Chomsky hierarchy

The Chomsky hierarchy, in the fields of formal language theory, computer science, and linguistics, is a containment hierarchy of classes of formal grammars. This hierarchy of grammars was described by Noam Chomsky in 1956.
— Wikipedia

The Chomsky hierarchy is a concept of theoretical computer science. Noam Chomsky looked at grammars of regular language and classified them into four levels, numbered 0 to 3 (4 groups, according to their conditional complexity).

Group 0 consists of regular expressions without restriction, while groups 1 to 3 contain restrictions. Grammars in higher-numbered levels also satisfy the constraints of all levels below them.
— Simple English Wikipedia