vim: unable to match character after new line properly

Question

In vim, I'm trying to match all lines that has ^A. but not followed by a B. on the next line.
I'm matching it with /\v^A\. .*$\n[^B]. This correctly identifies the relevant portion in question 2., but not in question 3. I'm not able to figure out why. The ending matches \n\n. And [^B] should match \n but it doesn't. What am I doing wrong?

1. Hello?
A. aaaaaaaa
B. bbbbbb
C. ccccccc
D. ddddd

2. Hello?
A. aaaaaaaa B. bbbbbb
C. ccccccc
D. ddddd


3. Hello?
A. aaaaaaaa B. bbbbbb C. ccccccc D. ddddd

4. Hello?

Answer 1

As :help /[\n] documents, a collection like [^B] does not match an end-of-line character.

This makes it Vi compatible: Without the "_" or "\n" the collection does not match an end-of-line.

You have two options: Either explicitly include a newline, so that your line followed by an empty line also matches:

/\v^A\. .*$\n([^B]|$)/

Or do a negative lookahead match with /\@!:

/\v^A\. .*$\nB@!/