The current specification can be found at XPath 2.0. Specifications for XPath 2.0 functions can be found at XPath 2.0 functions

XPath is the XML Path Language used to select parts of an XML instance document for processing by various other XML technologies such as XSLT, XPointer, XQuery and XForms. XPath provides a language for modelling XML instance documents as a set of hierarchical nodes with axes used to navigate from one node to another. A number of functions are available for processing contents of nodes and predicates can be used to further filter node identifiers.

<!-- A single step location path -->
child::Phone[@type="Home"]
<!-- A two step location path -->
child::Person/child::Phone[@type="Home"]

In XPath, the context is defined as the location, size and position of the node currently being processed. Relative XPath locations are specified in relation to the current context

XPath Nodes

In XPath version 1.0 there are seven types of nodes:

Root Node

An XML document has one and only one root element sometimes called the document element. The root element is a child of the root node. The root node may have other child elements consisting of comment nodes or processing instruction nodes. In XPath, the root node represents the document itself. The text value of the root node is the concatenation of all text values in descendant nodes.

Element Node

Each element in an XML instance document is represented as an element node. The name for each element node consists of a namespace URI or the namespace prefix and the localpart of its name, seperated with a colon. The text content of an element node is the concatenation of all text values in descendant nodes.

Attribute Node

Each element attribute is represented as an attribute node. Although the element node to which it belongs is the parent node, the attribute node is not a child node of the element. Attribute nodes can not therefore be accessed via the 'child' axis of its parent element, but instead using the 'attribute' axis. The parent axis can however be used to access the parent element from an attribute node.

Text Node

The text content of an element node is represented as a text node.

Namespace Node

All in-scope namespaces of a node are represented as namespace nodes. The name() function returns the namespace prefix associated with a node. The 'self::node()' expression (or '.') returns the namespace URI.

Comment Node

Comment nodes represent comments in the XPath data model

Processing Instruction Node

Processing instruction nodes represent comments in the XPath data model

XPath Axes

XPath axes are used to navigate the node tree of the XPath data model. There are 13 axes available in XPath version 1.0:

child axis

The default axis in XPath. Selects immeadiate child nodes of the context node. Because child is the default axis, location paths can be expressed as either 'child::itemname' or simply 'itemname'. 'child::*' or '*' returns all child nodes with a name (that is elements only) of the current context node. To select all nodes use 'child::node()' or simply 'node()'. To select text node children only use 'child::text()' or 'text()'.

attribute axis

Selects attribute nodes associated with an element node. 'attribute::*' can be abbreviated to '@*'. To select a specific attribute only use 'attribute::attname' or '@attname'

ancestor axis

Recursively selects all parent nodes for the current context node up to and including the root node.

ancestor-or-self axis

Returns all ancestor nodes plus the context node.

descendant axis

Recursively returns the child nodes of the current context node.

descendant-or-self axis

Returns all descendant nodes plus the current context node.

following axis

Returns all nodes that come after the context node in document order, but excludes descendant, attribute and namespace nodes associated with the context node.

following-sibling axis

Returns all following nodes that share the same parent as the context node.

namespace axis

Returns all in-scope namespace nodes for context node.

parent axis

Returns the parent node for the context node.

preceding axis

Returns all nodes that come before the context node in document order, excluding ancestor, attribute and namespace nodes.

preceding-sibling axis

Returns all preceding nodes that share the same parent as the context node.

self axis

Returns the context node. Can be specified as 'self::node()' or simply '.'.

Functions

A built-in function library exists as part of the XPath specification that can be used with predicates to add further filtering to an XPath expression.

Boolean Functions

boolean()

Tests argument and returns true or false

false()

Returns false

lang()

Returns true if context node language matches string argument

not()

Returns opposite boolean value of its argument

true()

Returns true

Node-Set Functions

count()

Returns number of nodes in node-set

id()

Returns node-set of nodes with id attribute equal to its argument

last()

Returns context size

local-name()

Returns localpart of the name of the node set argument, or of the context node if no argument given

name()

Returns name of element in prefix::localpart format

namespace-uri()

Returns namespace URI for node-set argument, or for context node if no argument provided

position()

Returns value equal to context position

Numeric Functions

ceiling()

Returns smallest integer value greater than numeric argument

floor()

Returns smallest integer value less than numberic argument

number()

Returns numberic value of argument

round()

Rounds its argument

sum()

Returns sum of its node-set argument's values

String Functions

concat()

Returns concatenation of string arguments

contains()

Returns true if first string argument contains the second string argument

normalize-space()

Strips leading and trailing space and replaces consecutive whitespace with a single space character

starts-with()

Returns true if first arguement string starts with second arguement string

string()

Returns string value of argument

string-length()

Returns length of string argument

substring()

Returns a string from the first argument beginning at a number specified in second argument and optionally ending at number specified in third argument

substring-after()

Returns string from first argument that occurs after second string argument

substring-before()

Returns string from first argument that occurs before second string argument

translate()

Returns first string argument, with characters from second argument translated to corresponding characters in third argument