  In this work we implement the Minimal Geometric Deformation method to obtain
the isotropic sector and the decoupler matter content of any anisotropic
solution of the Einstein field equations with cosmological constant in $2+1$
dimensional space--times. We obtain that the solutions of both sectors can be
expressed analytically in terms of the metric functions of the original
anisotropic solutions instead of formal integral as in its $3+1$ counterpart.
As a particular example we study a regular black hole solution and we show
that, depending on the sign of the cosmological constant, the solutions
correspond to regular black holes violating the null energy condition or to a
non--regular black hole without exotic hair. The exotic/non--exotic and the
regular/non--regular black hole dualities are discussed.
