  Let $X$ be an arbitrary set and let $T(X)$ denote the full transformation
monoid on $X$. We prove that an element of $T(X)$ is unit-regular if and only
if it is semi-balanced. For infinite $X$, we discuss regularity of the
submonoid of $T(X)$ consisting of all injective (resp. surjective)
transformations. For a partition $\mathcal{P}$ of $X$, we characterize
unit-regular elements in the monoid $T(X, \mathcal{P})$, under composition,
defined as \[T(X, \mathcal{P}) = \{f\in T(X)\mid (\forall X_i \in \mathcal{P})
(\exists X_j \in \mathcal{P})\; X_i f \subseteq X_j\}.\] We also characterize
(unit-)regular elements in various known submonoids of $T(X, \mathcal{P})$.
