XSLT 2.0 and XPath 2.0 Programmer's Reference, 4th Edition (660 page)

The character following the backslash, for example, \? matches a question mark

A
multicharacter escape
(
MultiCharEsc
) is a
\
followed by one of the characters shown below. Each multicharacter escape matches one of a number of different characters (but it only matches one character at a time).

Multicharacter escape	Matches
\s	space (x20) , tab (x09) , newline (x0A) , or carriage return (x0D)
\i	An initial name character: specifically, a character classified in the XML specification as a Letter, or : or _
\c	A name character, as defined by the NameChar production in the XML specification
\d	A decimal digit: anything that matches \p{Nd}