This patch is now based on Python 2.6rc2.

I've reduced the number of macros and used functions instead, provided
that it didn't cost much in terms of speed. In many cases it should be
faster than the current release, and at worst no slower. More speed
tests and tweaks needed.

BTW, the impression I got was that look-behind was fixed width because
the matching operations could only match forwards through the text, so
in order to look behind it had to step back through the text and then
match forwards. For simplicity and speed it insisted that it must be
able to determine the size of the step beforehand, hence fixed-width. My
addition was to add matching operations which worked matched backwards
and also reverse the order of the matching for look-behinds, an idea
which I got from a page on how it could be implemented in Perl 6!