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)
PathEnq XPath 2.0 online evaluator — nice for try&see

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.:

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)
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
PathEnq XPath 2.0 online evaluator
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>
       <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 -->
  <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"?>
<a>
     <b/> <!-- this "b" too -->
     <b> <!-- this "b" too -->
         <c> <!-- this "c" too -->
             <d/> <!-- this "d" too -->
         </c>
     </b>
     <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
*: 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

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.
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.

XPath 2.0 - examples

String functions: http://www.fi.muni.cz/~tomp/xml03/xpath20/string.html
Numeric functions: http://www.fi.muni.cz/~tomp/xml03/xpath20/numeric.html
Sequence functions: http://www.fi.muni.cz/~tomp/xml03/xpath20/sequence.html
Boolean functions: http://www.fi.muni.cz/~tomp/xml03/xpath20/boolean.html

Other resources on XPath

Programming in XPath 3.0 (D. Novatchev)
XPath functions (Mozilla)