Wrong or missing exception when compiling regexes with recursive named backreferences

[dhaffey]

BPO	24580
Nosy	@benjaminp, @ezio-melotti, @serhiy-storchaka
Files	re_open_group_symbolic_ref.patch re_open_group_symbolic_ref-3.4.patch

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

Show more details

GitHub fields:

assignee = 'https://github.com/serhiy-storchaka'
closed_at = <Date 2015-12-19.21:26:32.930>
created_at = <Date 2015-07-07.01:25:41.905>
labels = ['expert-regex', 'type-bug']
title = 'Wrong or missing exception when compiling regexes with recursive named backreferences'
updated_at = <Date 2015-12-19.21:26:32.928>
user = 'https://bugs.python.org/dhaffey'

bugs.python.org fields:

activity = <Date 2015-12-19.21:26:32.928>
actor = 'serhiy.storchaka'
assignee = 'serhiy.storchaka'
closed = True
closed_date = <Date 2015-12-19.21:26:32.930>
closer = 'serhiy.storchaka'
components = ['Regular Expressions']
creation = <Date 2015-07-07.01:25:41.905>
creator = 'dhaffey'
dependencies = []
files = ['39879', '39880']
hgrepos = []
issue_num = 24580
keywords = ['patch']
message_count = 5.0
messages = ['246390', '246396', '246397', '246913', '256740']
nosy_count = 6.0
nosy_names = ['benjamin.peterson', 'ezio.melotti', 'mrabarnett', 'dhaffey', 'python-dev', 'serhiy.storchaka']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue24580'
versions = ['Python 2.7', 'Python 3.4', 'Python 3.5', 'Python 3.6']

[dhaffey]

Error reporting for recursive backreferences in regexes isn't consistent across both types of backref. Here's the exception for a recursive numeric backref:

>>> import re
>>> re.compile(r'(\1)')
Traceback (most recent call last):
    ...
sre_constants.error: cannot refer to an open group at position 1

Here's what I'm seeing on the 3.5 branch for a named backref:

>>> re.compile(r'(?P<spam>(?P=spam))')
Traceback (most recent call last):
    ...
RecursionError: maximum recursion depth exceeded

Which is an improvement over 3.4 and below, where compilation succeeds and appears to treat (?P=spam) as valid but unmatchable.

[serhiy-storchaka]

Here is a patch that forbids symbolic references to opened groups in 3.5+.

[serhiy-storchaka]

It is questionable if an exception should be raised in older Python versions. Here is a patch for 3.4 that just issues a warning.

[python-dev]

New changeset 361d7af9396e by Serhiy Storchaka in branch '3.5':
Issue bpo-24580: Symbolic group references to open group in re patterns now are
https://hg.python.org/cpython/rev/361d7af9396e

New changeset 4d3557500019 by Serhiy Storchaka in branch 'default':
Issue bpo-24580: Symbolic group references to open group in re patterns now are
https://hg.python.org/cpython/rev/4d3557500019

[serhiy-storchaka]

It is too late for 3.4, and I left 2.7 as is. Reopen issue if you think that it is worth to add a warning to 2.7.