In a Formula field, a regex with \b never matches anything.
The above Formula should match these entities, but doesn’t match any:
It seems you’re right, but I don’t know why 
In the meantime, for your specific case, could you just use
[\W^]PM[\W$]
instead?
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Maybe a Unicode-related bug somewhere? Or escaping/quoting? SQL?
I think it would need to be something more like
(^|\W)PM($|^W)
because you can’t put ^ $ in a character class - it’s just a literal there.
You’re probably right - regex is not my strong point to be honest!