XPath Axes Overview

This table provides a detailed overview of XPath axes, their relationships, and how they can be combined. Use this as a reference to understand how to navigate XML/HTML document structures effectively.

Axis	Description	Relationship to Context Node	Inverse Axis	Node Test	Example
self	Selects the context node	Is the context node	self	Any	`self::div`
child	Selects all children of the context node	Direct children	parent	Element, Text, Comment, Processing Instruction	`child::p`
parent	Selects the parent of the context node	Direct parent	child	Element	`parent::div`
ancestor	Selects all ancestors of the context node	All nodes above in hierarchy	descendant	Element	`ancestor::section`
ancestor-or-self	Selects the context node and all its ancestors	Self and all nodes above	descendant-or-self	Element	`ancestor-or-self::div`
descendant	Selects all descendants of the context node	All nodes below in hierarchy	ancestor	Any	`descendant::li`
descendant-or-self	Selects the context node and all its descendants	Self and all nodes below	ancestor-or-self	Any	`descendant-or-self::p`
following-sibling	Selects all siblings after the context node	Same level, after	preceding-sibling	Element	`following-sibling::div`
preceding-sibling	Selects all siblings before the context node	Same level, before	following-sibling	Element	`preceding-sibling::h1`
following	Selects everything after the closing tag of the context node	After in document order	preceding	Any	`following::section`
preceding	Selects everything before the opening tag of the context node	Before in document order	following	Any	`preceding::p`
attribute	Selects attributes of the context node	Attributes of the node	None	Attribute	`attribute::href`
namespace	Selects namespace nodes of the context node	Namespace nodes	None	Namespace	`namespace::*`

Additional Information:

Context Node: The node currently being processed or the node from which the path expression is anchored.
Document Order: The hierarchical order of elements as they appear in the XML document, from top to bottom.
Axis Specifiers: In XPath expressions, axis specifiers can be abbreviated:
- child:: is the default axis (can be omitted)
- @ is short for attribute::
- // is short for /descendant-or-self::node()/
- . is short for self::node()
- .. is short for parent::node()
Node Tests:
- node(): Matches any node of any type
- text(): Matches text nodes
- comment(): Matches comment nodes
- processing-instruction(): Matches processing instruction nodes
Predicate Usage: Predicates (filters) can be applied to any axis, e.g., ancestor::div[1] selects the nearest ancestor that is a div.
Axis Combinations: Axes can be combined in a single XPath expression, e.g., ancestor::div/following-sibling::p selects all p elements that are siblings of any ancestor div.
Performance Considerations:
- Forward axes (child, descendant, following, etc.) are generally faster than reverse axes (parent, ancestor, preceding, etc.).
- descendant:: is usually slower than child:: because it searches deeper in the hierarchy.
Special Cases:
- The namespace axis is not supported in all XPath processors and is less commonly used.
- The attribute axis cannot have children, descendants, or siblings.

This table and additional information provide a comprehensive overview of XPath axes and their relationships, suitable for both beginners and advanced users exploring XPath capabilities.