http://docs.python.org/py3k/library/re.html
"(?P<name>...)

    Similar to regular parentheses, but the substring matched by the group is accessible within the rest of the regular expression via the symbolic group name name. Group names must be valid Python identifiers, and each group name must be defined only once within a regular expression."


>>> chr(255)
'ÿ'
>>> 'ÿ'.isidentifier()
True
>>> import re
>>> re.search(r'(?P<ÿ>a)', 'abc')
Traceback (most recent call last):
  File "/usr/local/lib/python3.2/functools.py", line 176, in wrapper
    result = cache[key]
KeyError: (<class 'str'>, '(?P<ÿ>a)', 0)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.2/re.py", line 158, in search
    return _compile(pattern, flags).search(string)
  File "/usr/local/lib/python3.2/re.py", line 255, in _compile
    return _compile_typed(type(pattern), pattern, flags)
  File "/usr/local/lib/python3.2/functools.py", line 180, in wrapper
    result = user_function(*args, **kwds)
  File "/usr/local/lib/python3.2/re.py", line 267, in _compile_typed
    return sre_compile.compile(pattern, flags)
  File "/usr/local/lib/python3.2/sre_compile.py", line 491, in compile
    p = sre_parse.parse(p, flags)
  File "/usr/local/lib/python3.2/sre_parse.py", line 692, in parse
    p = _parse_sub(source, pattern, 0)
  File "/usr/local/lib/python3.2/sre_parse.py", line 315, in _parse_sub
    itemsappend(_parse(source, state))
  File "/usr/local/lib/python3.2/sre_parse.py", line 552, in _parse
    raise error("bad character in group name")
sre_constants.error: bad character in group name
>>>


also:
(?P=name)
(?(name)yes|no)
\g<name>

There are two options here:

1. fix the doc;
2. fix the code;

Matthew, do you have any opinion on this?  Does this work on regex?

It doesn't work in regex, but it probably should. IMHO, if it's a valid identifier, then it should be allowed.

Here is a patch which make re to use for groups the same rule as for Python 3 identifiers. In Python 2 the implementation confirms the documentation.

New changeset 2fa27a3818a2 by Georg Brandl in branch '3.3':
Closes #14462: allow any valid Python identifier in sre group names, as documented.
http://hg.python.org/cpython/rev/2fa27a3818a2