XSLT 2.0 and XPath 2.0 Programmer's Reference, 4th Edition (662 page)

\C	Any character that does not match \c
\D	Any character that does not match \d
\W	Any character that does not match \w

The two constructs
\i
and
\c
are defined by reference to the XML specification. The definitions in question have changed slightly between XML 1.0 and XML 1.1. The XPath specification leaves it to the implementor to decide whether to use the XML 1.0 or XML 1.1 definitions. Some products (Saxon is an example) might give you the choice, as a configuration option.