it cannot be put inside a bracketed character class; C</[\R]/> is an error.
C<\R> is introduced in perl 5.10.
-Mnemonic: none really. C<\R> was picked because PCRE already uses C<\R>.
+Mnemonic: none really. C<\R> was picked because PCRE already uses C<\R>,
+and more importantly because Unicode recommends such a regular expression
+metacharacter, and suggests C<\R> as the notation.
=item \X
include (but are not restricted to) I<combining characters> and
I<vowel signs>.
+C<\X> matches quite well what normal (non-Unicode-programmer) usage
+would consider a single character: for example a base character
+(the C<\PM> above), for example a letter, followed by zero or more
+diacritics, which are I<combining characters> (the C<\pM*> above).
+
Mnemonic: eI<X>tended Unicode character.
=back