  We analyse the physics of nonlinear gravitational processes inside a
spherical charged black hole perturbed by a self-gravitating massless scalar
field. For this purpose we created an appropriate numerical code. Throughout
the paper, in addition to investigation of the properties of the mathematical
singularities where some curvature scalars are equal to infinity, we analyse
the properties of the physical singularities where the Kretschmann curvature
scalar is equal to the planckian value. Using a homogeneous approximation we
analyse the properties of the spacetime near a spacelike singularity in
spacetimes influenced by different matter contents namely a scalar field,
pressureless dust and matter with ultrarelativistic isotropic pressure. We also
carry out full nonlinear analyses of the scalar field and geometry of spacetime
inside black holes by means of an appropriate numerical code with adaptive mesh
refinement capabilities. We use this code to investigate the nonlinear effects
of gravitational focusing, mass inflation, matter squeeze, and these effects
dependence on the initial boundary conditions. It is demonstrated that the
position of the physical singularity inside a black hole is quite different
from the positions of the mathematical singularities. In the case of the
existence of a strong outgoing flux of the scalar field inside a black hole it
is possible to have the existence of two null singularities and one central
$r=0$ singularity simultaneously.
