XPath: Location paths

XPath is a declarative language for addressing (used in XPointer) and pattern matching (used in XSLT).

The central construct is the location path, which is a sequence of location steps separated by /.

A location step is evaluated wrt. some context resulting in a set of nodes.
A location path is evaluated compositionally, left-to-right, starting with some initial context.
Each node resulting from evaluation of one step is used as context for evaluation of the next, and the results are unioned together.

A context consists of:

a context node,
a context position and size (two integers, 1<=position<=size),
variable bindings, a function library, and a set of namespace declarations.

Initial context: defined externally (e.g. by XPointer or XSLT).

(Location paths starting with / always use the document root as initial context node!)

Example:

child::section[position()<6]/descendant::cite/attribute::href

selects all href attributes in cite elements in the first 5 sections of an article.