Notes on empty string corner case of category \B

Signed-off-by: y5c4l3 <[email protected]>

Author: y5c4l3

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::
+
+      However, ``\B`` does not match an empty string, which acts a little bit
+      different compared to other implementations. This behavior is currently
+      kept for compatibility considerations.
+
 .. index:: single: \d; in regular expressions
 
 ``\d``