Negative lookahead/behind is regular in the mathematical sense. Anything that can be achieved by a finite combination of FSMs is, including OR, AND and NOT on t

Negative lookahead/behind is regular in the mathematical sense. Anything that can be achieved by a finite combination of FSMs is, including OR, AND and NOT on their accepting states. What isn't regular is stipulation that a group matches the same string as another group. E.g. "Same word twice" is not expressible by formal regular languages.