You might mean something like if you restrict the set of all valid inputs to N characters then the language is regular, but that's a much stronger condition tha

You might mean something like if you restrict the set of all valid inputs to N characters then the language is regular, but that's a much stronger condition than requiring the input to be finite. Furthermore, such a restriction makes the language non-recursive; it's effectively a trivial language. In standard automata theory, all inputs to automatons are finite.