  We present a semi-numerical algorithm to calculate one-loop virtual
corrections to scattering amplitudes. The divergences of the loop amplitudes
are regulated using dimensional regularization. We treat in detail the case of
amplitudes with up to five external legs and massless internal lines, although
the method is more generally applicable. Tensor integrals are reduced to
generalized scalar integrals, which in turn are reduced to a set of known basis
integrals using recursion relations. The reduction algorithm is modified near
exceptional configurations to ensure numerical stability. To test the procedure
we apply these techniques to one-loop corrections to the Higgs to four quark
process for which analytic results have recently become available.
