Python 2.7 re.IGNORECASE broken in re.sub?

[Alex Willmer]

On Aug 16, 12:23 pm, Steven D'Aprano <st... at REMOVE-THIS-
cybersource.com.au> wrote:
> On Sun, 15 Aug 2010 17:36:07 -0700, Alex Willmer wrote:
> > On Aug 16, 1:07 am, Steven D'Aprano <st... at REMOVE-THIS-
> > cybersource.com.au> wrote:
> >> You're passing re.IGNORECASE (which happens to equal 2) as a count
> >> argument, not as a flag. Try this instead:
>
> >> >>> re.sub(r"python\d\d" + '(?i)', "Python27", t)
> >> 'Python27'
>
> > Basically right, but in-line flags must be placed at the start of a
> > pattern, or the result is undefined.
>
> Pardon me, but that's clearly not correct, as proven by the fact that the
> above example works.

Undefined includes 'might work sometimes'. I refer you to the Python
documentation:

"Note that the (?x) flag changes how the expression is parsed. It
should be used first in the expression string, or after one or more
whitespace characters. If there are non-whitespace characters before
the flag, the results are undefined."
http://docs.python.org/library/re.html#regular-expression-syntax