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When PEP 414 restored support for explicit Unicode literals in Python 3, the "ur" string prefix was deemed to be a synonym for the "r" prefix.
However, "ur" in 2.x was only kinda-sorta-raw, since it still supported Unicode escapes:
$ python
Python 2.7.3 (default, Apr 30 2012, 21:18:11) 
[GCC 4.7.0 20120416 (Red Hat 4.7.0-2)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> print(ur'\u03B3')
γ
By contrast, they really are raw strings in 3.x, because the default UTF-8 source code encoding allows arbitrary Unicode characters to be included directly in the literal:
$ ./python
Python 3.3.0a4+ (default:cfbf6aa5c9e3+, Jun 17 2012, 15:25:45) 
[GCC 4.7.0 20120507 (Red Hat 4.7.0-5)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> r"γ"
'γ'
>>> "\U000003B3"
'γ'
>>> r"\U000003B3"
'\\U000003B3'
>>> ur"\U000003B3"
'\\U000003B3'
Rather than reintroducing this weird legacy not-really-raw string behaviour, I'd prefer to revert this aspect of the PEP 414 changes completely.

+1
Raw unicode strings are rarely used and it's easy to overcome the limitation. I've created a patch that disabled ur'' syntax, updates the docs and adds some tests for u'' syntax. 
I couldn't find any tests for the syntax ... shame on you! :)

> I've created a patch that disabled ur'' syntax, updates the docs and adds some tests for u'' syntax. 
Don't forget tokenize module (see issue15054).

If you commit this, please do so before beta1.

I'll submit an updated patch tomorrow.

New patch:
* corrected comment in Parser/tokenizer.c
* added test for ub"" and bu"" raising SyntaxError
* removed handling of ur"" from tokenize module
Serhiy, could you please review my changed to the tokenize module? I think I found all relevant places but I'm not absolutely sure.

New changeset 8e47e9af826e by Christian Heimes in branch 'default':
Issue #15096: Drop support for the ur string prefix
http://hg.python.org/cpython/rev/8e47e9af826e 

Thanks for you review, Serhiy. I removed the last remains from test_tokenize and even found another missing sentence in the docs.