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A '''regular expression''' is used in computer software to define a sequence of characters.<ref>"A regular expression, or regex for short, is a pattern describing a certain amount of text." [http://www.regular-expressions.info/quickstart.html Regular Expression Quick Start] </ref>
A '''regular expression''' is used in computer software to define a sequence of characters.<ref>"A regular expression, or regex for short, is a pattern describing a certain amount of text." [http://www.regular-expressions.info/quickstart.html Regular Expression Quick Start]</ref>
In general, any character will match itself, but there are a dozen special characters, including the [[escape character]].
In general, any character will match itself, but there are a dozen special characters, including the [[escape character]].
To match any of 2 or more characters, enclose them in square brackets. For example,
To match any of 2 or more characters, enclose them in square brackets. For example,
gr[ae]y
will match ''gray'' or ''grey''.
will match ''gray'' or ''grey''.
Regular expressions have also been extended by many languages, some of which extend them to the point where they are able to match a wider range of languages than is specified by a regular language.
Regular expressions have also been extended by many languages, some of which extend them to the point where they are able to match a wider range of languages than is specified by a regular language.
== Examples ==
== Examples ==
A '''regular expression''' is particular instance of a non-[[deterministic]] [[finite]] state [[automaton]]. Regular expressions are a [[type-3 grammar]] in the [[Chomsky hierarchy]] of [[language]].
A '''regular expression''' is particular instance of a non-[[deterministic]] [[finite]] state [[automaton]]. Regular expressions are a [[type-3 grammar]] in the [[Chomsky hierarchy]] of [[language]].
A regular expression is not able to count. This is because there is a finite number of states. Consider the language that is specified by a<sup><small>n</small></sup>ba<sup><small>n</small></sup>. Examples of this language include b, aba, aabaa, aaabaaa, etc... A regular expression - being a [[finite state automaton]] itself - has a finite number of states that it can be in. If there a point at which the state loops back on itself it is no longer able to match that language.
A regular expression is not able to count. This is because there is a finite number of states. Consider the language that is specified by a<sup>n</sup>ba<sup>n</sup>. Examples of this language include b, aba, aabaa, aaabaaa, etc... A regular expression - being a [[finite state automaton]] itself - has a finite number of states that it can be in. If there a point at which the state loops back on itself it is no longer able to match that language.
== See also ==
== See also ==
== Further reading ==
== Further reading ==
* http://www.regular-expressions.info/reference.html
* http://www.regular-expressions.info/reference.html
<references/>
[[Category:Computer Science]]
[[Category:Computer Science]]
[[Category:language]]
[[Category:Language]]