fnmatch.translate() can produce a pattern which emits a nested set warning

[timgraham]

BPO	32775
Nosy	@serhiy-storchaka, @timgraham
PRs	bpo-32775: Fix regular expression warnings in fnmatch. #5583 [3.7] bpo-32775: Fix regular expression warnings in fnmatch. (GH-5583) #5596

^{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 = 'https://github.com/serhiy-storchaka'
closed_at = <Date 2018-02-09.11:57:42.153>
created_at = <Date 2018-02-05.20:07:22.506>
labels = ['3.7', 'type-bug', 'library']
title = 'fnmatch.translate() can produce a pattern which emits a nested set warning'
updated_at = <Date 2018-02-09.11:57:42.152>
user = 'https://github.com/timgraham'

bugs.python.org fields:

activity = <Date 2018-02-09.11:57:42.152>
actor = 'serhiy.storchaka'
assignee = 'serhiy.storchaka'
closed = True
closed_date = <Date 2018-02-09.11:57:42.153>
closer = 'serhiy.storchaka'
components = ['Library (Lib)']
creation = <Date 2018-02-05.20:07:22.506>
creator = 'Tim.Graham'
dependencies = []
files = []
hgrepos = []
issue_num = 32775
keywords = ['patch']
message_count = 4.0
messages = ['311687', '311689', '311878', '311881']
nosy_count = 2.0
nosy_names = ['serhiy.storchaka', 'Tim.Graham']
pr_nums = ['5583', '5596']
priority = 'high'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue32775'
versions = ['Python 3.7']

[timgraham]

As discussed in bpo-30349#msg311684, fnmatch.translate() can produce a pattern which emits a nested set warning:

>>> import fnmatch, re
>>> re.compile(fnmatch.translate('[[]foo]'))
__main__:1: FutureWarning: Possible nested set at position 10
re.compile('(?s:\\(.s:[[]foo\\\\\\]\\)\\\\Z)\\Z')

[serhiy-storchaka]

It is easy to fix sets starting with '['. It is more hard to fix sets with doubled '-', '&', '|' or '~'.

>>> fnmatch.fnmatch('#', '[#--]')
/home/serhiy/py/cpython/Lib/fnmatch.py:46: FutureWarning: Possible set difference at position 6
  return re.compile(res).match
True

[serhiy-storchaka]

New changeset 23cdbfa by Serhiy Storchaka in branch 'master':
bpo-32775: Fix regular expression warnings in fnmatch. (bpo-5583)
23cdbfa

[serhiy-storchaka]

New changeset 0e36173 by Serhiy Storchaka (Miss Islington (bot)) in branch '3.7':
bpo-32775: Fix regular expression warnings in fnmatch. (GH-5583) (GH-5596)
0e36173