"\W" pattern with re.ASCII flag is not equivalent to "[^a-zA-Z0-9_]"

[owentrigueros]

BPO	45458
Nosy	@ezio-melotti, @serhiy-storchaka, @owentrigueros

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2021-10-13.09:28:14.946>
created_at = <Date 2021-10-13.09:10:37.218>
labels = ['expert-regex', 'type-bug', '3.8', '3.9', '3.10', '3.7', 'invalid']
title = '"\\W" pattern with re.ASCII flag is not equivalent to "[^a-zA-Z0-9_]"'
updated_at = <Date 2021-10-13.09:28:14.945>
user = 'https://github.com/owentrigueros'

bugs.python.org fields:

activity = <Date 2021-10-13.09:28:14.945>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = True
closed_date = <Date 2021-10-13.09:28:14.946>
closer = 'serhiy.storchaka'
components = ['Regular Expressions']
creation = <Date 2021-10-13.09:10:37.218>
creator = 'owentrigueros'
dependencies = []
files = []
hgrepos = []
issue_num = 45458
keywords = []
message_count = 2.0
messages = ['403810', '403811']
nosy_count = 4.0
nosy_names = ['ezio.melotti', 'mrabarnett', 'serhiy.storchaka', 'owentrigueros']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue45458'
versions = ['Python 3.6', 'Python 3.7', 'Python 3.8', 'Python 3.9', 'Python 3.10']

[owentrigueros]

"\W" regex pattern, when used with re.ASCII, is expected to have the same behavior as "[^a-zA-Z0-9_]" (see [1]).

For example, the following sub() call

>>> re.sub('\W', '', '½ a', re.ASCII)
'½a'

should return the same as this one:

>>> re.sub('[^a-zA-Z0-9_]', '', '½ a', re.ASCII)
'a'

But it does not.

[1] https://docs.python.org/3/library/re.html#regular-expression-syntax

[serhiy-storchaka]

It works as expected:

>>> re.sub(r'\W', '', '½ a', 0, re.ASCII)
'a'

You just passed re.ASCII as the count argument, not as the flags argument.

>>> help(re.sub)
Help on function sub in module re:

sub(pattern, repl, string, count=0, flags=0)
    Return the string obtained by replacing the leftmost
    non-overlapping occurrences of the pattern in string by the
    replacement repl.  repl can be either a string or a callable;
    if a string, backslash escapes in it are processed.  If it is
    a callable, it's passed the Match object and must return
    a replacement string to be used.