According to the rules of syntax.

(relatively rare) syntax

Of, related to or connected with syntax. / Containing morphemes that are combined in the same order as they would be if they were separate words e.g. greenfinch

syntactic, related to syntax

(computing) Additions to the syntax of a computer language that make code easier for humans to read or write, but that do not change the functionality or expressiveness of the language.

(computing) Additions to a computer language that make it harder to write bad code.