On the regular Sylow $p$-subgroups of Chevalley groups over $\mathbb Z_{p^m}$

We prove that a Sylow $p$-subgroup of the general linear group of dimension $n$ over the residue ring modulo $p^m$ is regular for $n^2<p$ and powerful if and only if $n=2$ and $m=1$. We obtain similar results for the Sylow $p$-subgroups of normal types over the same ring.