  The derived algebra of a symmetrizible Kac-Moody algebra $\lie g$ is
generated (as a Lie algebra) by its root spaces corresponding to real roots. In
this paper, we address the natural reverse question: given any subset of real
root vectors, is the Lie subalgebra of $\lie g$ generated by these again the
derived algebra of a Kac-Moody algebra? We call such Lie subalgebras root
generated, give an affirmative answer to the above question and show that there
is a one-to-one correspondence between them, real closed subroot systems and
$\pi$-systems contained in the positive system of $\lie g$. Finally, we apply
these identifications to all untwised affine types in order to classify
symmetric regular subalgebras first introduced by Dynkin in the
finite-dimensional setting. We show that any root generated subalgebra
associated to a maximal real closed subroot system can be embedded into a
unique maximal symmetric regular subalgebra.
