Given a word, say deregulate, is there a prefix to denote the opposite, rather than simply saying regulate? It seems fairly illogical to have one but I was wondering if something existed.
Merriam-Webster's Eleventh Collegiate Dictionary (2003) lists several words in which the prefix un- negates a word that de- has previously negated:
Such instances are quite rare, however, in comparison to the number of words that use re- to override a de- negation. Thus, for example, if a doctor decontaminates a wound with antiseptic, but bacteria later reappear, we say that the new germs "recontaminate" the wound, not that they "undecontaminate" it. Among the word pairs that follow this model are:
Not all of these pairs are exact opposites (and certainly not in all senses of each term), but a number of them are very nearly so.
Still, noting the existence of this group of paired opposites is very far from saying that replacing a de- prefix with a re- prefix negates the word that de- was attached to. I suspect that Teresa, who contributed an earlier answer nominating en- as a possible negating prefix for words containing the de- prefix, had a similar (albeit smaller) set of opposed word pairs in mind:
Unfortunately, the two instances that Teresa put forward to show en- in direct opposition to de- weren't good examples; but her argument about en- does have some validity, as the five word pairs above demonstrate.
Ultimately, the simple answer to the general question raised in the original post is that no prefix consistently and reliably undoes the negation that de- (or un- or dis-, for that matter) introduces.
Regulate means to control or direct according to rule, principle, or law.
Deregulate means to remove regulations.
Re-regulate could mean to add regulations again, possibly after they've had been removed.
If you're doing something for the first time, simply using the root word is sufficient. If you're doing something a second time, you could use a re- prefix, but check for each word.