Chomsky hierarchy

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.
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. The concept was developed in the 1950s.