I think "simple string literals" undersells it. I think that description works for engines like RE2 or Go's regex engine, but not Hyperscan or Rust regex. (And I would put Hyperscan in another category than even Rust regex.) Granted, it is arguably difficult to be succinct here since it's a heuristic with difficult-to-predict failure points. But something like: "patterns from which a small number of string literals can be extracted."