Calculation of box graph with lightlike particles

A basic Feynman graph of the box type, relevant to several applications of perturbative quantum chromodynamics and of quantum electrodynamics, is calculated in an elegant manner using dimensional regularization. With $q$ the four-momentum of one of the external particles and with all other particles lightlike, we present results in simple and compact forms for $q^2=0\mathrm{}$ as well as for $q^2\ne 0$.