Some exact results for regular Tur\'an problems

As a variant of the famous Tur\'an problem, we study $\mathrm{rex}(n,F)$, the maximum number of edges that an $n$-vertex regular graph can have without containing a copy of $F$. We determine $\mathrm{rex}(n,K_{r+1})$ for all pairs of integers $r$ and large enough $n$. For every tree $T$, we determine $\mathrm{rex}(n,T)$ for every $n$ large enough.