.net - Match string not preceded by another with a regular expression

Question

The statement

Regex.Replace("XB", "([^A])B", "$1AB")

produces XAB, as expected. Can someone explain me why

Regex.Replace("XBB", "([^A])B", "$1AB")

does not produce XABAB, but XABB? It's like the regex parser no longer has knownledge of the preceding character when it reaches the second B.

Ultimately, I want to replace all Bs not preceded by a A by AB.

Answer 1

All B's not preceded by a A by AB.

Find: (?<!A)B
Replace: AB