XPath

1
XPath

• Tao Wan
• March 04, 2002

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.

3
Outline

• Introduction
• Data Model
• Xpath Syntax
• Location Path
• General Xpath Expressions
• Core Function Library
• XPath utilities
• Conclusion

4
Introduction

• W3C Recommendation. November 16, 1999
• Latest version http//www.w3.org/TR/xpath
• 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.

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

• lt?xml version1.0gt
• lt?xml-stylesheet typetext/xsl hrefbib.xsl
  ?gt
• lt! -- simple XML document --gt
• ltbibgtltbook price25.00 pages400gt
• ltpublishergt IDG
  bookslt/publishergt ltauthorgt
  ltfirst-namegtRicklt/first-namegt
  ltlast-namegt Hull lt/last-namegt
  lt/authorgt ltauthorgt Simon
  Northlt/authorgt lttitlegt XML complete
  lt/titlegt ltyeargt 1997
  lt/yeargtlt/bookgtltbookgt ltpublishergt
  Freeman lt/publishergt ltauthorgt
  Jeffrey D. Ullman lt/authorgt lttitlegt
  Principles of Database lt/titlegt
  ltyeargt 1998 lt/yeargtlt/bookgt
• lt/bibgt

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.

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)

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. Childbib/childbook (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)
• childbook selects the book
  element children of the context
• node
• child selects all element
  children of the context node
• attributeprice selects the price
  attribute of the context node
• descendantbook selects all book
  descendants of the context node
• selfbook selects the context
  node if it is a book element (otherwise
  selects nothing)
• child/childbook 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 childbookposition( )1 child is the
name of the axis, book is the node test, and
position()1 is a predicate
ex descendantbookposition( )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
• 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. Childtext() select the text node
  children of context node.
• Childbook select book element
  children of context node.
• Attribute select all attribute
  children of context node.

Weve only seen these, so far
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. Childbookattributeprice25
• (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
• descendantpublisher
• (selects the publisher elements that are
  descendant of the context node)
• attributes
• (selects all attributes of the context node)
• Basis and Predicate
• childbook3
• (selects the 3rd book of the children of the
  context node)
• childselfauthor or selfyearposition()la
  st()
• (selects the last author or year child of the
  context node)
• childbookattributepage4005
• (selects the fifth book child of the context node
  that has a page attribute
• with value 400)

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 childbib/childbook
attribute can be abbreviated to _at_ ex.
Book_at_price25 is short for childbookattribu
teprice25
// is short for /descendant-or-selfnode()/ ex.
Book//author is short for book/descendant-or-self
node()/childauthor
A location step of . is short for
selfnode()ex .//book is short for
selfnode()/descendant-or-selfnode()/childboo
k
Location step of .. is short for
parentnode() ex. ../title is short for
parentnode()/childtitle
Location Path
17
Expressions

• 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
  nodes position in the context node list as a
  number.

Expressions
19
Expressions

• Function Calls
• Node-sets
• Booleans
• Numbers
• Strings

Expressions
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

Expressions
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
• , !
• lt, lt, gt, gt
• Ex. BookXML complete or bookPrinciples of
  Database

Expressions
23
Expressions

• Function Calls
• Node-sets
• Booleans
• Numbers
• Strings

Expressions
24
Numbers

• A number represents a floating-point number, no
  pure integers exist in Xpath.
• The basic arithmetic operators include
• , -, , div and mod.
• Ex. _at_id div 10

Expressions
25
Expressions

• Function Calls
• Node-sets
• Booleans
• Numbers
• Strings

Expressions
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
• http//www.xmlsoftware.com/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
• http//www.oreilly.com/catalog/xmlnut/chapter/
  
• ch09.html
• Managing XML and Semistructured Data
  http//www.cs.washington.edu/homes/suciu/COURSES/5
  90DS/06xpath.htm
• Xpath utilities
• http//www.xmlsoftware.com/xpath/

Xpath Reference