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A metacharacter is a character that has a special meaning to a computer program, such as a shell interpreter or a regular expression (regex) engine.. In POSIX extended regular expressions, there are 14 metacharacters that must be escaped — preceded by a backslash (\) — in order to drop their special meaning and be treated literally inside an expression: opening and closing square brackets ...
It can signify exponentiation, the bitwise XOR operator, string concatenation [citation needed], and control characters in caret notation, among other uses. In regular expressions, the caret is used to match the beginning of a string or line; if it begins a character class, then the inverse of the class is to be matched.
/ is special because it indicates the end of the regex. For example, insource:/yes/no/ is treated the same as insource:/yes/ no (because the keyword search for no/ ignores punctuation). The / character must be backslash-escaped everywhere it appears inside a regex – even inside square brackets or quotation marks.. matches any single character.
Regular Expression Flavor Comparison – Detailed comparison of the most popular regular expression flavors; Regexp Syntax Summary; Online Regular Expression Testing – with support for Java, JavaScript, .Net, PHP, Python and Ruby; Implementing Regular Expressions – series of articles by Russ Cox, author of RE2; Regular Expression Engines
Brackets: Angle bracket, Parenthesis • Bullet: Interpunct ‸ ⁁ ⎀ Caret (proofreading) Caret (computing) (^) Chevron (non-Unicode name) Caret, Circumflex, Guillemet, Hacek, Glossary of mathematical symbols ^ Circumflex (symbol) Caret (The freestanding circumflex symbol is known as a caret in computing and mathematics) Circumflex ...
Every regular expression can be written solely in terms of the Kleene star and set unions over finite words. This is a surprisingly difficult problem. As simple as the regular expressions are, there is no method to systematically rewrite them to some normal form. The lack of axiom in the past led to the star height problem.
In SQL, wildcard characters can be used in LIKE expressions; the percent sign % matches zero or more characters, and underscore _ a single character. Transact-SQL also supports square brackets ([and ]) to list sets and ranges of characters to match, a leading caret ^ negates the set and matches only a character not within the list.
Perl Compatible Regular Expressions (PCRE) is a library written in C, which implements a regular expression engine, inspired by the capabilities of the Perl programming language. Philip Hazel started writing PCRE in summer 1997. [ 3 ]