Regular expressions with empty named groups break isname check

[GabrielRodrguezAlberich]

BPO	16578
Nosy	@ezio-melotti, @serhiy-storchaka
Superseder	bpo-12759: "(?P=)" input for Tools/scripts/redemo.py raises unnhandled exception

^{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 2012-11-29.17:32:22.052>
created_at = <Date 2012-11-29.17:16:26.267>
labels = ['expert-regex', 'type-crash']
title = 'Regular expressions with empty named groups break isname check'
updated_at = <Date 2012-11-29.17:35:09.963>
user = 'https://bugs.python.org/GabrielRodrguezAlberich'

bugs.python.org fields:

activity = <Date 2012-11-29.17:35:09.963>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = True
closed_date = <Date 2012-11-29.17:32:22.052>
closer = 'Gabriel.Rodr\xc3\xadguez.Alberich'
components = ['Regular Expressions']
creation = <Date 2012-11-29.17:16:26.267>
creator = 'Gabriel.Rodr\xc3\xadguez.Alberich'
dependencies = []
files = []
hgrepos = []
issue_num = 16578
keywords = []
message_count = 3.0
messages = ['176664', '176665', '176666']
nosy_count = 4.0
nosy_names = ['ezio.melotti', 'mrabarnett', 'serhiy.storchaka', 'Gabriel.Rodr\xc3\xadguez.Alberich']
pr_nums = []
priority = 'normal'
resolution = 'duplicate'
stage = 'resolved'
status = 'closed'
superseder = '12759'
type = 'crash'
url = 'https://bugs.python.org/issue16578'
versions = ['Python 2.7', 'Python 3.2']

[GabrielRodrguezAlberich]

>>> import re
>>> re.compile("(?P<>)")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.7/re.py", line 190, in compile
    return _compile(pattern, flags)
  File "/usr/lib/python2.7/re.py", line 240, in _compile
    p = sre_compile.compile(pattern, flags)
  File "/usr/lib/python2.7/sre_compile.py", line 500, in compile
    p = sre_parse.parse(p, flags)
  File "/usr/lib/python2.7/sre_parse.py", line 673, in parse
    p = _parse_sub(source, pattern, 0)
  File "/usr/lib/python2.7/sre_parse.py", line 308, in _parse_sub
    itemsappend(_parse(source, state))
  File "/usr/lib/python2.7/sre_parse.py", line 544, in _parse
    if not isname(name):
  File "/usr/lib/python2.7/sre_parse.py", line 218, in isname
    if not isident(name[0]):
IndexError: string index out of range

isname() shouldn't be blindly accessing name[0].

[serhiy-storchaka]

Fixed in bpo-12759.

[GabrielRodrguezAlberich]

Uh, sorry about that. Thanks.