Regular constraint

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A global constraint that is defined on a fixed-length sequence of finite-domain variables and stating that the values taken by this sequence of variables belongs to a given regular language. It allows us to express relations between the variables of a sequence such as the maximum length sub-sequence of identical values.

Formally, a deterministic finite automaton (DFA) is described by a 5-tuple LaTeX: M = (Q, \Sigma, \delta, q_0, F) where LaTeX: Q is a finite set of states, LaTeX: \Sigma is an alphabet, LaTeX: \delta: Q \times \Sigma \rightarrow Q is a transition function, LaTeX: q_0 \in Q is the initial state, and LaTeX: {F} \subseteq Q is the set of final (or accepting) states. Given an input string, the automaton starts in the initial state LaTeX: q_{0} and processes the string one symbol at the time, applying the transition function LaTeX: \delta at each step to update the current state. If the last state reached belongs to the set of final states LaTeX: F, then the string is accepted. All strings that are accepted by M generates the language LaTeX: {L}(M).

Now, let LaTeX: M = (Q, \Sigma, \delta, q_0, F) be a DFA and let LaTeX: X = \{x_1, s_2, \dots x_n\} be a set of variables with domain LaTeX: D(x_{i}) \subseteq \Sigma for LaTeX: 1\leq i \leq n. Then LaTeX: regular(X,M) = \{ (d_1, \dots, d_n) | \forall i, d_i \in D(x_i), d_1d_2 \dots d_n \in L(M) \}.


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