DFAs, Regex, PDAs, and Turing Machines.
Models Deterministic Finite Automata (DFA) \(M = (Q, \Sigma, \delta, q_0, F)\) where \(Q\) is states, \(\Sigma\) is alphabet, \(\delta: Q \times \Sigma \to Q\) is transition function, \(q_0\) is start state, and \(F \subseteq Q\) is accept states. Kleene's Theorem establishes equivalence between DFAs, Non-Deterministic Automata (NFA), and Regular Expressions. The Pumping Lemma proves non-regularity by stating \(\forall s \in L\) with \(|s| \ge p\), \(s = xyz\) with \(|xy| \le p, |y| > 0\) such that \(xy^i z \in L, \forall i \ge 0\).