The doctest.IGNORE_EXCEPTION_DETAIL optionflag does not seem to have the
desired behavior when the exception does not provide a message, due to the
regular expressions in doctest.DocTestRunner.__run expecting a colon in the
second group::

                elif self.optionflags & IGNORE_EXCEPTION_DETAIL:
                    m1 = re.match(r'(?:[^:]*\.)?([^:]*:)', example.exc_msg)
                    m2 = re.match(r'(?:[^:]*\.)?([^:]*:)', exc_msg)
                    if m1 and m2 and check(m1.group(1), m2.group(1),
                                           self.optionflags):
                        outcome = SUCCESS

Normally this wouldn't matter, as there's no need to ignore the exception
detail if there is no detail to normalize, but since
http://bugs.python.org/issue7490 it looks like the blessed method of
normalizing Python 2 and 3 exceptions in doctests is to use
IGNORE_EXCEPTION_DETAIL.  This doesn't work for any exceptions which do
not have a message.

Example::

    >>> def f(x):
    ...     r'''
    ...     >>> from http.client import HTTPException
    ...     >>> raise HTTPException() #doctest: +IGNORE_EXCEPTION_DETAIL
    ...     Traceback (most recent call last):
    ...     foo.bar.HTTPException
    ...     '''
    >>> test = doctest.DocTestFinder().find(f)[0]
    >>> doctest.DocTestRunner(verbose=True).run(test)
    Failed example:
        raise HTTPException() #doctest: +IGNORE_EXCEPTION_DETAIL
    Expected:
        Traceback (most recent call last):
        foo.bar.HTTPException
    Got:
        Traceback (most recent call last):
        ...
        http.client.HTTPException

I've attached a test and a very naive fix of the regular expression.

I agree this is a bug, and at a first scan your fix looks good.  I'll make it a priority to pay more attention to it now ;-)

On second thought, I don't want to use a regexp for this.  The mandatory colon _was_ a kind of absolute wall, and the various instances of "[^:]" exploited that to avoid unintended matches.

But "possibly dotted name followed possibly by a colon" is straightforward to get right with a pair of simple string searches, so I'll introduce a new `_strip_exception_details()` function instead.

New changeset c80083ad142d by Tim Peters in branch 'default':
Issue #19138: doctest's IGNORE_EXCEPTION_DETAIL now allows no detail at all.
http://hg.python.org/cpython/rev/c80083ad142d

New changeset 5e14a3435f52 by Tim Peters in branch '2.7':
Issue #19138: doctest's IGNORE_EXCEPTION_DETAIL now allows no detail at all.
http://hg.python.org/cpython/rev/5e14a3435f52

New changeset 017d7c27a4f6 by Tim Peters in branch '3.3':
Issue #19138: doctest's IGNORE_EXCEPTION_DETAIL now allows no detail at all.
http://hg.python.org/cpython/rev/017d7c27a4f6