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, <br /><br /> gr[ae]y<br /><br /> 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:
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.
== See also ==
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[[Category:Computer Science]]
[[Category:languageLanguage]]