Regex \B doesn't match empty string

Bug report

Bug description:

>>> import re
>>> list(re.finditer(r'\b', 'e'))
[<re.Match object; span=(0, 0), match=''>, <re.Match object; span=(1, 1), match=''>]
>>> list(re.finditer(r'\B', 'e'))
[]
>>> list(re.finditer(r'\b', '%'))
[]
>>> list(re.finditer(r'\B', '%'))
[<re.Match object; span=(0, 0), match=''>, <re.Match object; span=(1, 1), match=''>]
>>> list(re.finditer(r'\b', ''))
[]
>>> list(re.finditer(r'\B', ''))
[]

Apparently the empty string neither is nor isn't a word boundary. Is that supposed to happen? \B matches the empty string in every other language I can think of.

Online reproducer: https://godbolt.org/z/8q6fehss7

CPython versions tested on:

3.11, 3.12

Operating systems tested on:

Linux

Linked PRs

gh-124130: Notes on empty string corner case of category \B #124133
[3.13] gh-124130: Notes on empty string corner case of category \B (GH-124133) #124328
[3.12] gh-124130: Notes on empty string corner case of category \B (GH-124133) #124329
gh-124130: Increase test coverage for \b and \B in regular expressions #124330
[3.13] gh-124130: Increase test coverage for \b and \B in regular expressions (GH-124330) #124413
[3.12] gh-124130: Increase test coverage for \b and \B in regular expressions (GH-124330) #124414
gh-124130: Fix a bug in matching regular expression \B in empty string #127007