Which type of grammar is most restricted according to the Chomsky hierarchy?

Regular grammar is considered the most restricted type of grammar according to the Chomsky hierarchy. This hierarchy classifies grammars into four levels based on their generative power. Regular grammars, which can be represented by regular languages, are defined by very simple production rules; they can only create linear patterns and are represented by finite automata. This limitation means they cannot handle nested structures or dependencies between elements, which are possible in more complex grammars.

In contrast, context-free grammars are more powerful, capable of describing a wider range of languages, particularly those requiring hierarchical structures, such as parentheses in mathematical expressions. Context-sensitive grammars allow for even more complexity and are capable of generating languages that require context to determine their structure. Unrestricted grammars have no restrictions and can generate any language that a Turing machine can recognize, making them the most powerful type in this hierarchy.

The simplicity and restrictions of regular grammar make it suitable for specific applications, such as lexical analysis in programming languages or defining simple patterns in text processing. However, its limitations are precisely what establishes it as the most restricted option within the Chomsky hierarchy.