XPath Intro
Patricia O'Connor
XML Tree Structure
An XML file starts from "the root", and branches out in complex ways to the "leaves".
Parent
Siblings
What is XPath?
XPath is a language that we write to select parts (or nodes) of an XML document
An XPath expression enables a user to describe, find, and navigate to information inside an XML document
XML languages that employ XPath to find and manipulate information in XML documents:
XSLT (eXtensible Stylesheet Language Transformations)
XQuery (XML Query language)
Nodes
A node is a piece of information in the XML tree, such as an element, an attribute, a string of text, or a comment.
element()
attribute()
text()
comment()
@
XPath Components
| Component | Purpose |
|---|---|
| path expression | Describe the location of some nodes in a tree. |
| axis | Describe the direction in which one looks in the tree. An axis is part of a path expression. |
| predicate | Filter the results of a path expression. |
| function | Do something with the information retrieved from the document instead of just returning it as received. |
Path Expressions
| Expression | Description |
|---|---|
| nodename | Selects all nodes with the name "nodename" |
| / | Selects from the root node |
| // | Selects nodes in the document from the current node that match the selection no matter where they are |
| . | Selects the current node |
| .. | Selects the parent of the current node |
| @ | Selects attributes |
| Describes the location of some nodes in an XML file. |
|
Axes
| Expression | Description |
|---|---|
| ancestor:: | The ancestor axis sends you to parents and above, all the way up to the root node. |
| . or self:: |
The self axis designating the current context node and the current location in a path. |
| .. or parent:: | The parent axis sends you up a short distance, to the immediate parent of the context node |
| / or child:: | The child axis (the default) sends you down to the immediate child of the context node. |
| // or descendant:: | The descendant axis sends you down to the children and their children etc. |
| @ or attribute:: | The attribute (@) axis for locating attributes and attribute values |
Predicates
Predicates are always embedded in square brackets.
Filter the results of path expressions
Predicates are used to find a specific node or a node that contains a specific value.
//div[@type="letter"]
//div/p[1]
//persName[@ref="#Chaucer"]
//div[@type="letter"]/p[1]
//div[@type="letter"]/p[1]/persName[1]
Predicates & Axes
//persName[parent::title]
//persName[ancestor::body]
//persName[descendant::forename]
//place[child::placeName]
Unknown Nodes
| Wildcard | Description |
|---|---|
| * | Matches any element node |
| @* | Matches any attribute node |
| node() | Matches any node of any kind |
| XPath Expression | Description |
|---|---|
| //* | Selects all elements in the document |
| //title[@*] | Selects all title elements which have at least one attribute of any kind |
Function
Do something with the information retrieved from the document instead of just returning it as received.
Retrieve all of the <paragraph> elements in a <div> element (that has a type attribute value of "letter") but instead of returning the actual elements, return just a count of how many there are. This uses the count() function.
count(//div[@type="letter"]/p)
//div[@type="letter"]/p => count()
Functions
Retrieve all of the <persName> elements in a <div> element (that has a type attribute value of "letter") and return just a count of how many there are.
count(//div[@type="letter"]//persName)
//div[@type="letter"]//persName => count()
Functions
| Function | Description |
|---|---|
| distinct-values() | eliminate repetition in a list of results |
| last() | Returns the last |
|
lower-case() upper-case() |
Changes case of string |
| not() | Inverts the truth value of the argument. //p[not(q)] returns all <p> elements that do not have any <q> child element. |
| normalize-space() | normalize the white space (spaces, tabs, new lines, etc.) in text |
| string-length() | returns the length of text by counting characters. |
Functions
//div[@type="letter"]//persName=>distinct-values()
//div[@type="letter"]//p[last()]
//div[@type="letter"]//p[1]
//placeName=> distinct-values()
//div[@type="letter"]//persName[not(@ref="#Webb_Mary_younger")]
Simple Map & Arrow
| Operator | Description |
|---|---|
| ! | The simple map operator (!) means do the thing on the right once for each item on the left. |
| => | The arrow operator (=>) means apply the function on the right to the entire sequence (all at once) on the left. |
//addrLine ! count(descendant::persName)
//persName => distinct-values()
//div[@type="letter"] ! count(p)
Comparison Operators
| Value | General | Description |
|---|---|---|
| eq | = | equal to |
| ne | != | not equal to |
| gt | > | > greater than (may also be written >) |
| ge | >= | greater than or equal to (not less than; may also be written >=) |
| lt | < | less than (may also be written <) |
| le | <= | less than or equal to (not greater than; may also be written <=) |