Regular expression

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In computing, regular expressions, also referred to as regex or regexp, provide a concise and flexible means for matching strings of text, such as particular characters, words, or patterns of characters. A regular expression is written in a formal language that can be interpreted by a regular expression processor, a program that either serves as a parser generator or examines text and identifies parts that match the provided specification.

The following examples illustrate a few specifications that could be expressed in a regular expression:

  • The sequence of characters "car" appearing consecutively in any context, such as in "car", "cartoon", or "bicarbonate"
  • The sequence of characters "car" occurring in that order with other characters between them, such as in "Icelander" or "chandler"
  • The word "car" when it appears as an isolated word
  • The word "car" when preceded by the word "blue" or "red"
  • The word "car" when not preceded by the word "motor"
  • A dollar sign immediately followed by one or more digits, and then optionally a period and exactly two more digits (for example, "$10" or "$245.99").

Regular expressions can be much more complex than these examples.

Regular expressions are used by many text editors, utilities, and programming languages to search and manipulate text based on patterns. Some of these languages, including Perl, Ruby, Awk, and Tcl, have fully integrated regular expressions into the syntax of the core language itself. Others like C, C++,.NET, Java, and Python instead provide access to regular expressions only through libraries. Utilities provided by Unix distributions—including the editor ed and the filter grep—were the first to popularize the concept of regular expressions.

As an example of the syntax, the regular expression \bex can be used to search for all instances of the string "ex" that occur after "word boundaries" (signified by the \b). In layman's terms, \bex will find the matching string "ex" in two possible locations, (1) at the beginning of words, and (2) between two characters in a string, where one is a word character and the other is not a word character. Thus, in the string "Texts for experts", \bex matches the "ex" in "experts" but not in "Texts" (because the "ex" occurs inside a word and not immediately after a word boundary).

Many modern computing systems provide wildcard characters in matching filenames from a file system. This is a core capability of many command-line shells and is also known as globbing. Wildcards differ from regular expressions in generally expressing only limited forms of patterns.

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