Invalid expression error if a regex ends with a backslash

[mlesniew]

BPO	13409
Nosy	@ezio-melotti

^{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/ezio-melotti'
closed_at = <Date 2011-11-15.19:16:18.103>
created_at = <Date 2011-11-15.19:01:05.153>
labels = ['expert-regex', 'type-bug', 'invalid']
title = 'Invalid expression error if a regex ends with a backslash'
updated_at = <Date 2011-11-15.19:16:18.102>
user = 'https://bugs.python.org/mlesniew'

bugs.python.org fields:

activity = <Date 2011-11-15.19:16:18.102>
actor = 'ezio.melotti'
assignee = 'ezio.melotti'
closed = True
closed_date = <Date 2011-11-15.19:16:18.103>
closer = 'ezio.melotti'
components = ['Regular Expressions']
creation = <Date 2011-11-15.19:01:05.153>
creator = 'mlesniew'
dependencies = []
files = []
hgrepos = []
issue_num = 13409
keywords = []
message_count = 2.0
messages = ['147696', '147697']
nosy_count = 2.0
nosy_names = ['ezio.melotti', 'mlesniew']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue13409'
versions = ['Python 2.7']

[mlesniew]

If a regular expression ends with a backslash, an exception is raised. Of course, the backslash has to be escaped. The simplest example, that causes the error is a regular expression, that should match only a single backslash:

    import re
    r = re.compile("\\")

[ezio-melotti]

You either have to double escape it (once for python and then for the regex engine) or use raw strings:
>>> re.match("\\\\", '\\').group()
'\\'
>>> re.match(r"\\", '\\').group()
'\\'