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Structure of a syntactically well-formed, although nonsensical, English sentence ("historical example from Chomsky 1957).

In "mathematics, "computer science, and "linguistics, a formal language is a "set of "strings of "symbols together with a set of rules that are specific to it.

The "alphabet of a formal language is the set of symbols, letters, or tokens from which the strings of the language may be formed.[1] The strings formed from this alphabet are called words, and the words that belong to a particular formal language are sometimes called well-formed words or "well-formed formulas. A formal language is often defined by means of a "formal grammar such as a "regular grammar or "context-free grammar, also called its "formation rule.

The field of formal language theory studies primarily the purely "syntactical aspects of such languages—that is, their internal structural patterns. Formal language theory sprang out of linguistics, as a way of understanding the syntactic regularities of "natural languages. In computer science, formal languages are used among others as the basis for defining the grammar of "programming languages and formalized versions of subsets of natural languages in which the words of the language represent concepts that are associated with particular meanings or "semantics. In "computational complexity theory, "decision problems are typically defined as formal languages, and "complexity classes are defined as the sets of the formal languages that can be "parsed by machines with limited computational power. In "logic and the "foundations of mathematics, formal languages are used to represent the syntax of "axiomatic systems, and "mathematical formalism is the philosophy that all of mathematics can be reduced to the syntactic manipulation of formal languages in this way.



The first formal language is thought to be the one used by "Gottlob Frege in his "Begriffsschrift (1879), literally meaning "concept writing", and which Frege described as a "formal language of pure thought."[2]

"Axel Thue's early "semi-Thue system, which can be used for rewriting strings, was influential on "formal grammars.

Words over an alphabet[edit]

An alphabet, in the context of formal languages, can be any "set, although it often makes sense to use an "alphabet in the usual sense of the word, or more generally a "character set such as "ASCII or "Unicode. The elements of an alphabet are called its letters. Alphabets may be "infinite;[3] however, most definitions in formal language theory specify finite alphabets, and most results only apply to them.

A word over an alphabet can be any finite sequence (i.e., "string) of letters. The set of all words over an alphabet Σ is usually denoted by Σ* (using the "Kleene star). The length of a word is the number of letters it is composed of. For any alphabet there is only one word of length 0, the empty word, which is often denoted by e, ε, λ or even Λ. By "concatenation one can combine two words to form a new word, whose length is the sum of the lengths of the original words. The result of concatenating a word with the empty word is the original word.

In some applications, especially in "logic, the alphabet is also known as the vocabulary and words are known as formulas or sentences; this breaks the letter/word metaphor and replaces it by a word/sentence metaphor.


A formal language L over an alphabet Σ is a "subset of Σ*, that is, a set of words over that alphabet. Sometimes the sets of words are grouped into expressions, whereas rules and constraints may be formulated for the creation of 'well-formed expressions'.

In computer science and mathematics, which do not usually deal with "natural languages, the adjective "formal" is often omitted as redundant.

While formal language theory usually concerns itself with formal languages that are described by some syntactical rules, the actual definition of the concept "formal language" is only as above: a (possibly infinite) set of finite-length strings composed from a given alphabet, no more nor less. In practice, there are many languages that can be described by rules, such as "regular languages or "context-free languages. The notion of a "formal grammar may be closer to the intuitive concept of a "language," one described by syntactic rules. By an abuse of the definition, a particular formal language is often thought of as being equipped with a formal grammar that describes it.


The following rules describe a formal language L over the alphabet Σ = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, +, = }:

Under these rules, the string "23+4=555" is in L, but the string "=234=+" is not. This formal language expresses "natural numbers, well-formed additions, and well-formed addition equalities, but it expresses only what they look like (their "syntax), not what they mean ("semantics). For instance, nowhere in these rules is there any indication that "0" means the number zero, or that "+" means addition.


For finite languages one can explicitly enumerate all well-formed words. For example, we can describe a language L as just L = {"a", "b", "ab", "cba"}. The "degenerate case of this construction is the empty language, which contains no words at all (L = ").

However, even over a finite (non-empty) alphabet such as Σ = {a, b} there are an infinite number of finite-length words that can potentially be expressed: "a", "abb", "ababba", "aaababbbbaab", …. Therefore, formal languages are typically infinite, and describing an infinite formal language is not as simple as writing L = {"a", "b", "ab", "cba"}. Here are some examples of formal languages:

Language-specification formalisms[edit]

Formal languages are used as tools in multiple disciplines. However, formal language theory rarely concerns itself with particular languages (except as examples), but is mainly concerned with the study of various types of formalisms to describe languages. For instance, a language can be given as

Typical questions asked about such formalisms include:

Surprisingly often, the answer to these decision problems is "it cannot be done at all", or "it is extremely expensive" (with a characterization of how expensive). Therefore, formal language theory is a major application area of "computability theory and "complexity theory. Formal languages may be classified in the "Chomsky hierarchy based on the expressive power of their generative grammar as well as the complexity of their recognizing "automaton. "Context-free grammars and "regular grammars provide a good compromise between expressivity and ease of "parsing, and are widely used in practical applications.

Operations on languages[edit]

Certain operations on languages are common. This includes the standard set operations, such as union, intersection, and complement. Another class of operation is the element-wise application of string operations.

Examples: suppose L1 and L2 are languages over some common alphabet.

Such "string operations are used to investigate "closure properties of classes of languages. A class of languages is closed under a particular operation when the operation, applied to languages in the class, always produces a language in the same class again. For instance, the "context-free languages are known to be closed under union, concatenation, and intersection with "regular languages, but not closed under intersection or complement. The theory of "trios and "abstract families of languages studies the most common closure properties of language families in their own right.[4]

Closure properties of language families ( Op where both and are in the language family given by the column). After Hopcroft and Ullman.
Operation Regular "DCFL "CFL "IND "CSL "recursive "RE
"Union Yes No Yes Yes Yes Yes Yes
"Intersection Yes No No No Yes Yes Yes
"Complement Yes Yes No No Yes Yes No
"Concatenation Yes No Yes Yes Yes Yes Yes
Kleene star Yes No Yes Yes Yes Yes Yes
Homomorphism Yes No Yes Yes No No Yes
e-free Homomorphism Yes No Yes Yes Yes Yes Yes
Substitution Yes No Yes Yes Yes No Yes
Inverse Homomorphism Yes Yes Yes Yes Yes Yes Yes
Reverse Yes No Yes Yes Yes Yes Yes
Intersection with a "regular language Yes Yes Yes Yes Yes Yes Yes


Programming languages[edit]

A compiler usually has two distinct components. A "lexical analyzer, generated by a tool like "lex, identifies the tokens of the programming language grammar, e.g. "identifiers or "keywords, which are themselves expressed in a simpler formal language, usually by means of "regular expressions. At the most basic conceptual level, a "parser, usually generated by a "parser generator like "yacc, attempts to decide if the source program is valid, that is if it belongs to the programming language for which the compiler was built.

Of course, compilers do more than just parse the source code — they usually translate it into some executable format. Because of this, a parser usually outputs more than a yes/no answer, typically an "abstract syntax tree. This is used by subsequent stages of the compiler to eventually generate an "executable containing "machine code that runs directly on the hardware, or some "intermediate code that requires a "virtual machine to execute.

Formal theories, systems and proofs[edit]

This diagram shows the "syntactic divisions within a "formal system. "Strings of symbols may be broadly divided into nonsense and "well-formed formulas. The set of well-formed formulas is divided into "theorems and non-theorems.

In "mathematical logic, a formal theory is a set of "sentences expressed in a formal language.

A formal system (also called a logical calculus, or a logical system) consists of a formal language together with a "deductive apparatus (also called a deductive system). The deductive apparatus may consist of a set of "transformation rules, which may be interpreted as valid rules of inference, or a set of "axioms, or have both. A formal system is used to "derive one expression from one or more other expressions. Although a formal language can be identified with its formulas, a formal system cannot be likewise identified by its theorems. Two formal systems and may have all the same theorems and yet differ in some significant proof-theoretic way (a formula A may be a syntactic consequence of a formula B in one but not another for instance).

A formal proof or derivation is a finite sequence of well-formed formulas (which may be interpreted as "propositions) each of which is an axiom or follows from the preceding formulas in the sequence by a "rule of inference. The last sentence in the sequence is a theorem of a formal system. Formal proofs are useful because their theorems can be interpreted as true propositions.

Interpretations and models[edit]

Formal languages are entirely syntactic in nature but may be given "semantics that give meaning to the elements of the language. For instance, in mathematical "logic, the set of possible formulas of a particular logic is a formal language, and an "interpretation assigns a meaning to each of the formulas—usually, a "truth value.

The study of interpretations of formal languages is called "formal semantics. In mathematical logic, this is often done in terms of "model theory. In model theory, the terms that occur in a formula are interpreted as objects within "mathematical structures, and fixed compositional interpretation rules determine how the truth value of the formula can be derived from the interpretation of its terms; a model for a formula is an interpretation of terms such that the formula becomes true.

See also[edit]


Citation footnotes[edit]

  1. ^ See e.g. Reghizzi, Stefano Crespi (2009), Formal Languages and Compilation, Texts in Computer Science, Springer, p. 8, "ISBN "9781848820500, An alphabet is a finite set .
  2. ^ Martin Davis (1995). "Influences of Mathematical Logic on Computer Science". In Rolf Herken. The universal Turing machine: a half-century survey. Springer. p. 290. "ISBN "978-3-211-82637-9. 
  3. ^ For example, "first-order logic is often expressed using an alphabet that, besides symbols such as ∧, ¬, ∀ and parentheses, contains infinitely many elements x0x1x2, … that play the role of variables.
  4. ^ Hopcroft & Ullman (1979), Chapter 11: Closure properties of families of languages.

General references[edit]

External links[edit]

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