We prove that the L p ′ -solvability of the homogeneous Dirichlet problem for an elliptic operator L = − div A∇ with real and merely bounded coefficients is equivalent to the L p ′ -solvability of the Poisson Dirichlet problem Lw = H − div F, assuming that H and F lie in certain Carleson-type spaces, and that the domain Ω ⊂ R n+1 , n ≥ 2, satisfies the corkscrew condition and has n-Ahlfors regular boundary. The L p ′solvability of the Poisson problem (with an L p ′ estimate on the non-tangential maximal function) is new even when L = −∆ and Ω is the unit ball. In turn, we use this result to show that, in a bounded domain with uniformly n-rectifiable boundary that satisfies the corkscrew condition, L p ′ -solvability of the homogeneous Dirichlet problem for an operator L = − div A∇ satisfying the Dahlberg-Kenig-Pipher condition (of arbitrarily large constant) implies solvability of the L p -regularity problem for the adjoint operator L * = − div A T ∇, where 1/p + 1/p ′ = 1 and A T is the transpose matrix of A. Contents 4.2. The starlike Lipschitz subdomains Ω ± R 31 4.3. The corona decomposition of Ω 36 4.4. The properties of Ω R 38 5. The almost L-elliptic extension 41 6. The Regularity Problem for DKP operators 42 6.1. A n.t. maximal function estimate for the almost L-elliptic extension 44 6.2. Proof of Proposition 6.2 49 Appendix A. Proofs of auxiliary results 54 References 63