Difference between revisions of "Regular expression"
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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 read from left to right and is processed one piece 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 | ||
| + | * '^' - begining of a string | ||
| + | * '$' - end of a string | ||
| + | |||
| + | There also exists a wide range of special character classes distinguished with a backslash (a small list): | ||
| + | * '\\' - a literal backslash | ||
| + | * '\.' - a literal period | ||
| + | * '\s' - any whitespace character | ||
| + | * '\b' - a word boundary | ||
| + | * '\d' - a digit | ||
| + | |||
| + | Regular expressions have also been extended by many languages, some of which extend them to the point where they are able to match a wider range of languages than is specified by a regular language. | ||
| + | |||
| + | == Examples == | ||
| + | |||
| + | * /b[ae]t/ - matches 'bat', 'bet', 'batik', 'abet' | ||
| + | * /Mrs?\. Smith/ - matches 'Mr. Smith', 'Mrs. Smith' | ||
| + | * /^a*b*$/ - 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 | ||
| + | |||
| + | == Limitations of regular expressions == | ||
| + | |||
| + | 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 == | ||
| + | |||
| + | * [[non-deterministic finite automaton]] | ||
| + | * [[deterministic finite automaton]] | ||
| + | |||
| + | == Further reading == | ||
| + | * http://www.regular-expressions.info/reference.html | ||
| + | |||
| + | [[Category:computer science]] | ||
| + | [[Category:language]] | ||
Revision as of 23:18, June 26, 2007
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 read from left to right and is processed one piece 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
- '^' - begining of a string
- '$' - end of a string
There also exists a wide range of special character classes distinguished with a backslash (a small list):
- '\\' - a literal backslash
- '\.' - a literal period
- '\s' - any whitespace character
- '\b' - a word boundary
- '\d' - a digit
Regular expressions have also been extended by many languages, some of which extend them to the point where they are able to match a wider range of languages than is specified by a regular language.
Examples
- /b[ae]t/ - matches 'bat', 'bet', 'batik', 'abet'
- /Mrs?\. Smith/ - matches 'Mr. Smith', 'Mrs. Smith'
- /^a*b*$/ - 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
Limitations of regular expressions
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.