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Chomsky Hierarchy in Theory of Computation

Last Updated on September 22, 2023 by Mayank Dham

The Chomsky Hierarchy in Theory of Computation, named after the renowned linguist and cognitive scientist Noam Chomsky, is a fundamental concept in the field of theoretical computer science. It classifies formal grammars and languages into four distinct levels, each with increasing expressive power. This hierarchy provides valuable insights into the capabilities and limitations of computational models, shedding light on the nature of computation itself.

Introduction to Formal Grammars and Languages

Formal grammars serve as a foundation for describing the structure of languages in a precise and systematic manner. They consist of a set of rules that define how strings of symbols can be generated. In this context, language refers to a set of strings composed from a given alphabet, which is a finite set of symbols.

The Four Levels of the Chomsky Hierarchy in TOC

The Chomsky Hierarchy in TOC is composed of four levels, organized in increasing order of complexity and generative power

a. Type 3: Regular Languages
At the lowest level of the hierarchy, we have regular languages. These languages can be described using regular expressions or recognized by finite automata. Regular languages have simple rules for pattern recognition and are suited for tasks like lexical analysis in programming languages.

b. Type 2: Context-Free Languages
Context-free languages are one level higher in the hierarchy. They can be described using context-free grammars. These grammars consist of rules that define how non-terminal symbols can be replaced by sequences of terminal and non-terminal symbols. Context-free languages are commonly used to define the syntax of programming languages and are recognized by pushdown automata.

c. Type 1: Context-Sensitive Languages
Context-sensitive languages extend the expressive power of context-free languages by allowing rules that take into account the context in which symbols appear. This level of the hierarchy is recognized by linear-bounded automata, which have limited tape space for computation. Context-sensitive languages capture certain linguistic constructs that context-free grammars cannot.

d. Type 0: Recursively Enumerable Languages
At the highest level of the Chomsky Hierarchy in TOC, we have recursively enumerable languages. These languages can be generated by unrestricted grammars or recognized by Turing machines. Recursively enumerable languages encompass all possible languages that can be computed, making them the most powerful class in the hierarchy. However, this power comes at the cost of undecidability and uncomputability for many problems.

Implications and Significance of Chomsky Hierarchy in Theory of Computation

The Chomsky Hierarchy in TOC has profound implications for the theory of computation and linguistics:

a. Computational Complexity: As we move up the hierarchy, the complexity of recognizing languages increases. Regular languages can be recognized in linear time, context-free languages in polynomial time, and context-sensitive languages in non-deterministic polynomial time. Recursively enumerable languages can take an infinite amount of time for recognition, depending on the specific Turing machine configuration.

b. Language Classification: The hierarchy allows us to classify languages based on their generative power. This classification is not only relevant for computer science but also for linguistics. Natural languages, such as English or Spanish, are generally more expressive than the formal languages described by regular or context-free grammars.

c. Limits of Computation: The Chomsky Hierarchy in TOC establishes the limits of what can be computed within different computational models. For example, certain problems are inherently beyond the scope of context-free grammars or even Turing machines. This understanding has direct applications in algorithm design, complexity analysis, and the study of undecidability.


The Chomsky Hierarchy in TOC stands as a fundamental framework for understanding the capabilities and limitations of computational models. Its division of formal grammars and languages into four distinct levels provides a structured perspective on the complexity of language recognition and generation. From regular expressions used in text search to the complexity of programming language syntax, the Chomsky Hierarchy in TOC’s influence is pervasive and enlightening, shaping our understanding of computation’s boundaries and potentials.

Frequently Asked Questions (FAQs)
Here are some frequently asked questions on the Chomsky Hierarchy in TOC.

Q1. What is the Chomsky Hierarchy?
The Chomsky Hierarchy is a classification system that categorizes formal grammars and languages into four levels based on their complexity and generative power. These levels range from regular languages (simple patterns) to recursively enumerable languages (infinite computations), providing insights into the capabilities and limitations of computational models.

Q2. How does the Chomsky Hierarchy relate to programming languages?
The Chomsky Hierarchy is relevant to programming languages as it helps define and understand their syntax. Context free languages, the second level of the hierarchy, are commonly used to describe programming language grammars. This aids in developing compilers, interpreters, and syntax analyzers, ensuring accurate language parsing and translation.

Q3. Can you provide real world examples of languages from each hierarchy level?
Here are some of the real world examples:

  • Regular Language: The set of all strings containing an even number of ‘0’s and ‘1’s.
  • Context Free Language: The language of well formed arithmetic expressions.
  • Context language: Natural languages like English or Spanish, which have complex grammatical rules influenced by context.
  • Recursively Enumerable Language: The set of all valid Turing machine descriptions that halt on input ‘0’.

Q4. What’s the significance of the Chomsky Hierarchy in linguistics?
The Chomsky Hierarchy has had a profound impact on linguistics. It showcases the formal hierarchy of language complexity, helping linguists understand the structural variations between different languages. Moreover, it clarifies the inherent limitations of formal languages in capturing the full complexity of natural languages like English or French.

Q5. Are there practical implications of the Chomsky Hierarchy beyond theory?
The hierarchy’s insights have practical implications across computer science. It aids in designing efficient algorithms for parsing and recognizing languages, ensuring compilers and interpreters work correctly. Additionally, the hierarchy establishes limits of computation, highlighting problems that are fundamentally unsolvable by certain computational models, which is crucial in understanding algorithmic complexity and the boundaries of computability.

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