XSLT 2.0 and XPath 2.0 Programmer's Reference, 4th Edition (650 page)

Character Groups

A
character group
always appears within square brackets; it defines a set of permitted characters, and matches a single character from the input string if it is a member of that set. There are various ways the set of permitted characters can be defined, using the following syntax:

Construct	Syntax
charGroup	posCharGroup | negCharGroup | charClassSub
posCharGroup	( charRange | charClassEsc )+
negCharGroup	∧ posCharGroup
charClassSub	( posCharGroup | negCharGroup ) - charClassExpr

A character group is either a
positive group
, a
negative group
, or a
subtraction
. Examples of the three kinds are
[a-z]
,
[
∧
0-9]
, and
[a-z-[pqr]]
.

A
positive group
(
posCharGroup
) consists of a sequence of one or more parts, each of which is either a
character range
or a
character class escape
. A positive group matches a character if any one of its parts matches the character. For example,
[a-zA-Z0-9%#]
matches any character that falls in one of the ranges
a
to
z
,
A
to
Z
, or
0
to
9
, as well as the
%