XSLT 2.0 and XPath 2.0 Programmer's Reference, 4th Edition (300 page)

Whitespace is allowed between the
$
sign and the
QName
, though it is rarely used (it was not permitted in XPath 1.0).

Usage

The
QName
must match the name of a variable that is in scope at the point where the expression containing the variable name appears. A variable can be declared either within a containing
for
,
some
, or
every
expression, or (in XSLT) in an

or

element; note that if the name contains a namespace prefix, it is the namespace URI that must match, not necessarily the prefix. The XPath expression must be within the scope of this

or

element—the scope rules are given in Chapter 6 on page 502.

The value of the variable reference is whatever value has been assigned to it by the matching
for
,
some
, or
every
expression, or (with XSLT) the matching

or

declaration. The value may be of any type: any sequence containing nodes, atomic values, or a mixture of both.

A variable reference can be used anywhere in an XPath expression where a value is required. It cannot be used to represent concepts of the language other than values; for example, you can't use a variable in place of a name, a node type, or an axis. Nor can you use a variable to hold an entire expression.

A common misunderstanding about variables is to write a path expression such as:

thinking that if the value of
$property
is the string “title”, then this is equivalent to writing:

You can do this sort of thing in a shell scripting language, where variables work by textual substitution: in that kind of language the content of a variable can hold any part of an expression. But in XPath, variables hold values, not pieces of an expression. The actual meaning of the above expression is:

which will return the string “title” once for each book element.

The way to achieve the desired effect is to write:

Some processors (including Saxon) offer an
evaluate()
extension function, which allows you to construct an XPath expression at runtime, from a string which might be held in a variable. But this capability is not present in the standard.