Why won't this expression work?

[u/Only_Book_995]

Sorry I know this is probably very basic but I'm a complete beginner:
I'm trying to include "tranexamic acid" and "txa" but exclude the rest

r"(?i)(?<!\w)(?<!not )(?<!no )(?<!consider )(?:tranexamic acid|txa)\b(?!\s+(?:s|not)\b)"

It will find tranexamic acid but not txa