We studied the photoluminescence from GaAsN/GaAs, with the nitrogen content of less than 0.5%. The low-temperature photoluminescence spectra are composed of several excitons bound to nitrogen complexes, each associated with different composition or configuration. These features were studied as a function of the excitation intensity, temperature, concentration, and growth conditions. The dependence of the binding energy of the dominant recombination center on the nitrogen concentration is interpreted in terms of a hierarchy of nitrogen complexes, from centers composed of at least two nitrogen atoms to more extended clusters. These excitonic transitions are very sensitive to growth parameters and can be used to study the statistical distribution of nitrogen in nominally uniform layers. We also show that the transition from nitrogen doping to alloy formation occurs for nitrogen concentrations above 0.25%.