Elastic optical networking that enables flexible spectrum resource allocation and bitrate/distance-adaptive modulation is one of the hot trends in optical networks to cope with both legacy low-bitrate services and emerging new bandwidth-abundant super-channel services. This paper evaluates spectrum utilization and regenerating spectrum requirement in regeneration capable elastic optical networks with different modulation format assignment schemes under various network and traffic conditions. The spectrum efficiency of elastic optical networks is also analyzed in comparison with that of conventional WDM networks. Finally, numerical simulation is applied to verify the performance of elastic optical networks against conventional WDM networks. The results prove that elastic optical networks are more spectrum-efficient than conventional WDM networks and the network performance significantly depends on the regeneration capability.