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# Chomsky Hierarchy in Theory of Computation

• Difficulty Level : Easy
• Last Updated : 29 Aug, 2022

According to Chomsky hierarchy, grammar is divided into 4 types as follows:

1. Type 0 is known as unrestricted grammar.
2. Type 1 is known as context-sensitive grammar.
3. Type 2 is known as a context-free grammar.
4. Type 3 Regular Grammar.

Type 0: Unrestricted Grammar:

Type-0 grammars include all formal grammar. Type 0 grammar languages are recognized by turing machine. These languages are also known as the Recursively Enumerable languages.

Grammar Production in the form of    where

\alpha         is ( V + T)* V ( V + T)*
V : Variables
T : Terminals. 
is ( V + T )*.

In type 0 there must be at least one variable on the Left side of production.

For example:

Sab --> ba
A --> S

Here, Variables are S, A, and Terminals a, b.

Type 1: Context-Sensitive Grammar

Type-1 grammars generate context-sensitive languages. The language generated by the grammar is recognized by the Linear Bound Automata

In Type 1

• First of all Type 1 grammar should be Type 0.
• Grammar Production in the form of

|\alpha |<=|\beta |

That is the count of symbol in is less than or equal to

Also β  ∈ (V + T)+

i.e. β can not be ε

For Example:

S --> AB
AB --> abc
B --> b  

Type 2: Context-Free Grammar: Type-2 grammars generate context-free languages. The language generated by the grammar is recognized by a Pushdown automata.  In Type 2:

• First of all, it should be Type 1.
• The left-hand side of production can have only one variable and there is no restriction on

|\alpha         | = 1.

For example:

S --> AB
A --> a
B --> b 

Type 3: Regular Grammar: Type-3 grammars generate regular languages. These languages are exactly all languages that can be accepted by a finite-state automaton. Type 3 is the most restricted form of grammar.

Type 3 should be in the given form only :

V --> VT / T          (left-regular grammar)
(or)
V --> TV /T          (right-regular grammar)

For example:

S --> a

The above form is called strictly regular grammar.

There is another form of regular grammar called extended regular grammar. In this form:

V --> VT* / T*.        (extended left-regular grammar)
(or)
V --> T*V /T*          (extended right-regular grammar)

For example :

S --> ab.