All regular expression match groups are the empty string

[adiba]

BPO	35653

^{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 2019-01-04.01:13:35.866>
created_at = <Date 2019-01-03.21:29:10.584>
labels = ['expert-regex', 'invalid', 'type-bug', '3.7']
title = 'All regular expression match groups are the empty string'
updated_at = <Date 2019-01-05.11:05:10.761>
user = 'https://github.com/adiba'

bugs.python.org fields:

activity = <Date 2019-01-05.11:05:10.761>
actor = 'adiba'
assignee = 'none'
closed = True
closed_date = <Date 2019-01-04.01:13:35.866>
closer = 'mrabarnett'
components = ['Regular Expressions']
creation = <Date 2019-01-03.21:29:10.584>
creator = 'adiba'
dependencies = []
files = []
hgrepos = []
issue_num = 35653
keywords = []
message_count = 2.0
messages = ['332948', '332956']
nosy_count = 0.0
nosy_names = []
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue35653'
versions = ['Python 3.7']

[adiba]

This is the regular expression: ^(?:(\d*)(\D*))*$
This is the test string: 42AZ
This is the expectation for the match groups: ('42', 'AZ')
This is the actual return value: ('', '')

https://gist.github.com/adiba/791ba943a1102994d43171dc98aaecd0

[mrabarnett]

Look at the spans of the groups:

>>> import re
>>> re.search(r'^(?:(\d*)(\D*))*$', "42AZ").span(1)
(4, 4)
>>> re.search(r'^(?:(\d*)(\D*))*$', "42AZ").span(2)
(4, 4)

They're telling you that the groups are matching twice (because of the outer *). The first time, they match ('42', 'AZ'); the second time, they match ('', '') at the end of the string.

Not a bug.