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clean up
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]].
To match any of 2 or more characters, enclose them in square brackets. For example,
gr[ae]y
will match ''gray'' or ''grey''.
A regular expression is matched from left to right and is processed one [[token]] at a time. Certain characters have special meanings within the description of a regular language:
* '*' - the previous construction is matched 0 or more times
* '+' - the previous construction is matched 1 or more times
* '?' - the previous construction is matched 0 or 1 times
* '(...)' - the contents between the parentheses is considered a single construction
* '...|...' - the construction on the left of the '|' may be matched or the construction on the right of the '|' may be matched.
* '<nowiki>[...]</nowiki>' - match one of the characters contained within the square braces.
* '.' - match any single character
* '^' - beginning of a string
* '$' - end of a string
There also exists a wide range of special character classes distinguished with a backslash (a small list):
* '\\' - a literal backslash
* '\.' - a literal period
Regular expressions have also been extended by many languages, some of which extend them to the point where they are able to match a wider range of languages than is specified by a regular language.
== Examples ==
* /Mrs?\. Smith/ - matches 'Mr. Smith', 'Mrs. Smith'
* /^a*b*$/ - matches 'a', 'ab', 'b', 'aaaab', 'abbbbb', 'aaabbb', ''
* /[1-9][0-9]*(\.0|([1-9][0-9]*))+/ - matches version numbers: 1, 1.0, 2.3, 103.4, 42.5.6.8.9, 7.0.8.9
== Formal definition and Limitations ==
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>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 ==
* [[non-deterministic finite automaton]]
* [[deterministic finite automaton]]
== Further reading ==
* http://www.regular-expressions.info/reference.html
<references/>
[[Category:Computer Science]]
[[Category:Language]]