re.sub replaces twice

[spz1st]

BPO	41555
Nosy	@animalize

^{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 2020-08-17.23:40:00.719>
created_at = <Date 2020-08-14.22:28:33.971>
labels = ['3.8', 'type-bug', '3.7', 'invalid']
title = 're.sub replaces twice'
updated_at = <Date 2020-09-05.12:21:11.896>
user = 'https://bugs.python.org/spz1st'

bugs.python.org fields:

activity = <Date 2020-09-05.12:21:11.896>
actor = 'spz1st'
assignee = 'none'
closed = True
closed_date = <Date 2020-08-17.23:40:00.719>
closer = 'pablogsal'
components = []
creation = <Date 2020-08-14.22:28:33.971>
creator = 'spz1st'
dependencies = []
files = []
hgrepos = []
issue_num = 41555
keywords = []
message_count = 6.0
messages = ['375436', '375444', '375459', '375463', '375475', '376429']
nosy_count = 2.0
nosy_names = ['malin', 'spz1st']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue41555'
versions = ['Python 3.7', 'Python 3.8']

[spz1st]

The following command produced "name.tsvtsv" with version 3.7.1 and 3.8.5 instead of the expected "name.tsv" from version 2.7.5, 3.5.6, and 3.6.7. Changing * to + produced expected "name.tsv".

python -c 'import re; v="name.txt";v = re.sub("[^\\.]*$", "tsv", v);print(v)'

[animalize]

The re.sub() doc said:
Changed in version 3.7: Empty matches for the pattern are replaced when adjacent to a previous non-empty match.

IMO 3.7+ behavior is more reasonable, and it fixed a bug, see bpo-25054.

[spz1st]

Thanks. But if talking about empty matches, there would be endless empty
matches at the end in such cases. So in my opinion, [^\\.]*$ should match
txt plus the empty match because the greedy rule applies here.

On Fri, Aug 14, 2020 at 10:37 PM Ma Lin <report@bugs.python.org> wrote:

Ma Lin <malincns@163.com> added the comment:

The re.sub() doc said:
Changed in version 3.7: Empty matches for the pattern are replaced when
adjacent to a previous non-empty match.

IMO 3.7+ behavior is more reasonable, and it fixed a bug, see bpo-25054.

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nosy: +malin

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Python tracker <report@bugs.python.org>
<https://bugs.python.org/issue41555\>

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[animalize]

There can be at most one empty match at a position. IIRC, Perl's regex engine has very similar behavior.
If don't want empty match, use + is fine.

[spz1st]

Okay. Thanks.

On Sat, Aug 15, 2020 at 9:07 AM Ma Lin <report@bugs.python.org> wrote:

Ma Lin <malincns@163.com> added the comment:

There can be at most one empty match at a position. IIRC, Perl's regex
engine has very similar behavior.
If don't want empty match, use + is fine.

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Python tracker <report@bugs.python.org>
<https://bugs.python.org/issue41555\>

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[spz1st]

Thanks.

On Mon, Aug 17, 2020 at 7:40 PM Pablo Galindo Salgado <
report@bugs.python.org> wrote:

Change by Pablo Galindo Salgado <pablogsal@gmail.com>:

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resolution: -> not a bug
stage: -> resolved
status: open -> closed

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Python tracker <report@bugs.python.org>
<https://bugs.python.org/issue41555\>

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