Regular black holes and their singular families

Regular black holes without curvature singularity can arise in Einstein gravity with appropriate matter energy-momentum tensor. We show that these regular solutions represent only a special case of a much broader family of black holes with a free mass parameter. The regularity is achieved only at a specific mass value, and any deviation from the fine-tuned parameter inevitably results in curvature singularity. As a concrete example, we consider nonlinear electrodynamics (NLED) as matter sources. A new NLED theory is proposed that is a generalization of the Bardeen class and the Hayward class. New regular black holes and their singular counterparts are obtained. Significant distinctions between regular black holes and their singular counterparts are analyzed. These findings provide new insights into regular black holes.