PB138 — XPath
XPath basic principles
• XPath is a syntax used to specify parts of XML documents (primitive values, nodes, sequences of
values or nodes
• XPath does not allow to specify parts of text nodes.
• Its name is derived from path expression providing a means of hierarchic addressing of the
nodes in an XML tree.
• XPath uses syntax similar to file system path.
• XPath offers standard functions library, as well as user defined functions in either some XPath
2.0 or even XPath 1.x processors.
• XPath does not use XML syntax (it would be too long).
XPath specifications
• XPath 1.0 (revised Sep 7, 2015),
• XML Path Language (XPath) 2.0 and
• XML Path Language (XPath) 3.0 are W3C Recommendations (Apr 08, 2014)
• XPath 3.1 is a Recommendation (Mar 21, 2017)
• Backward compatibility: nearly all XPath 1.0 expressions continue to deliver the same result
with XPath 3.0 (for exceptions see http://www.w3.org/TR/xpath-30/#id-backwards-compatibility)
XPath in other XML technologies
• XPath is used as a base for XSLT since version 1.0 and
• in XQuery since XPath version 2.0.
Crucial Learning Resources
• XPath Tutorial @W3Schools
• Zvon XPath 1.0 Tutorial to learn step by step (by Miloslav Nič)
• Online XPath Tester - Evaluator by ExtendsClass
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XPath domain: Advanced XML Data
navigation
<?xml version="1.0"?>
<a>
  <b/> <!-- this is the //b[count(./*)=0] -->
  <b> <!-- this is the //b[./c] -->
  <c/>
  </b>
  <b> <!-- this is the //b[3] -->
  <!-- and also //b[./c] -->
  <c/>
  </b>
</a>
• Select the 3rd node b: //b[3]
• Select a node "b", which has a child node "c": //b[./c]
• Select an empty (eg. no child elements) node b: //b[count(./*)=0]
XPath domain: Transformation (XSLT)
• Select nodes that have to be processed next: <xsl:apply-templates match="para"/>
• Select value: <xsl:value-of select="para/@id"/>
XPath domain: Selection parts in XQuery
• (F)or part, eg. for $para in $doc//para selects all para in the document doc
• (L)et part, eg. let $mypara := $doc//para[@owner='myself']
• (W)here part, eg. where $para[@class='task']
• (O)rder part, eg. order by $para/@created
XPath domain: Modeling languages
• Schematron
• XML Schema
XPath paths and locations
Path describes (or "navigates" to) an XML document location. Paths syntax is constructed a similar
way to paths in file systems, i.e.:
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relative
related to a context node (CN), see further, or
absolute
related to the root element but predicates are evaluated in relation to CN.
XPath data types
• Since XPath 3.0 unified with the XML Schema and XQuery datatypes
• XQuery and XPath Data Model 3.0, W3C Recommendation 08 April 2014
Axes
• Axes (singular axis, plural axes) are sets of document elements, related to (usually relatively) to
context.
• Context is formed by a document and the current (context) node (CN).
List of Axes (1)
child
contains direct child nodes of CN
descendant
contains all descendants of CN except attributes.
parent
contains the CN parent nod (if it exists)
ancestor
contains all ancestors of CN - means parents, grandparents, etc to a root element (if the CN is not
the root element itself)
following-sibling
contains all following siblings of CN (the axis is empty for NS and attributes)
preceding-sibling
dtto, but it contains the preceding sibling.
List of Axes (2)
following
contains all nodes following the CN (except the attributes, child nodes and NS nodes)
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preceding
dtto, but contains preceding nodes (except ancestors, attributes, NS)
attribute
contains attributes (for elements only)
namespace
contains all NS nodes of CN (for elements only)
self
the CN itself
descendant-or-self
contains the union of descendant and self axes
ancestor-or-self
contains the union of ancestor and self axes
XPath online testers
• It is possible to try evaluation of XPath expressions upon a provided XML document by using
many online testers without the need of (local PC) installation.
• Such as http://codebeautify.org/Xpath-Tester or
• XPath online tester also allows to evaluate XPath against an XML document
Example //b/child::*
<?xml version="1.0"?>
<a>
  <b/>
  <b>
  <c/> <!-- this "c" will be selected -->
  </b>
  <b>
  <c/> <!-- and this "c" too -->
  </b>
</a>
Example //b/descendant::*
<?xml version="1.0"?>
<a>
  <b/>
  <b>
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  <c> <!-- everything "under b" will be selected -->
  <d/> <!-- i.e. this "d" too -->
  </c>
  </b>
  <b>
  <c/> <!-- and this "c" too -->
  </b>
</a>
Example //d/parent::*
<?xml version="1.0"?>
<a>
  <b/>
  <b>
  <c> <!-- this "c" is the parent of "d" -->
  <d/>
  </c>
  </b>
  <b>
  <c/>
  </b>
</a>
Example //d/ancestor::*
<?xml version="1.0"?>
<a> <!-- this "a" is ancestor of "d" -->
  <b/>
  <b> <!-- this "b" is ancestor of "d" -->
  <c> <!-- this "c" is ancestor of "d" -->
  <d/>
  </c>
  </b>
  <b>
  <c/>
  </b>
</a>
Example //b/following-sibling::*
<?xml version="1.0"?>
<a>
  <b/> <!-- every child of "a" after this "b" is following-sibling -->
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  <b> <!-- this "b" too -->
  <c>
  <d/>
  </c>
  </b>
  <b> <!-- this "b" too -->
  <c/>
  </b>
</a>
Example //b/preceding-sibling::*
<?xml version="1.0"?>
<a>
  <b/> <!-- this "b" too -->
  <b>
  <c>
  <d/>
  </c>
  </b> <!-- this "b" is preceding-sibling -->
  <b> <!-- every child of "a" before this "b" is preceding-sibling -->
  <c/>
  </b>
</a>
Example /a/b/c/following::*
<?xml version="1.0"?>
<a>
  <b/>
  <b>
  <c>
  <d/>
  </c> <!-- every element starting after "c" is following -->
  <e/> <!-- such as this "e" -->
  </b>
  <b> <!-- this "b" too -->
  <c/> <!-- this "c" too -->
  </b>
</a>
Example /a/b/e/preceding::*
<?xml version="1.0"?>
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<a>
  <b/> <!-- this "b" too -->
  <b> <!-- this "b" too -->
  <c> <!-- this "c" too -->
  <d/> <!-- this "d" too -->
  </c>
  </b>
Example /a/b/e/preceding::*
  <b>
  <d/> <!-- such as this "d" -->
  <e/> <!-- every element starting before "e" is preceding -->
  </b>
</a>
Predicates
• Figure: /article/para[3] — selects the 3rd paragraph (element para) of article (element article)
• Simplest predicate expression is proximity position specification — see preceding.
• Attention at reverse axes (ancestor, preceding, …) - position is numbered always from the
context node, means opposite to document physical location directions.
• Position specification 3 can be replace by the expression position()=3.
Expressions
• Used in predicates for calculations. Expressions may contain XPath functions. Expressions may
operate on:
◦ text strings
◦ numbers (floating-point numbers)
◦ logical values (boolean)
◦ nodes
◦ sequences.
Short notation — examples 1
para
selects all child nodes of context node with name para
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*
selects all element children of the context node
text()
selects all text node children of the context node
@name
selects the name attribute of the context node
@*
selects all the attributes of the context node
para[1]
selects the first para child of the context node
para[last()]
selects the last para child of the context node
*/para
selects all para grandchildren of the context node
Short notation — examples 2
/doc/chapter[5]/section[2]
selects the second section of the fifth chapter of the doc
chapter//para
selects all descendants of element chapter with name para
//para
selects all elements para in the document
//olist/item
selects all elements item with parent element olist
.//para
selects all descendant nodes of the context node with name para
..
selects the parent node of the context node
../@lang
selects a lang attribute of the context node parent node
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XPath — short notation (2)
Most common used short notation is at child axis
• we use article/para instead of child::article/child::para.
• at attribute:we use para[@type="warning"] instead of child::para[attribute::type="warning"]
• The next used short notation is // instead of /descendant-or-self::node()/
• and of course shortcuts . and ..
For clarity, we keep sometimes the longer form: Do not fight it at all costs!
XPath 2.0
• Final specification available at http://www.w3.org/TR/xpath20/
• Different point of view on return values of XPatch expressions: everything is a sequence (even
containing a single element) → removes the set node order problems
• Introduces conditional expressions and cycles.
XPath 2.0
• Introduces user-defined functions (dynamically evaluate XPath expressions)
• Users can uses general and existential quantifiers, for example exist student/name="Fred", all
student/@id
• For more details see http://www.saxonica.com/, pages contains the XPath/XSLT/XQuery
processor Saxon as well.
Other resources on XPath
• Programming in XPath 3.0 (D. Novatchev)
• XPath functions (Mozilla)
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