All articles (example) I read till now about regular expressions and NFAs explain three operations:sequencealternation (union)repetition (Kleene star)No one talks about negation.  Is the negation not regular?Update:@RemcoGerlich What does it mean swap the states. Can you explain it with the Kleene closure?How would the Kleene closure look with swapped states?
Is the negation of a regular expression still regular?
regular expressions
Yes; first, every non-deterministic finite automaton can be converted to a deterministic finite automaton that accepts the same language.Now, just make all accepting states non-accepting and vice versa: this new DFA accepts a string exactly when the original DFA wouldn't, so its language is the complement of the orignal language.