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A regular expression (shortened as regex or regexp), [1] sometimes referred to as rational expression, [2] [3] is a sequence of characters that specifies a match pattern in text. Usually such patterns are used by string-searching algorithms for "find" or "find and replace" operations on strings , or for input validation .
An actual backslash is produced by a double backslash \\. Regular expression languages used it the same way, changing subsequent literal characters into metacharacters and vice versa. For instance \||b searches for either '|' or 'b', the first bar is escaped and searched for, the second is not escaped and acts as an "or".
The slash is also used as the default regular expression delimiter, so to be used literally in the expression, it must be escaped with a backslash \, leading to frequent escaped slashes represented as \/. If doubled, as in URLs, this yields \/\/ for an escaped //.
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 ...
Most commonly this is backslash; in addition to other characters, a key point is that backslash itself can be encoded as a double backslash \\ and for delimited strings the delimiter itself can be encoded by escaping, say by \" for ". A regular expression for such escaped strings can be given as follows, as found in the ANSI C specification: [1 ...
/ 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.
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 ]
Since UNCs start with two backslashes, and the backslash is also used for string escaping and in regular expressions, this can result in extreme cases of leaning toothpick syndrome: an escaped string for a regular expression matching a UNC begins with 8 backslashes – \\\\\ – because the string and regular expression both require escaping.