regular expression

A regular expressionMathworldPlanetmath is a particular meta-syntax for specifying regular grammars, which has many useful applications.

While variations abound, fundamentally a regular expression consists of the following pieces:

  • Parentheses can be used for grouping and nesting, and must contain a fully-formed regular expression.

  • The | symbol can be used for denoting alternatives. Some specifications do not provide nesting or alternatives.

  • There are also a number of postfix operators. The ? operator means that the preceding element can either be present or non-present, and corresponds to a rule of the form AB|λ.

  • The * operator means that the preceding element can be present zero or more times, and corresponds to a rule of the form ABA|λ.

  • The + operator means that the preceding element can be present one or more times, and corresponds to a rule of the form ABA|B.

Note that while these rules are not immediately in regularPlanetmathPlanetmathPlanetmathPlanetmathPlanetmathPlanetmath form, they can be transformed so that they are.

Formally, let S={,,*,(,)} and Σ an alphabet disjoint from S. Consider the languagePlanetmathPlanetmath L(Σ) over ΣS specified below

  1. 1.


  2. 2.

    aL(Σ) for each aΣ,

  3. 3.

    if uL(Σ), then u*L(Σ),

  4. 4.

    if u1,u2L(Σ), then (u1u2) and (u1u2) are both in L(Σ), and

  5. 5.

    among all languages over ΣS satisfying conditions 1-4, L(Σ) is the smallest.

Then any element uL(Σ) is called a regular expression over Σ.

Here is an example of a regular expression that specifies a grammarMathworldPlanetmath that generates the binary representation of all multiples of 3 (and only multiples of 3).


This specifies the context-free grammar (in BNF):

S ::= AB
A ::= CD
B ::= 0B|λ
C ::= 0C|λ
D ::= 1E𝟷
E ::= FE|λ
F ::= 0G𝟶
G ::= 1G|λ

A little further work is required to transform this grammar into an acceptable form for regular grammars, but it can be shown that this grammar (and any grammar specified by a regular expression) is equivalentMathworldPlanetmathPlanetmathPlanetmathPlanetmathPlanetmath to some regular grammar.

One can understand the language described by a regular expression in another way, by viewing the regular expression operators as shorthand for various set-theoretic operationsMathworldPlanetmath. Formally, the language L(u) over Σ associated with a regular expression u over Σ is inductively defined as follows:

  • L()=,

  • L(a)={a} whenever aΣ,

  • L(u*)=L(u)*, where the * on the right side is the Kleene star operation on sets,

  • L((u1u2))=L(u1)L(u2), where the right side denotes the concatenationMathworldPlanetmath of two sets, and

  • L((u1u2))=L(u1)L(u2), where on the right side is the union operation on sets.

A language L over Σ is regular iff there is a regular expression u over Σ such that L=L(u).

With this interpretationMathworldPlanetmathPlanetmath, it is quite straightforward to design a non-deterministic finite automaton that recognizes the language described by a regular expression. Of course, for computer implementations, one must transform this into a deterministic finite automaton, but there are various algorithmsMathworldPlanetmath for doing this efficiently. This process, production of a non-deterministic automaton and conversion to an equivalent deterministic automaton is approximately what is done in software packages implementing regular expression searching. In fact, most such packages implement operations impossible for a finite automaton, such as requiring a later part of the string to be the same as a previous part (the language {AnBAn for n0} is not regular but can be matched by most “regular expression” software; such capabilities are called “extended regular expressions”. None of these systems are powerful enough to recognize the language of balanced parentheses.

Regular expressions have many applications. Quite often they are used for powerful string matching and substitution features in many text editors and programming languages.

Title regular expression
Canonical name RegularExpression
Date of creation 2013-03-22 12:26:56
Last modified on 2013-03-22 12:26:56
Owner CWoo (3771)
Last modified by CWoo (3771)
Numerical id 17
Author CWoo (3771)
Entry type Definition
Classification msc 20M35
Classification msc 68Q70
Related topic RegularLanguage
Related topic KleeneStar
Related topic KleeneAlgebra