template match="text()|@*"> <xsl:value-of select="."/> </xsl:template>5 XPaths with predicatesLet us now scale up a bit.A predicate is an expression that can be true or falseIt is appended within [...] to a given location path and will refine resultsMore than one predicate can be appended to and within (!) a location pathExpressions can contain mathematical or boolean operatorsFind element number N in a listSyntax: XML_element_name [ N ]/project/participants/participant[2]/project/participants/participant[2]/FirstName Find elements that have a given attributeSynatx: XML_element_name [ @attribute_name ]//difficulty[@level] Find elements that have a given element as childSyntax XML_element_name [ XML_element_name ]//participant[FoodPref]Note: this is not the same as //participant/FoodPref. The latter would return a list of FoodPref elements, whereas the former returns a list of participantMathematical expressionsUse the standard operators, except div instead of / (for obvious reasons)- * div mod mod is interesting if you want to display a long list in table format5 mod 2 returns 1, as will "7 mod 2" and "3 mod 2" Boolean operators (comparison, and, or)List of operators (according to precedence)<=, <, >=, >=,!=and, orExamplesReturn all exercise titles with a grade bigger than 5.//exercise[note>5]/titleFind elements that have a given attribute with a given valueRecall of the Syntax: XML_element_name [ @attribute_name="value"]//solutions/item[@val="low"]Example XSLT template that will match all item elements with val="low".<xsl:template match=""//item[@val="low"]"> <xsl:value-of select="." /></xsl:template>Usually expressions also contain functions as we shall see below, examples:Return the last five elements of a listauthor [(last() - 4) <= position()) and (position() <= last())] Return all Participant nodes with a contents of FirstName bigger than 7 characters:"//Participant[string-length(FirstName)>=8]"5.1 Example: Retrieve selected elementsThe following example will retrieve the following:All persons who do have a food preferenceAll items that have an a "priority" attribute set to "high"The XSLT stylesheet (filexpath-jungle-3.xsl)<?xml version="1.0" encoding="utf-8"?><xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0"> <xsl:output method="html"/> <xsl:template match="/"> <html> <body bgcolor="#FFFFFF"> <h1>Retrieve selected elements</h1> Here is the name of participant two: <ul><li><xsl:value-of select=".//participant[2]/FirstName"/></li></ul> Here are all participant"s firstnames that have a food preference: <ul><xsl:apply-templates select=".//participant[FoodPref]"/></ul> Here are all items that have a value of "high" <ul><xsl:apply-templates select=".//item[@val="high"]"/></ul> </body> </html> </xsl:template><xsl:template match="participant"> <li><xsl:value-of select="FirstName"/></li></xsl:template><xsl:template match="item"> <li><xsl:value-of select="."/></li></xsl:template></xsl:stylesheet>HTML result<html> <body bgcolor="#FFFFFF"> <h1>Retrieve selected elements</h1> Here is the name of participant two: <ul> <li>Jonathan</li> </ul> Here are all participant"s firstnames that have a food preference: <ul> <li>Daniel</li> <li>Jonathan</li> </ul> Here are all items that have a value of "high" <ul> <li>Register for a XSLT course and do exercices</li> <li>Register for a XPath course and do exercices</li> </ul> </body></html>Live example:xpath-jungle-3.xmlxpath-jungle-3.xslBelow is a more complex example. One in short notation and the other (to do) in it equivalent long notations:/outputTree/command/pivotTable/dimension//category[@text="Measurement"]/dimension/category/cell[@text="Nominal"]6 XPath functionsXPath defines a certain number of functions. You can recognize a function because it has appended "()".Functions are programming constructs that will return various kinds of informations, e.g.true / falsea numbera stringa list of nodesIt is not obvious to understand what all of these functions do. For example, there are restrictions on how you can use functions (stick to examples or the reference)last()last() gives the number or nodes within a contextposition()position() returns the position of an element with respect to other children in the same parentWarning: The result will include empty nodes create from whitespaces between elements. To avoid this you should strip the parent nodes using<xsl:strip-space elements="name_of_the_parent_node"/>count(node-set)count gives the number of nodes in a node setWe got <xsl:value-of select="count(//problem)"/> problems, i.e. it will count N elements retrieved and not the child elements of problemLive example:xpath-jungle-6.xml -xpath-jungle-6.xslstarts-with(string, string)returns TRUE if the second string is part of the first and starts off the first//Participant[starts-with(Firstname,"Berna")]" contains(string, string)returns TRUE if the second string is part of the first//Participant[contains(FirstName,"nat")] string-length(string)returns the length of a stringnumber(string)transforms a string into a numbersum(node-set)computes the sum of a given set of nodes.If necessary, does string conversion with number()round(number)round a number, e.g. 1.4 becomes 1 and 1.7 becomes 2translate(string1, string2, string3)translates string1 by substituting string2 elements with string3 elements6.1 Example: Computation of an averageWe would like to compute the average of participant"s qualifications<participant><FirstName>Daniel</FirstName> ""<qualification>8</qualification>"" </participant>The XSLT stylesheet (filexpath-jungle-4.xslWe compute the sum of a node-set and then divide by the number of nodes<?xml version="1.0" encoding="utf-8"?><xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0"> <xsl:output method="html"/> <xsl:template match="/"> <html> <body bgcolor="#FFFFFF"> <h1>Qualification level of participants</h1> Average is <xsl:value-of select="sum(.//participant/qualification) div count(.//participant/qualification)"/> </body> </html> </xsl:template></xsl:stylesheet>HTML result<html> <body bgcolor="#FFFFFF"> <h1>Qualification level of participants</h1> Average is 5.75 </body> </html>Live example:xpath-jungle-4.xml6.2 Example: Find first names containing "nat"The XSLT stylesheet (filexpath-jungle-5.xsl<?xml version="1.0" encoding="utf-8"?><xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0"> <xsl:output method="html"/> <xsl:template match="/"> <html> <body bgcolor="#FFFFFF"> <h1>Do we have a "nat"?</h1> First Names that contain "nat": <ul><xsl:apply-templates select=".//participant[contains(FirstName,"nat")]"/></ul> First Names that contain "nat" and "Nat": <ul><xsl:apply-templates select=".//participant[contains(translate(FirstName,"N","n"),"nat")]"/></ul> </body> </html> </xsl:template><xsl:template match="participant"> <li><xsl:value-of select="FirstName"/></li></xsl:template></xsl:stylesheet>Live examplexpath-jungle-5.xml7 Union of XPathsUnion Xpaths combine more than one XPath (and all the resulting nodes are returned). A typical example that we already introduced above is the default rule which means that the template matches either the root element (i.e. "/" or just any element),<xsl:template match="*|/"> <xsl:apply-templates/> </xsl:template>Often, this construct is used to simplify apply-templates or even templates themselves. E.g. the following rules applies to both "description" and "para" elements.<xsl:template match="para|description"> <p><xsl:apply-templates/></p> </xsl:template>8 List of commonly used XPath expressionsSyntaxelement(Type of path)Example pathExample matchesnamechild element nameproject<project> ...... </project>/child / childproject/title<project> <title> ... </title>/(root element)//descendantproject//title<project><problem> <title>....</title>//title<root>... <title>..</title> (any place)*"wildcard"*/title<bla> <title>..</title> and <bli> <title>...</title>|"or operatortitle|head<title>...</title> or <head> ...</head>*|/|@*All elements: root, children and attributes.current element.../parent element../problem<project>@attrattribute name@id<xyz id="test">...</xyz>element/@attrattribute of childproject/@id<project id="test" ...> ... </project>@attr="value"value of attributelist[@type="ol"]<list type="ol"> ...... </list>position()position of elementin parentposition()last()number of elements within a contextlast()position()!=last()Important:The XML standard requires that an XML parser returns emtpy space between elements as empty nodes. In other words, functions such as location() or last() will also count empty nodes! Tell XSLT to remove these within the elements within which you need to count.Good code:<xsl:strip-space elements="name_of_the_parent"/>Bad code:you forgot to use this ..Example fragment where position in a list is used to compute position on the screen with SVG in HTML5:<xsl:strip-space elements="list"/><xsl:template match="list"> <rect x="10" y="105" width_="{10 * count(item)}" height="5" fill="black" stroke="red"/> <xsl:apply-templates/></xsl:template>Live example:intro-html5.xml9 Links9.1 Introductory tutorialsXpath (Wikipedia)Zvon tutorial (lots of examples)XPath for .NET Developers by Darshan Singh9.2 OtherXPath Visualizer A windows program you can install to train. Alternatively, just use a XML editor with Xpath support.Liquid XML has anXPath builder
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