1. XPath

2. What is XPath?
A language designed to be used by XSL Transformations (XSLT), Xlink, Xpointer and XML Query.
Primary purpose: Address ‘part’ of an XML document, and provide basic facilities for manipulation of strings, numbers and booleans.
Because Xpaht is cooperation between XSL and Xpointer working groups it has a broader definition so that both groups can use it. Different implementations specific to each language are defined within that language so that Xpath can be used across several languages.
Xpath is used because we needed a way to point to a specific thing or a set of things in an XML doc.

3. Outline Introduction Data Model Xpath Syntax XPath utilities
Location Path
General Xpath Expressions
Core Function Library
XPath utilities
Conclusion

4. Introduction W3C Recommendation. November 16, 1999
Latest version:
XPath uses a compact, string-based, rather than XML element-based syntax.
Operates on the abstract, logical structure of an XML document rather than its surface syntax.
Uses a path notation (like in URLs) to navigate through this hierarchical tree structure.
Introduction

5. Introduction Cont.
Xpath models an XML doc as a tree of nodes and defines a way to compute a string-value for each type of node.
Supports Namespaces.
Expression (Expr) is the primary syntactic construct of Xpath.
Namespace portion if the name of a node is optional, NULL if not specified
Introduction

6. Data Model The way to represent an XML document.
This tree consists of 7 nodes:
Root Node
Element Nodes
Attribute Nodes
Namespace Nodes
Processing Instruction Nodes
Comment Nodes
Text Nodes
The tree structure is ordered in order of the occurrence of nodes’ start-tag in the XML doc.
Data Model

7. Data Model Example <?xml version=“1.0”>
<?xml-stylesheet type=“text/xsl” href=“bib.xsl” ?>
<! -- simple XML document -->
<bib> <book price=“25.00” pages=“400”>
<publisher> IDG books</publisher> <author> <first-name>Rick</first-name> <last-name> Hull </last-name> </author> <author> Simon North</author> <title> XML complete </title> <year> 1997 </year> </book> <book> <publisher> Freeman </publisher> <author> Jeffrey D. Ullman </author> <title> Principles of Database </title> <year> 1998 </year> </book>
</bib>
Children of DOC are a processing-instruciton node and two para element nodes
Four nodes comprise the descendents of the 1st <para> node: 2 text nodes, and <em>node >>Children
<em> has child text node >> Grandchild of <para>
?Pub Caret?: Pub Caret is the xml declaration and contains special info for the xml processor.
Tree order is top to bottom and left to right. Is possible to count nodes and select them by ordinal position w/respect to document order. Ex. Address and select 2nd <para> child of doc.
Data Model

8. Xpath Syntax Expression is the primary syntactic construct in XPath
Evaluated to yield an object of 4 basic types.
node-set (unordered collection of nodes without duplicates).
boolean (true/false)
number (float)
string (sequence of UCS chars)
Expression Evaluation occurs will respect to a context. (XSLT/XPointer specified context)
Location path is one important kind of expression.
Location paths select a set of nodes relative to the context node.
node (the context node)
pair of non-zero positive integers (context position and context size)
set of variable bindings (mapping from variable names to variable values)
function library (mapping function names to functions) XPath implementations support a core Function Library. XSLT/XPointer extend XPath by defining additional functions
set of namespace declarations (mapping from prefixes to namespace URIs)
Expression

9. Location Path
Location Path provides the mechanism for ‘addressing’ parts of an XML doc, similar to file system addressing.
Ex: /book/year (select all the year elements that have a book parent)
Every location path can be expressed using a straightforward but rather verbose syntax:
unabbreviated syntax (verbose syntax)
Ex: child::* (select all element children of the context node)
abbreviated syntax
Ex. * (equivalent to unabbreviation above)
(special case of an Expr)
Location Path: describes path from 1pt to another in XML doc
RelativeLocationPath ::= Step | RelativeLocationPath ‘/’ Step
| AbbreviatedRelativeLocationPath
=>The initial sequence of steps selects a set of nodes relative to a context node. Each node in that set is used as a context node for the following step. The set of nodes identified by the composition of the steps is this union.
ex: child::div/child::para
selects the para element children of the div element children of the context node
para element grandchildren that have div parents
AbsoluteLocationPath ::= ‘/’ RelativeLocationPath?
| AbbreviatedAbsoluteLocationPath
AbbreviatedAbsoluteLocationPath
=> / by itself selects the root node of the document containing the context node. If followed by a relative location path, then the location path selects the set of nodes relative to the root node of the document containing the context node.
Location Path

10. Location Path Cont. Two types of paths: Relative & Absolute
Relative location path: consists of a sequence of one or more location steps separated by /
absolute location path: consists of / optionally followed by a relative location path
Composed of a series of steps (1 or more)
Ex. Child::bib/child::book (select the book element children of the
bib element children of the context node)
Ex. / (select the root node of the document containing the
context node)
Location Path

11. Location Path Examples
Verbose syntax (has syntactic abbreviations for common cases) Examples (unabbreviated)
child::book selects the book element children of the context
node
child::* selects all element children of the context node
attribute::price selects the price attribute of the context node
descendant::book selects all book descendants of the context node
self::book selects the context node if it is a book element (otherwise selects nothing)
child::*/child::book selects all book grandchildren of the context node
/ selects the document root (which is always the
parent of the document element)
Location Path

12. Location Steps
3 parts
axis (specifies relationship btwn selected nodes and the context node)
node test (specifies the node type and expanded-name of selected nodes)
predicates (arbitrary expressions to refine the selected set of nodes)
The syntax for location step is the axis name and node test separated by a double colon followed by zero or more expressions, each in square bracket.
Evaluate a location step is to generate an initial node-set from axis (relationship to context node) and node-test (node-type and expanded-name), then filter that node-set by each of the predicates in turn.
ex: child::book[position( )=1]
child is the name of the axis, book is the node test, and [position()=1] is a predicate
ex: descendant::book[position( )=1]
selects the all book element descendants of the context node firstly, then filter the one
which is first book descendant of context node.
Location Step

13. Location Steps Axes We’ve only seen these, so far Node test
13 axes defined in XPath
Ancestor, ancestor-or-self
Attribute
Child
Descendant, descendant-or-self
Self
Following
Preceding
Following-sibling, preceding-sibling
Namespace
Parent
Node test
Identifies type and expanded-name of node.
Can use a name, wildcard or function to evaluate/verify type and name.
ex. Child::text() select the text node children of context node.
Child::book select book element children of context node.
Attribute::* select all attribute children of context node.
We’ve only seen these, so far
Node Tests
Every Axis has a principal node type.
Principal node type is the type of the nodes that the axis can contain.
attribute axis (principal node type is attribute)
In relation to CONTEXT NODE
Ancestor, ancestor–or-self //Ancestors of context node…or-self includes context node
Attribute //Specific attribute of the context node
Child //children of the context node
Descsndant, descendant-or-self //descendants of …or-self includes context node
Following //all elements that com after the context node excluding descentants
//elements whose start tags come AFTER the end tag of the context node in doc order
Preceding //elements coming before ContextNode excluding ancestors
Folling-dibling, preceding-sibling //any sibling preceding(before)or following(after) contextnode LtoR order
// in tree structure
Namespace //all open namespaces at context node
Empty if ContextNode is NOT an element
Parent //parent of context node
Self //context node itself
Location step

14. Location Step Cont. Predicate
A predicate filters a node-set with respect to an axis to produce a new node-set.
Use XPath expressions (normally, boolean expressions) in square brackets following the basis (axis & node test).
Ex. Child::book[attribute::price=“25”]
(select all book children of the context node that have a price attribute with value 25.
A predicateExpr is evaluated by evaluating the Expr and converting the result to a boolean (True or False)

15. Examples Axis and Node Test: Basis and Predicate:
descendant::publisher
(selects the publisher elements that are descendant of the context node)
attributes::*
(selects all attributes of the context node)
Basis and Predicate:
child::book[3]
(selects the 3rd book of the children of the context node)
child::*[self::author or self::year][position()=last()]
(selects the last author or year child of the context node)
child::book[attribute::page=“400”][5]
(selects the fifth book child of the context node that has a page attribute
with value 400)
Basis: axis & nodetest
Location Path

16. Abbreviated Syntax
Abbreviated syntax is the simpler way to express location path.
For common case, abbreviation can be used to express concisely (not every case).
Each abbreviation can be converted to unabbreviated one.
child:: can be omitted from a location step (child is the default axis) ex. bib/book is equivalent to child::bib/child::book
attribute:: can be abbreviated
ex. is short for child::book[attribute::price=“25”]
// is short for /descendant-or-self::node()/
ex. Book//author is short for book/descendant-or-self::node()/child::author
A location step of . is short for self::node() ex: .//book is short for self::node()/descendant-or-self::node()/child::book
Location step of .. is short for parent::node()
ex. ../title is short for parent::node()/child::title
Location Path

17. Expressions Function Calls Function Calls Node-sets Booleans Numbers
Strings
Function Calls
Expressions

18. Function Calls
Function call expression is evaluated by using the FunctionName to identify a function in the expression evaluation context function library.
An argument is converted
to type string (as if calling the string function),
to type boolean (as if calling the Boolean function),
to type number (as if calling the number function),
An argument that is not of type node-set cannot be converted to a node-set.
Ex. position() function returns the current node’s position in the context node list as a number.
Expressions

19. Expressions Function Calls Node-sets Booleans Numbers Strings

20. Node-sets A location path can be used as an expression.
The expression returns the set of nodes selected by the path.
Expressions

21. Expressions Function Calls Node-sets Booleans Numbers Strings

22. Booleans A boolean can only have two values: true or false
The following operators can be used in boolean expressions or combine two boolean expressions according to the usual rules of boolean logic: or
and
=, !=
<=, <, >=, >
Ex. Book=‘XML complete’ or book=‘Principles of Database
Expressions

23. Expressions Function Calls Node-sets Booleans Numbers Strings

24. Numbers
A number represents a floating-point number, no pure integers exist in Xpath.
The basic arithmetic operators include:
+, -, *, div and mod.
div 10
Expressions

25. Expressions Function Calls Node-sets Booleans Numbers Strings

26. Strings Strings consist of a sequence of zero or more character.
May be enclosed in either single or double quotes.
Comparison operators: =, !=
Expressions

27. Core Function Library
XPath defines a core set of functions to evaluate expressions.
All implementations of Xpath must implement the core function library.
Four type of functions:
Node Set Functions: operate on or return info about node sets.
String Functions: are used for basic string operations.
Ex. substring(“12345”, 0, 3) returns “12”
Boolean Functions: all return true or false.
Number Functions: are used for basic number operations.
Core Library

28. Xpath Utilities Miscellaneous utilities related to Xpath
XPath Visualiser:
This is a powerful tool for the evaluation of an XPath expression and visual presentation of the resulting node-set.
allowing you to experiment with XPath for finding the correct expression.
The display of the XML source document is similar to the default
IE display with the same syntax color and collapsible & expandable container nodes.
very straightforward XPath learning process.
Xpath Utilities

29. XPath Visualiser Context Node Xpath input Tree View of XML Doc
Xpath evaluating
result
Result is highlighted
Xpath Utilities

30. Conclusion Xpath is complete pattern match language.
Provides an concise way for addressing parts of an XML document.
Base for XSLT, Xpointer and XML Query WG. Supported by W3C.
Implementing XPath basically requires learning the abbreviated syntax of location path expressions and the functions of the core library.
Conclusion

31. Reference XML Path Language (XPath) V1.0 http://www.w3.org/TR/xpath
XML in a Nutshell
ch09.html
Managing XML and Semistructured Data
Xpath utilities
Xpath Reference