Regular has a technical meaning here, that is, Chomsky’s grammar hierarchy. It’s where the “regular” in “regular expression” comes from. That said, I’m using it in an imprecise way here to mean “simpler to process.” (This is because regular languages are simpler to process than say, context-sensitive languages.)

Location of the type is about grammar complexity. Rust’s grammar plays into its type inference capabilities, and the pattern syntax. There’s an underlying uniformity with syntax elsewhere.