An Introduction to XPath

1
An Introduction to XPath
Nick Davies
Tuesday 20th January 2004
2
XPath Introduction

• XPath is a W3C recommendation (1999).
• According to W3C, it is a language for
  addressing parts of an XML document
• It can be thought of as query language, like
  SQL, however it operates on XML documents, not
  databases, by specifying path expressions in
  order to identify nodes in the document
• XPath models XML documents as trees of nodes
  using the Document Object Model (DOM), text order
  is maintained in the tree
• XPath's primary intention was that of a
  component that can be used by other
  specifications (such as XPointer and XSLT)

3
XPath Syntax
XPath uses path expressions to locate nodes with
XML documents Here is a simple XML document,
representing a DVD collection.
lt?xml version1.0 encodingISO-8859-1?gt ltcolle
ctiongt ltdvd titleFawlty Towersgt ltgenregtcomed
ylt/genregt ltyeargt1975lt/yeargt ltcertgtPGlt/certgt lt
/dvdgt ltdvd titleQuadropheniagt ltgenregtcultlt/g
enregt ltyeargt1979lt/yeargt ltlengthgt114lt/lengthgt
lt/dvdgt ltdvd titleGoldfingergt ltyeargt1964lt/yea
rgt ltcertgtPGlt/certgt ltlengthgt105lt/lengthgt lt/dvd
gt ltcollectiongt
4
XPath Syntax
We can now describe the XPath syntax with
examples from the previous XML document
lt?xml version1.0 encodingISO-8859-1?gt ltcolle
ctiongt ltdvd titleFawlty Towersgt ltgenregtcomed
ylt/genregt ltyeargt1975lt/yeargt ltcertgtPGlt/certgt lt
/dvdgt ltdvd titleQuadropheniagt ltgenregtcultlt/g
enregt ltyeargt1979lt/yeargt ltlengthgt114lt/lengthgt
lt/dvdgt ltdvd titleGoldfingergt ltyeargt1964lt/yea
rgt ltcertgtPGlt/certgt ltlengthgt105lt/lengthgt lt/dvd
gt ltcollectiongt
Locating nodes /collection/dvd/cert
5
XPath Syntax
We can now describe the XPath syntax with
examples from the previous XML document
lt?xml version1.0 encodingISO-8859-1?gt ltcolle
ctiongt ltdvd titleFawlty Towersgt ltgenregtcomed
ylt/genregt ltyeargt1975lt/yeargt ltcertgtPGlt/certgt lt
/dvdgt ltdvd titleQuadropheniagt ltgenregtcultlt/g
enregt ltyeargt1979lt/yeargt ltlengthgt114lt/lengthgt
lt/dvdgt ltdvd titleGoldfingergt ltyeargt1964lt/yea
rgt ltcertgtPGlt/certgt ltlengthgt105lt/lengthgt lt/dvd
gt ltcollectiongt
Select Unknown Values using wildcards ()
/collection/dvd/
6
XPath Syntax
We can now describe the XPath syntax with
examples from the previous XML document
lt?xml version1.0 encodingISO-8859-1?gt ltcolle
ctiongt ltdvd titleFawlty Towersgt ltgenregtcomed
ylt/genregt ltyeargt1975lt/yeargt ltcertgtPGlt/certgt lt
/dvdgt ltdvd titleQuadropheniagt ltgenregtcultlt/g
enregt ltyeargt1979lt/yeargt ltlengthgt114lt/lengthgt
lt/dvdgt ltdvd titleGoldfingergt ltyeargt1964lt/yea
rgt ltcertgtPGlt/certgt ltlengthgt105lt/lengthgt lt/dvd
gt ltcollectiongt
Selecting Branches using square
brackets /collection/dvd1 /collection/dvdlas
t()
7
XPath Syntax
We can now describe the XPath syntax with
examples from the previous XML document
lt?xml version1.0 encodingISO-8859-1?gt ltcolle
ctiongt ltdvd titleFawlty Towersgt ltgenregtcomed
ylt/genregt ltyeargt1975lt/yeargt ltcertgtPGlt/certgt lt
/dvdgt ltdvd titleQuadropheniagt ltgenregtcultlt/g
enregt ltyeargt1979lt/yeargt ltlengthgt114lt/lengthgt
lt/dvdgt ltdvd titleGoldfingergt ltyeargt1964lt/yea
rgt ltcertgtPGlt/certgt ltlengthgt105lt/lengthgt lt/dvd
gt ltcollectiongt
Select Several Paths using the
operator /collection/dvd/genre
/collection/dvd/cert
8
XPath Syntax
We can now describe the XPath syntax with
examples from the previous XML document
lt?xml version1.0 encodingISO-8859-1?gt ltcolle
ctiongt ltdvd titleFawlty Towersgt ltgenregtcomed
ylt/genregt ltyeargt1975lt/yeargt ltcertgtPGlt/certgt lt
/dvdgt ltdvd titleQuadropheniagt ltgenregtcultlt/g
enregt ltyeargt1979lt/yeargt ltlengthgt114lt/lengthgt
lt/dvdgt ltdvd titleGoldfingergt ltyeargt1964lt/yea
rgt ltcertgtPGlt/certgt ltlengthgt105lt/lengthgt lt/dvd
gt ltcollectiongt
Selecting Attributes using the _at_
prefix //dvd_at_titleFawlty Towers
9
XPath Location Paths

• One of the most important XPath expressions is
  the location path.
• A location path expression results in a node-set.
• Each location path consists of one or more
  location steps. Each location step has
• an axis
• a node test
• one or more predicates (optional)
• Location Step Syntax and Example
• axisnamenodetestpredicate
  childdvdgenrecomedy

10
XPath Location Paths Axes
There are several axes in XPath each of which
selects a different subset of the nodes in the
document, depending on the context node. They are
Ancestor, ancestor-or-self, attribute, child,
descendant, descendant-or-self, following,
following-sibling, namespace, parent, preceding,
preceding-sibling, self.
collection
collection
dvd
dvd
dvd
dvd
dvd
dvd
year
cert
time
year
cert
time
The child axis with context node collection
The preceding-sibling axis with context node time
11
XPath Location Paths Node Tests

• Node Test
• Determines what kinds of nodes be selected along
  a given axis. The node test is applied to each
  node in the axis
• If the test succeeds the node is kept, if not the
  node is disregarded
• There are 7 types of node Example Location
  Paths
• root nodes childfoo - is there a foo child
  element?
• element nodes parenttext() - is the parent a
  text node?
• text nodes descendantcomment() - is there
  a comment
• attribute nodes node below?
• namespace nodes
• processing instruction nodes
• comment nodes

12
XPath Location Paths Predicates

• Predicates
• The predicate is a further test to retain or
  eliminate nodes. It filters the node set into a
  new node-set. Each node is evaluated in turn and
  if a predicate is true for a given node, it is
  kept in the new node set, if false it is removed.
  
• Consists of a well-formed expressions consisting
  of
• boolean operators
• functions
• numbers, strings
• comparison operators
• Examples
• childdvdgenrecomedy
• descendantdvdposition()2
• descendantcollectionattributetypetitle

13
XPath Expressions

• Expressions describes a set of nodes in a
  documents. Weve already seen one a location
  path.
• Path expressions may be absolute or relative
• absolute begin from the root node /collection/dv
  d/length
• relative begin from the context node dvd/genre
• XPath supports four types of expressions
• Numerical - div mod
• Equality !
• Relational lt gt lt gt
• Boolean or and

14
XPath Functions
XPath defines a number of useful functions for
converting and translating data. Some take
arguments, some dont. Those that dont, operate
on the context node. There are 25 functions.
Here are only a few examples Node Set
Functions count() returns the number of nodes
in a node set String Functions concat()
returns the concatenations of all its
arguments contains() returns true if the second
of 2 given strings is in the first Number
Functions sum() returns the total value of a
set of numeric values in a node-set Boolean
Functions not() returns true if the argument is
false, and false if the arguments is true
15
XPath Summary

• To sum up
• XPath is a straightforward declarative language
  for selecting particular subsets of nodes from an
  XML document
• XPath has a syntax for the selection of nodes,
  unknown elements, branches,
• multiple paths and attributes
• XPath queries are written in expressions which
  can returns doubles or strings and allow for
  arithmetic and relational operations on data
  types
• Location paths are an important subset of
  expressions, they consist of one or more location
  step which in turn consists of an axis, a node
  test, and (optionally) one or more predicates
• XPath also provides a core set of functions for
  operating on the data types

16
Thanks For Your Time Any Questions?