A bug related to matching the empty string

[lanyjie]

BPO	10532
Nosy	@ned-deily

^{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 2010-11-26.20:31:54.710>
created_at = <Date 2010-11-25.16:39:12.326>
labels = ['expert-regex', 'type-bug', 'invalid']
title = 'A bug related to matching the empty string'
updated_at = <Date 2010-11-26.20:31:54.708>
user = 'https://bugs.python.org/lanyjie'

bugs.python.org fields:

activity = <Date 2010-11-26.20:31:54.708>
actor = 'ned.deily'
assignee = 'none'
closed = True
closed_date = <Date 2010-11-26.20:31:54.710>
closer = 'ned.deily'
components = ['Regular Expressions']
creation = <Date 2010-11-25.16:39:12.326>
creator = 'lanyjie'
dependencies = []
files = []
hgrepos = []
issue_num = 10532
keywords = []
message_count = 3.0
messages = ['122380', '122384', '122476']
nosy_count = 3.0
nosy_names = ['ned.deily', 'mrabarnett', 'lanyjie']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = None
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue10532'
versions = ['Python 3.2']

[lanyjie]

Here are some puzzling results I have got (I am using Python 3, I suppose similar results for python 2).

When I do the following, I got an exception:
>>> re.findall('(d*)*', 'adb')
>>> re.findall('((d)*)*', 'adb')

When I do this, I am fine but the result is wrong:
>>> re.findall('((.d.)*)*', 'adb')
[('', 'adb'), ('', '')]

Why is it wrong?

The first mactch of groups:
('', 'adb')
indicates the outer group ((.d.)*) captured
the empty string, while the inner group (.d.)
captured 'adb', so the outer group must have
captured the empty string at the end of the
provided string 'adb'.

Once we have matched the final empty string '',
there should be no more matches, but we got
another match ('', '')!!!

So, findall matched the empty string in
the end of the string twice!!!

Isn't this a bug?

Yingjie

[mrabarnett]

The spans say this:

>>> for m in re.finditer('((.d.)*)*', 'adb'):
    print(m.span())

(0, 3)
(3, 3)

There's an non-empty match followed by an empty match.

IHMO, not a bug.

[ned-deily]

Closing this issue since it appears to not be a bug.