[Python-Dev] Behavior of matching backreferences

[Tim Peters]

[Tim]
> ..
> There's no such beast as "an optional group".  The
>
>     ^(a)
>
> part *must* match or the entire regexp fails, period, regardless
> of whether or not backreferences appear later.  The question mark
> following doesn't change this requirement. ...

Wow, yesterday's drugs haven't worn off yet <wink>.  The details of this
explanation were partly full of beans.  Let's consider a different regexp:

    ^(a)?b\1$

Should that match

    b

or not?  Python and Perl say "no" today, because \1 refers to a group that
didn't match.  Ir remains unclear to me whether Gustavo is saying it should,
but, if he is, that's too big a change, and

    ^(a?)b\1$

is the intended way to spell it.