XSLT 2.0 and XPath 2.0 Programmer's Reference, 4th Edition (658 page)

charProp	Category | IsBlock
Category	One of the two-character codes listed in the section Character Categories on page 924
IsBlock	Is BlockName
where BlockName is the name of one of the Unicode code blocks listed in the section Character Blocks on page 922

A
character class escape
defines a set of characters, and matches any single character from this set.

There are four kinds of character class escapes:
single character escapes
,
multicharacter escapes
,
category escapes
, and
complementary escapes
.

The published XPath Functions and Operators Recommendation defines a back-reference as a fifth kind of character class escape. This is incorrect, because a back-reference matches a sequence of characters rather than a single character, which means that it makes no sense within square brackets. This is fixed by an erratum to the spec.

A
single character escape
(
SingleCharEsc
) consists of a backslash followed by one of the characters shown below. The single character escape matches a single character, as shown in the table.

Single character escape	Matches
\n	newline (x0A)
\r	carriage return (x0D)
\t	tab (x09)
\\ \| \. \? \* \+ \(