Using Groups inside Braces with Regular Expressions

[Chris]

I'm trying to delimit  sentences in a block of text by defining the
end-of-sentence marker as a period followed by a space followed by an
uppercase letter or end-of-string.

I'd imagine the regex for that would look something like:
[^(?:[A-Z]|$)]\.\s+(?=[A-Z]|$)

However, Python keeps giving me an "unbalanced parenthesis" error for
the [^] part. If this isn't valid regex syntax, how else would I match
a block of text that doesn't the delimiter pattern?

Thanks,
Chris