Navigation in XML data; XPath
March 1, 2013
1 XPath
1.1 XPath - core principles
• XPath is a special language (non a XML markup) to specify parts of XML
documents (nodes, node sets, node sequences);
• Parts of text nodes cannot be specified using XPath.
• XPath uses a syntax resembling the one used for specifying (file) path in
a file system.
• XPath uses a library of standard functions
• in XPath 2.0 and some XPath 1.x processors may use a user-defined set
of functions
• XPath 1.0 is a base for XSLT, XPath 2.0 also for XQuery
• XPath syntax is NOT XML (it would be too verbose)
• XPath 1.0 and 2.0 are W3C Recommendations - http://www.w3.org/TR/
xpath
1.2 XPath - application areas
• advanced navigation in XML data¡?xml version=”1.0”?¿ ¡a¿ ¡b/¿ ¡b¿ ¡c/¿
¡/b¿ ¡b¿ ¡c/¿ ¡/b¿ ¡/a¿
1.3 XPath - application areas
• Select third node b://b[3]
• Select the node b, having an ancestor c://b[./c]
• Select empty node b://b[count(./*)=0]
1.4 XPath - application areas /1
• Transformation (XSLT (http://www.w3.org/TR/xslt))
– used to eg. selection of nodes to process: ¡xsl:apply-templates se-
lect=”./c”/¿
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1.5 XPath - application areas /2
• In ”selection part” of XML query languages (XQuery (http://www.w3.
org/XML/Query/))
• In some modeling languages (Schematron (http://www.schematron.com/),
XML Schema (http://www.w3.org/XML/Schema))
• ...
1.6 XPath - pojem cesty (paths) a lokace (locations)
Path defines (ie. navigates to) a location in a document. Paths are constructed
similarly as in a file system, ie.
relative evaluated from the so-called context node (CN), see later, or
absolute from the document root, but predicates (expressions) also in relation
to the CN
1.7 XPath - syntax
[20] PathExpr ::= AbsolutePathExpr — RelativePathExpr [22] AbsolutePathExpr
::= (”/” RelativePathExpr?) — (”//” RelativePathExpr) [23] RelativePathExpr
::= StepExpr ((”/” — ”//”) StepExpr)* [24] StepExpr ::= AxisStep — GeneralStep
[25] AxisStep ::= (Axis? NodeTest StepQualifiers) — AbbreviatedStep
1.8 XPath - osy (axes)
Osy (singular axis, plural axes) are sets (sequences) of document nodes, usually
but not exclusively, outgoing from the context.Contextis composed of document
and the current (context) node (CN).Axes:
child all child nodes of the CN
descendant all descendants of the CN. No attributes.
parent parent node to the CN
ancestor all ancestor (parent, parent of parent, etc.) nodes
following-sibling all following siblings of the CN (for NS node and attributes
this is empty)
preceding-sibling dtto, but preceding siblings
following all nodes located after CN (no attributes, descendants and CN)
preceding similarly, but before
attribute all attributes of the CN (must be an element)
namespace contains all NS nodes of the CN (just for elements)
self the CN itself
descendant-or-self union of descendant and self
ancestor-or-self union of os ancestor and self
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1.9 Example: child axis
//b/child::*¡?xml version=”1.0”?¿ ¡a¿ ¡b/¿ ¡b¿ ¡c/¿ ¡/b¿ ¡b¿ ¡c/¿ ¡/b¿ ¡/a¿
1.10 Example: descendant axis
//b/descendant::*¡?xml version=”1.0”?¿ ¡a¿ ¡b/¿ ¡b¿ ¡c¿ ¡d/¿ ¡/c¿ ¡/b¿ ¡b¿
¡c/¿ ¡/b¿ ¡/a¿
1.11 Example: parent axis
¡?xml version=”1.0”?¿ ¡a¿ ¡b/¿ ¡b¿ ¡c¿ ¡d/¿ ¡/c¿ ¡/b¿ ¡b¿ ¡c/¿ ¡/b¿ ¡/a¿
1.12 Example: ancestor axis
¡?xml version=”1.0”?¿ ¡a¿ ¡b/¿ ¡b¿ ¡c¿ ¡d/¿ ¡/c¿ ¡/b¿ ¡b¿ ¡c/¿ ¡/b¿ ¡/a¿
1.13 Example: axis following-sibling
¡?xml version=”1.0”?¿ ¡a¿ ¡b/¿ ¡b¿ ¡c¿ ¡d/¿ ¡/c¿ ¡/b¿ ¡b¿ ¡c/¿ ¡/b¿ ¡/a¿
1.14 Example: axis preceding-sibling
¡?xml version=”1.0”?¿ ¡a¿ ¡b/¿ ¡b¿ ¡c¿ ¡d/¿ ¡/c¿ ¡/b¿ ¡b¿ ¡c/¿ ¡/b¿ ¡/a¿
1.15 Example: axis following
¡?xml version=”1.0”?¿ ¡a¿ ¡b/¿ ¡b¿ ¡c¿ ¡d/¿ ¡/c¿ ¡e/¿ ¡/b¿ ¡b¿ ¡c/¿ ¡/b¿ ¡/a¿
1.16 Example: axis preceding
¡?xml version=”1.0”?¿ ¡a¿ ¡b/¿ ¡b¿ ¡c¿ ¡d/¿ ¡/c¿ ¡/b¿ ¡b¿ ¡d/¿ ¡e/¿ ¡/b¿ ¡/a¿
1.17 XPath predicates
Condition used to select (filter) nodes specified eg. by pathex.: /article/para[3]
- selects the third para of the article The simplest is (proximity position) - see
above
• Attention by reverse axes (ancestor, preceding...) - the position is calculated
always (outwards) from the CN
3 could equally be replaced by position()=3
1.18 XPath expressions
Used in predicates, calculation (aggregation), etc. Might contain XPath functions.Expressions
can be:
• string (characters)
• numeric (floating-point numbers)
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• logic (boolean)
• nodes
• sequences
1.19 XPath - Examples of shortened notation
• paraselect all ”para” child elements of the CN
• *selects all element children of the context node
• text()selects all text node children of the context node
• @nameselects 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
• */paraselects all para grandchildren of the context node
• /doc/chapter[5]/section[2]selects the second section of the fifth chapter
of the doc
• chapter//paraelect all ”para” descendant elements of ”chapter”
• //paraall ”para” elements from the document
• //olist/itemall item elements, having a parent ”olist”
• . selects the CN
• .//paraselect all ”para” descendants of the CN
• .. vybere parent element of the CN
• ../@langselects the attribute ”lang” from the parent node of CN
1.20 XPath - shortened notation (2)
Most frequently used is the shortening of child axis:
• like article/para instead of child::article/child::para.
• and attributes: we write para[@type="warning"] instead of child::para[attribute::type="warning
• use of // instead of /descendant-or-self::node()/
• and shorthands dot . and double-dot ..
Sometimes it is good to preserve the full (long) form. So, please, learn it!
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1.21 Infosources on XPath
• XPath / W3C: http://www.w3.org/TR/xpath
• Zvon XPath Tutorial: http://zvon.org/xxl/XPathTutorial/Output/
index.html
• XPath Tutorial / W3Schools: http://www.w3schools.com/xpath/xpath\
_intro.asp
1.22 XPath 2.0
• The Recommendation - http://www.w3.org/TR/xpath20/
• The return value of an XPath expression: all are sequences (even if one
item)
• so they define ORDER on the returned nodes
• Introduce conditional expressions and loops
• User functions (in fact, dynamically evaluated expressions in XPath)
• One can use general and existence quantifier, eg. exist student/name="Fred"
or all student/@id
• See further eg. http://www.saxonica.com/, where also the XPath/XSLT/XQuery
processor Saxon is located.
1.23 XPath 2.0 - samples
• String functions (http://www.fi.muni.cz/~tomp/xml03/xpath20/string.
html)
• Numerical 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)
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