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This example uses some of the following regular expression metacharacters (sed supports the full range of regular expressions): The caret (^) matches the beginning of the line. The dollar sign ($) matches the end of the line. The asterisk (*) matches zero or more occurrences of the previous character.
Sed regular expressions, particularly those using the "s" operator, are much similar to Perl (sed is a predecessor to Perl). The default delimiter is "/", but any delimiter can be used; the default is s / regexp / replacement /, but s: regexp: replacement: is also a valid form. For example, to match a "pub" directory (as in the Perl example ...
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 .
Standard examples of data-driven languages are the text-processing languages sed and AWK, [1] and the document transformation language XSLT, where the data is a sequence of lines in an input stream – these are thus also known as line-oriented languages – and pattern matching is primarily done via regular expressions or line numbers.
section for sed. The first added example (labeled "trivial") is not really a "Hello, world!" example, since the result depends on bash. The obscure bash "here string" syntax will certainly be confusing for most readers. This is an article about sed, not bash. The example actually says little or nothing about sed, since cat would work equally well.
For simple, context-independent normalization, such as removing non-alphanumeric characters or diacritical marks, regular expressions would suffice.For example, the sed script sed ‑e "s/\s+/ /g" inputfile would normalize runs of whitespace characters into a single space.
Greed, in regular expression context, describes the number of characters which will be matched (often also stated as "consumed") by a variable length portion of a regular expression – a token or group followed by a quantifier, which specifies a number (or range of numbers) of tokens. If the portion of the regular expression is "greedy", it ...
Multiple quoting is particularly useful with regular expressions that contain usual delimiters such as quotes, as this avoids needing to escape them. An early example is sed, where in the substitution command s/regex/replacement/ the default slash / delimiters can be replaced by another character, as in s,regex,replacement,.