  We investigate the regularization-scheme dependence of scattering amplitudes
in massless QCD and find that the four-dimensional helicity scheme (FDH) and
dimensional reduction (DRED) are consistent at least up to NNLO in the
perturbative expansion if renormalization is done appropriately. Scheme
dependence is shown to be deeply linked to the structure of UV and IR
singularities. We use jet and soft functions defined in soft-collinear
effective theory (SCET) to efficiently extract the relevant anomalous
dimensions in the different schemes. This result allows us to construct
transition rules for scattering amplitudes between different schemes (CDR, HV,
FDH, DRED) up to NNLO in massless QCD. We also show by explicit calculation
that the hard, soft and jet functions in SCET are regularization-scheme
independent.
