Solving String Constraints with Concatenation Using SAT

We present a decision procedure for solving quantifier-free first-order formulas over the theory of strings, involving equality, regular constraints, and concatenation of string terms. Our approach uses an eager reduction to the Boolean satisfiability problem and extends the NFA2SAT string solver. We describe a novel SAT encoding for word equations that iteratively expands the search space and leverages incremental SAT solving. For unsatisfiable formulas, we estimate the bounds on the smallest solution from arithmetic constraints derived from word equations. An experimental evaluation shows that our approach is competitive with state-of-the-art string solvers and complements existing methods in string solving.