(pumping lemma): for every regular language \(\mathcal{L}\), there exists \(\ell {\gt} 0\) (a.k.a. pumping length) s.t. every \(s\in \mathcal{L}\) with \(|s|\ge \ell \) can be written as \(s=xyz\) with with \(\forall i \ge 0 xy^iz\in \mathcal{L},|y|{\gt}0,|xy|\ne 0\)