regular expression

Also found in: Dictionary, Thesaurus, Medical, Legal, Acronyms, Wikipedia.

regular expression

[′reg·yə·lər ik′spresh·ən]
(computer science)
A formal description of a language acceptable by a finite automaton or for the behavior of a sequential switching circuit.

regular expression

(text, operating system)
(regexp, RE) One of the wild card patterns used by Perl and other languages, following Unix utilities such as grep, sed, and awk and editors such as vi and Emacs. Regular expressions use conventions similar to but more elaborate than those described under glob. A regular expression is a sequence of characters with the following meanings:

An ordinary character (not one of the special characters discussed below) matches that character.

A backslash (\) followed by any special character matches the special character itself. The special characters are:

"." matches any character except NEWLINE; "RE*" (where the "*" is called the "Kleene star") matches zero or more occurrences of RE. If there is any choice, the longest leftmost matching string is chosen, in most regexp flavours.

"^" at the beginning of an RE matches the start of a line and "$" at the end of an RE matches the end of a line.

[string] matches any one character in that string. If the first character of the string is a "^" it matches any character except the remaining characters in the string (and also usually excluding NEWLINE). "-" may be used to indicate a range of consecutive ASCII characters.

\( RE \) matches whatever RE matches and \n, where n is a digit, matches whatever was matched by the RE between the nth \( and its corresponding \) earlier in the same RE. Many flavours use ( RE ) used instead of \( RE \).

The concatenation of REs is a RE that matches the concatenation of the strings matched by each RE. RE1 | RE2 matches whatever RE1 or RE2 matches.

\< matches the beginning of a word and \> matches the end of a word. In many flavours of regexp, \> and \< are replaced by "\b", the special character for "word boundary".

RE\m\ matches m occurences of RE. RE\m,\ matches m or more occurences of RE. RE\m,n\ matches between m and n occurences.

The exact details of how regexp will work in a given application vary greatly from flavour to flavour. A comprehensive survey of regexp flavours is found in Friedl 1997 (see below).

[Jeffrey E.F. Friedl, "Mastering Regular Expressions, O'Reilly, 1997].

regular expression

Any description of a pattern composed from combinations of symbols and the three operators:

Concatenation - pattern A concatenated with B matches a match for A followed by a match for B.

Or - pattern A-or-B matches either a match for A or a match for B.

Closure - zero or more matches for a pattern.

The earliest form of regular expressions (and the term itself) were invented by mathematician Stephen Cole Kleene in the mid-1950s, as a notation to easily manipulate "regular sets", formal descriptions of the behaviour of finite state machines, in regular algebra.

[S.C. Kleene, "Representation of events in nerve nets and finite automata", 1956, Automata Studies. Princeton].

[J.H. Conway, "Regular algebra and finite machines", 1971, Eds Chapman & Hall].

[Sedgewick, "Algorithms in C", page 294].

regular expression

In programming, a set of symbols used to search for occurrences of text or to search and replace text. The simplest regular expressions are DOS/Windows wildcards; for example, *.html refers to all file names with HTML extensions. However, regular expression functions are available in many programming languages that allow for complex pattern matching and text manipulation. For example, replacing specific text within a sentence when the sentence begins with a certain word can be performed with a regular expression. See expression.
References in periodicals archive ?
Fast and memory-efficient regular expression matching for deep packet inspection.
The star height of a regular language L, denoted by h(L), is defined as the minimum nesting depth of stars in any regular expression describing L.
Figure 1 shows the main components of a typical regular expression search engine.
We might create an FSA for the regular expression a(b|c *) with the following code:
TIMEX2s are matched against regular expression patterns that extract the necessary information for normalization.
A realization of this model represents the educational software GANIFA on the theory of generating finite automata from regular expressions.
We use [Sigma] as a regular expression denoting any symbol in the alphabet.
If the regular expression is not too large and n is large, the effective running time is O(nk).
2 regular expression notation, however, there is the added benefit that they are all executed simultaneously.
SpamAI uses Regular Expression (Regex) rules, and users can access and alter these rules.
It performs Layer 2-4 classification and is optimized for full Layer-7 content inspection and regular expression matching at OC-48 line rates in web switches, load balancers, multi-service switches, IP services gateways, cable modem termination systems (CMTS), edge routers, protocol analyzers, and fixed broadband wireless access stations.
It was a breakthrough in comparison with standard Event Viewer or similar software, but on the other hand, it was not too easy to build correct regular expression - our customers often send their expressions to us for verification.