  We establish the existence and sharp global regularity results ($C^{0,
\gamma}$, $C^{0, 1}$ and $C^{1, \alpha}$ estimates) for a class of fully
nonlinear elliptic PDEs with unbalanced variable degeneracy. In a precise way,
the degeneracy law of the model switches between two different kinds of
degenerate elliptic operators of variable order, according to the null set of a
modulating function. Such sharp regularity estimates generalize and improve, to
some extent, earlier ones via geometric treatments. Our results are
consequences of geometric tangential methods and make use of compactness,
localized oscillating and scaling techniques. In the end, our findings are
applied in the study of a wide class of nonlinear models and free boundary
problems.
