A phenomenological study of the final combined HERA data on inclusive deep inelastic scattering has been performed. The data are presented and investigated for a kinematic range extending from values of the four-momentum transfer, $Q^2$, above 104 GeV$^2$ down to the lowest values observable at HERA of $Q^2$=0.045 GeV$^2$ and Bjorken x, x$_{Bj}$ = 6 $\cdot$10$^{-7}$. The data are well described by fits based on perturbative quantum chromodynamics (QCD) using collinear factorisation and evolution of the parton densities encompassed in the Dokshitzer-Gribov-Lipatov-Altarelli-Parisi formalism from the highest $Q^2$ down to $Q^2$ of a few GeV$^2$. The Regge formalism with the soft Pomeron pole can describe the data up to $Q^2$ ≈ 0.65 GeV$^2$. The complete data set can be described by a new fit using the Abramowicz-Levin-Levy-Maor parametrization. The region between the Regge and the perturbative QCD regimes is of particular interest.