XSLT 2.0 and XPath 2.0 Programmer's Reference, 4th Edition (423 page)

, the leading
.
becomes necessary to indicate that you want to start the selection at the context node, not at the root document node.

Expressions using
//
can be expensive to evaluate, because the XPath processor will often have to search the whole document to find the selected nodes. If you can specify a more restricted search, it is generally a good idea to do so—for example, if you know that all the

elements are children of the document element, then specifying
/*/book
will generally be much more efficient than writing
//book
. Of course, actual performance characteristics of different products may vary. In some XML database products, elements are indexed in such a way that
//book
becomes very fast.

Examples Using
//

Expression	Description
//figure	Selects all elements in the document.
//book[@category = ‘fiction’]	Selects all elements in the document that have a category attribute with the value fiction .
//*/*	Selects all element nodes that have an element as a parent, in other words all elements except those that are immediate children of the root node. Here * is a NameTest that matches any element.
//book/title	Selects all