> How about \p and \P ("P" for "pairwise groupings" or just "pairs")?
I'm afraid those are taken, too.
Symbol Atomic Meaning
------ ------ -------
C<\0> yes Match the null character (ASCII NUL).
C<\I<NNN>> yes Match the character given in octal, up to C<\377>.
C<\I<n>> yes Match R<n>th previously captured string (decimal).
C<\a> yes Match the alarm character (BEL).
C<\A> no True at beginning of string.
C<\b> yes Match the backspace character (BS).
C<\b> no True at word boundary.
C<\B> no True when not at word boundary.
C<\cR<X>> yes Match the control character Control-R<X> (C<\cZ>, C<\c[>).
C<\C> yes Match one byte (C C<char>) even in utf8 (dangerous).
C<\d> yes Match any digit character.
C<\D> yes Match any non-digit character.
C<\e> yes Match the escape character (ASCII ESC, not backslash).
C<\E> -- End case (C<\L>, C<\U>) or metaquote (C<\Q>).
C<\f> yes Match the form feed character (FF).
C<\G> no True at end-of-match position of prior C<m//g>.
C<\l> -- Lowercase next character only.
C<\L> -- Lowercase till C<\E>.
C<\n> yes Match the newline character (NL, CR on Macs).
C<\N{R<NAME>}> yes Match the named char (C<\N{greek:Sigma}>.
C<\p{R<PROP>}> yes Match any character with named property.
C<\P{R<PROP>}> yes Match any character without named property.
C<\Q> -- Quote (de-meta) metacharacters till C<\E>.
C<\r> yes Match the return character (CR, NL on Macs).
C<\s> yes Match any whitespace character.
C<\S> yes Match any non-whitespace character.
C<\t> yes Match the tab character (HT).
C<\u> -- Titlecase next character only.
C<\U> -- Uppercase (not titlecase) till C<\E>.
C<\w> yes Match any "word" character (alphanums plus "_").
C<\W> yes Match any non-word character.
C<\x{abcd}> yes Match the character given in hexadecimal.
C<\X> yes Match "combining character sequence" string.
C<\z> no True at end of string only.
C<\Z> no True at end of string or before optional newline.
