[3.12] gh-124130: Notes on empty string corner case of category \B (GH-124133) (#124329)

gh-124130: Notes on empty string corner case of category `\B` (GH-124133)
(cherry picked from commit d3e79d7)

Signed-off-by: y5c4l3 <y5c4l3@proton.me>
Co-authored-by: Y5 <124019959+y5c4l3@users.noreply.github.com>
Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>

Author: 3 people

Doc/library/re.rst

@@ -572,6 +572,12 @@ character ``'$'``.
    Word boundaries are determined by the current locale
    if the :py:const:`~re.LOCALE` flag is used.
 
+   .. note::
+
+      Note that ``\B`` does not match an empty string, which differs from
+      RE implementations in other programming languages such as Perl.
+      This behavior is kept for compatibility reasons.
+
 .. index:: single: \d; in regular expressions
 
 ``\d``