t = re.sub(".*", "*", src) has different ouput

[soyoo]

BPO	41605
Nosy	@ronaldoussoren, @ericvsmith, @ezio-melotti, @serhiy-storchaka, @graingert
PRs	bpo-32308: Replace empty matches adjacent to a previous non-empty match in re.sub(). #4846
Files	python3 on windows.png

^{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 2020-08-21.18:49:53.534>
created_at = <Date 2020-08-21.01:01:05.421>
labels = ['expert-regex', 'invalid', 'type-bug', '3.7']
title = 't = re.sub(".*", "*", src) has different ouput'
updated_at = <Date 2020-08-21.18:49:53.531>
user = 'https://bugs.python.org/soyoo'

bugs.python.org fields:

activity = <Date 2020-08-21.18:49:53.531>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = True
closed_date = <Date 2020-08-21.18:49:53.534>
closer = 'serhiy.storchaka'
components = ['Regular Expressions']
creation = <Date 2020-08-21.01:01:05.421>
creator = 'soyoo'
dependencies = []
files = ['49418']
hgrepos = []
issue_num = 41605
keywords = ['patch']
message_count = 4.0
messages = ['375741', '375744', '375758', '375765']
nosy_count = 7.0
nosy_names = ['ronaldoussoren', 'eric.smith', 'ezio.melotti', 'mrabarnett', 'serhiy.storchaka', 'graingert', 'soyoo']
pr_nums = ['4846']
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue41605'
versions = ['Python 3.7']

[soyoo]

import re

src = "123"  # can be anything
t = re.sub(".*", "*", src)

"""
The output of 't' is different in python2 and python3;
When python version is python3, it's depend on OS version.

When python version is python2, it seem always output single '*'.
When python version is python3, and OS is Linux, it output single '*';
When python version is python3, and OS is Mac or windows, it output double '**'.
"""

[ericvsmith]

I'm suspicious that this differs per-OS. Please provide exact Python version information for each OS you list in your initial report.

I suspect what you're seeing is related to this change:

Changed in version 3.7: Empty matches for the pattern are replaced when adjacent to a previous non-empty match.

[ronaldoussoren]

in 3.6 t == '*', in 3.7 and later it is '**'.

To be honest I don't like the 3.7 behaviour, it is very surprising even if it is technically correct (the first '*' is from a match from the entire string, the second one is from an empty match at the end of the string).

A workaround is to explicitly anchor the regular expression (for example by adding '^' to the start).

[graingert]

seems like a dupe? https://bugs.python.org/issue32308