[issue3281] support r"\"

[LI Daobing]

New submission from LI Daobing <lidaobing at gmail.com>:

currently r"\" or r'\' is not a valid string literal and it is already
documented.

but this exception is not simple enough, it make users confused when
they found that r'C:\Program\Python\' does not work as expected.

please consider support this, thanks.

----------
components: None
messages: 69249
nosy: lidaobing
severity: normal
status: open
title: support r"\"
type: feature request
versions: Python 2.6

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue3281>
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