Let $\textbf{Grph}$ be the category whose objects are graphs $G = (V,E)$ such that $V$ is a set and $E \subseteq \mathcal{P}_2(V) := \{\{a,b\} \subseteq V: a\neq b\}$. We sometimes write $E(G)$ for $E$. The morphisms are maps $f:G\to H$ such that whenever $\{v,w\}\in E(G)$ then $\{f(v),f(w)\}\in E(H)$.
Do the notions of regular and extremal monomorphisms coincide in $\textbf{Grph}$?
The answer is yes.
It is clear that the extremal monomorphisms $(V_0, E_0) \to (V_1, E_1)$ are precisely the full graph embeddings, i.e. maps $f : V_0 \to V_1$ such that $f$ is injective and $(a, b) \in E_0$ if and only if $(f (a), f (b)) \in E_1$. It is also clear that regular monomorphisms are graph embeddings, so it remains to be shown that every full graph embedding is a regular monomorphism. But this is straightforward: given a graph $(V_1, E_1)$ and a full subgraph $(V_0, E_0)$, we can easily construct a graph $(V_2, E_2)$ containing two isomorphic copies of $(V_1, E_1)$ as full subgraphs whose intersection is isomorphic to $(V_0, E_0)$; then $(V_0, E_0)$ is the equaliser of the two embeddings $(V_1, E_1) \to (V_2, E_2)$.
