bpo-32658: Regex docs: Fix metacharacter reference (GH-32230)

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>
(cherry picked from commit 43571a3)

Co-authored-by: Mike cm <mikecmcleod@gmail.com>

Doc/howto/regex.rst

@@ -89,7 +89,7 @@ is the same as ``[a-c]``, which uses a range to express the same set of
 characters.  If you wanted to match only lowercase letters, your RE would be
 ``[a-z]``.
 
-Metacharacters are not active inside classes.  For example, ``[akm$]`` will
+Metacharacters (except ``\``) are not active inside classes.  For example, ``[akm$]`` will
 match any of the characters ``'a'``, ``'k'``, ``'m'``, or ``'$'``; ``'$'`` is
 usually a metacharacter, but inside a character class it's stripped of its
 special nature.