  The black hole information paradox is related to the area of event horizon,
and potentially to the volume and singularity behind it. One example is the
complexity/volume duality conjectured by Stanford and Susskind. Accepting the
proposal of Christodoulou and Rovelli, we calculate the maximal volume inside
regular black holes, which are free of curvature singularity, in asymptotically
flat and anti-de Sitter spacetimes respectively. The complexity/volume duality
is then applied to anti-de Sitter regular black holes. We also present an
analytical expression for the maximal volume outside the de Sitter horizon.
