Difference between revisions of "Regular expression"
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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''. | ||
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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 | + | 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 == | ||
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<references/> | <references/> | ||
[[Category:Computer Science]] | [[Category:Computer Science]] | ||
| − | [[Category: | + | [[Category:Language]] |
Latest revision as of 15:04, June 23, 2016
A regular expression is used in computer software to define a sequence of characters.[1]
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.
- '[...]' - 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 anban. 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
Further reading
- ↑ "A regular expression, or regex for short, is a pattern describing a certain amount of text." Regular Expression Quick Start