posix vs perl regular expressions

[RN]

Hi,

I don't know if this is the right place to ask this, but -
what's the difference between posix and perl regular expressions?

A good example is "aa|bb". Will this match "aa" or "ab" in perl and
posix?

And for those POSIX experts, what do the following do?
[==] is for non-english languages and such?
[..] is for strings instead of characters in bracket expressions?
[::] what does this do?
any difference between [[]] and []?

I'm asking because I'm a contributer to wxWindows, and we're trying to
figure out what to do with the current regular expressions library
wxWindows is using (it's not unicode complaint, etc.).

The reason I ask the posix questions is except for those expression
types mentioned above (the 3/4 bracket types and alternation
operator), my own regex library meets posix standards.

Thanks,
RN