In the theory of formal languages, the pumping lemma may refer to: Pumping lemma for regular languages, the fact that all sufficiently long strings in such a language have a substring that can be repeated arbitrarily many times, usually used to prove that certain languages are not regular Pumping lemma for context-free languages, the fact that all sufficiently long strings in such a language have a pair of substrings that can be repeated arbitrarily many times, usually used to prove that certain languages are not context-free Pumping lemma for indexed languages Pumping lemma for regular tree languages See also Ogden's lemma, a stronger version of the pumping lemma for context-free languages