We consider rewriting of a regular language with a left-linear term rewriting system. We show two completeness theorems on equational tree automata completion. The first one shows that, if the set of reachable terms is regular, then completion can compute it. This was known to be true for some term rewriting system classes preserving regularity, but was still an open question in the general case. The proof is not constructive because it depends on the regularity of the set of reachable terms, which is undecidable. The second theorem states that, if there exists a regular over-approximation of the set of reachable terms then completion can compute it (or safely under-approximate it). To carry out those proofs we generalize and improve two results of completion: the Termination and the Upper-Bound theorems. Those theoretical results provide an algorithmic way to safely explore regular approximations with completion. This has been implemented in Timbuk and used to verify safety properties, automatically and efficiently, on first-order and higher-order functional programs.