We study the problem of minimizing $\int _\Omega F)\, dx \;$ over the functions $u\in W^{1,1}$ that assume given boundary values $\phi $ on $\Gamma := \partial \Omega $. The lagrangian $F$ and the domain $\Omega $ are assumed convex. A new type of hypothesis on the boundary function $\phi $ is introduced: the lower bounded slope condition. This condition, which is less restrictive than the familiar bounded slope condition of Hartman, Nirenberg and Stampacchia, allows us to extend the classical (...) Hilbert-Haar regularity theory to the case of semiconvex boundary data. We prove in particular that the solution is locally Lipschitz in $\Omega $. In certain cases, as when $\Gamma $ is a polyhedron or else of class $C^{1,1}$, we obtain in addition a global Hölder condition on $\,\overline{\!\,\Omega }$. (shrink)