  Spatial scale separation often leads to sharp interfaces that can be fully
localized pulses or transition layer fronts connecting different states. This
paper concerns the asymptotic interaction laws of pulses and fronts in the
so-called semi-strong regime of strongly differing diffusion lengths for
reaction-diffusion systems in one space dimension. An asymptotic expansion and
matching approach is applied in a model independent common framework. First
order semi-strong interaction is introduced as a general interface interaction
type. It is distinct from the semi-strong interaction studied over the past
decade, which is referred to as `second order' here. Laws of motion are derived
for pulses as well as fronts in abstract systems with attention to the effect
of symmetries. First order interaction for pulses is shown to be gradient-like
under conditions that are numerically checked for a class of equations
including the Gray-Scott and Schnakenberg models.
