Simple vs non-simple loops on random regular graphs

In this note we solve the ``birthday problem’’ for loops on random regular graphs. Namely, for fixed $d\ge 3$, we prove that on a random $d$-regular graph with $n$ vertices, as $n$ approaches infinity, with high probability: (i) almost all primitive non-backtracking loops of length $k\prec \sqrt{n}$ are simple, i.e. do not self-intersect, (ii) almost all primitive non-backtracking loops of length $k\succ \sqrt{n}$ self-intersect.