Deprecate unrecognized backslash+letter escapes in re

[serhiy-storchaka]

BPO	23622
Nosy	@pitrou, @ezio-melotti, @serhiy-storchaka
Files	re_deprecate_escaped_letters.patch

^{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 2015-03-24.21:18:22.382>
created_at = <Date 2015-03-09.13:28:27.035>
labels = ['expert-regex', 'type-feature', 'library']
title = 'Deprecate unrecognized backslash+letter escapes in re'
updated_at = <Date 2015-03-24.23:35:06.524>
user = 'https://github.com/serhiy-storchaka'

bugs.python.org fields:

activity = <Date 2015-03-24.23:35:06.524>
actor = 'python-dev'
assignee = 'serhiy.storchaka'
closed = True
closed_date = <Date 2015-03-24.21:18:22.382>
closer = 'serhiy.storchaka'
components = ['Library (Lib)', 'Regular Expressions']
creation = <Date 2015-03-09.13:28:27.035>
creator = 'serhiy.storchaka'
dependencies = []
files = ['38406']
hgrepos = []
issue_num = 23622
keywords = ['patch']
message_count = 3.0
messages = ['237645', '239181', '239197']
nosy_count = 6.0
nosy_names = ['pitrou', 'ezio.melotti', 'mrabarnett', 'cvrebert', 'python-dev', 'serhiy.storchaka']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue23622'
versions = ['Python 3.5']

[serhiy-storchaka]

Regular expressions use the backslash character for two functions:

1. to indicate special forms;
2. to allow special characters to be used without invoking their special meaning.

If backslash + character is not recognized as special form (1), it interpreted in meaning (2).

Usually new special forms have form backslash + ASCII letter, because unlike to other characters single ASCII letters do not have special meaning in any regular expression engine or programming language. This using the backslash with inner ASCII letter dangerous. Currently it means just this letter literally, but in future it can mean special form. For example \u and \U forms were added in 3.3 and this could break regular expression patters that use \u and \U before.

To avoid possible breaking it makes sense to reject unrecognized backslash + ASCII letter sequences. Proposed patch adds deprecation warnings when unknown escape of ASCII letter is used. The idea was proposed by Matthew Barnett [1].

[1] http://permalink.gmane.org/gmane.comp.python.devel/151657

[python-dev]

New changeset 014031a4d398 by Serhiy Storchaka in branch 'default':
Issue bpo-23622: Unknown escapes in regular expressions that consist of '\'
https://hg.python.org/cpython/rev/014031a4d398

[python-dev]

New changeset 7384db2fce8a by Serhiy Storchaka in branch 'default':
Fixed using deprecated escaping in regular expression in _strptime.py (bpo-23622).
https://hg.python.org/cpython/rev/7384db2fce8a