regex howto example in "Lookahead Assertions"

[Pavel]

BPO	25517
Nosy	@akuchling, @birkenfeld, @ezio-melotti, @serhiy-storchaka, @elenaoat
Files	issue25517.diff

^{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 2016-01-11.22:12:26.221>
created_at = <Date 2015-10-30.10:07:26.641>
labels = ['expert-regex', 'type-bug', 'docs']
title = 'regex howto example in "Lookahead Assertions"'
updated_at = <Date 2016-01-11.22:12:26.220>
user = 'https://bugs.python.org/Pavel'

bugs.python.org fields:

activity = <Date 2016-01-11.22:12:26.220>
actor = 'ezio.melotti'
assignee = 'docs@python'
closed = True
closed_date = <Date 2016-01-11.22:12:26.221>
closer = 'ezio.melotti'
components = ['Documentation', 'Regular Expressions']
creation = <Date 2015-10-30.10:07:26.641>
creator = 'Pavel'
dependencies = []
files = ['41566']
hgrepos = []
issue_num = 25517
keywords = ['patch']
message_count = 5.0
messages = ['253725', '253726', '257906', '258032', '258033']
nosy_count = 9.0
nosy_names = ['akuchling', 'georg.brandl', 'ezio.melotti', 'mrabarnett', 'docs@python', 'python-dev', 'serhiy.storchaka', 'Elena.Oat', 'Pavel']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue25517'
versions = ['Python 2.7', 'Python 3.5', 'Python 3.6']

[Pavel]

The example advises ".*..*$" expression "to match filenames where the extension is not bat". But here is an example which passes such check:

>>> re.match("(.*)[.](?!bat$).*$", "test.a.bat")
<_sre.SRE_Match object at 0x7ff221996f30>

To my mind use of negative lookbehind expressions (not covered so far in the HOWTO) is better:

>> re.match("(.*)[.].*(?<!.bat)$", "test.a.bat")
>>

[serhiy-storchaka]

Note that lookbehind assertions still are not documented in Regular Expression HOWTO.

The example that demonstrates negative lookahead assertion is ".*.[^.]$". But then we should explain why ".*" is changed to "[^.]".

[elenaoat]

Corrected the regular expression and documented why [^.]* is used.

[python-dev]

New changeset c6b5c03183e3 by Ezio Melotti in branch '2.7':
bpo-25517: fix regex in the regex howto. Patch by Elena Oat.
https://hg.python.org/cpython/rev/c6b5c03183e3

New changeset 6bd4a4907f66 by Ezio Melotti in branch '3.5':
bpo-25517: fix regex in the regex howto. Patch by Elena Oat.
https://hg.python.org/cpython/rev/6bd4a4907f66

New changeset 48e2f5915d49 by Ezio Melotti in branch 'default':
bpo-25517: merge with 3.5.
https://hg.python.org/cpython/rev/48e2f5915d49

[ezio-melotti]

Fixed, thanks for the patch!