  We present a uniform method for translating an arbitrary nondeterministic
finite automaton (NFA) into a deterministic mass action input/output chemical
reaction network (I/O CRN) that simulates it. The I/O CRN receives its input as
a continuous time signal consisting of concentrations of chemical species that
vary to represent the NFA's input string in a natural way. The I/O CRN exploits
the inherent parallelism of chemical kinetics to simulate the NFA in real time
with a number of chemical species that is linear in the size of the NFA. We
prove that the simulation is correct and that it is robust with respect to
perturbations of the input signal, the initial concentrations of species, the
output (decision), and the rate constants of the reactions of the I/O CRN.
