  We study the energies and optical spectra of excitons in twisted bilayers of
anisotropic van der Waals semiconductors exhibiting moir\'e patterns, taking
phosphorene as a case study. Following the electronic Hamiltonian introduced by
us in [Phys. Rev. B 105, 235421 (2022)], and leveraging the scale separation
between the moir\'e lengthscale and the exciton Bohr radii, we introduce a
continuous model for excitons that incorporates the spatial variation of their
binding energies. Our zone-folding calculations reveal a dimensionality
crossover for the exciton states, driven by the combined dispersion- and
moir\'e potential anisotropies, from quantum-dot-like (0D) lattices at twist
angles $\theta<\theta_*$, to quantum-wire-like (1D) arrays at
$\theta>\theta_*$, with crossover angle $\theta_*=4^\circ$. We identify clear
signatures of this dimensionality crossover in the twist angle dependence of
the excitonic absorption spectra, which allows experimental verification of our
theoretical results through standard optical measurements. Our results
establish two-dimensional anisotropic moir\'e semiconductors as versatile
solid-state platforms for exploring bosonic correlations across different
dimensionalities.
