from the jeffrey friedl report:

The way (?i) works now is that if it appears anywhere 
in the regex, it turns on case-insensative matching 
for the entire regex. This is different (and less 
useful) than how Perl or Sun's Java package does it

[I'm pretty sure SRE does it this way to exactly
match the version of PCRE used in 1.5.2, but maybe
it's time to move forward... /F]

This is still a valid issue. (As of Python 2.6a1+ (trunk:61434, Mar 17 
2008, 08:06:54).)

>>> bool(re.match("abc(?i)","AbC"))
True

The documentation says that the behavior of a regex with (?<letter>) not 
at the beginning is undefined.

Short of implementing Java/Perl behavior, this should be made an error 
rather than undefined so that users porting their code from Java/Perl 
get an early warning.

Implemenetd in #3825.