Regular description for $\{a^n\mid n\in D\}$, where $D$ is the set of distances of paths (with allowed repetition of nodes in the path)
from node $0$ to node $5$ in the directed graph with edges labelled with lengths whose set of nodes is $\{0,1,2,3,4,5\}$ and whose set of edges is
$\{0\xrightarrow{7}1,\;0\xrightarrow{4}2,\;1\xrightarrow{7}1,\;1\xrightarrow{9}3,\;2\xrightarrow{4}4,\;2\xrightarrow{3}5,\;3\xrightarrow{4}0,\;4\xrightarrow{5}2,\;4\xrightarrow{3}3,\;5\xrightarrow{9}4\}$