Regular Expressions

In computing, a regular expression provides a concise and flexible means for "matching" (specifying and recognizing) strings of text, such as particular characters, words, or patterns of characters. Abbreviations for "regular expression" include "regex" and "regexp". The concept of regular expressions was first popularized by utilities provided by Unix distributions, in particular the editor ed and the filter grep. A regular expression is written in a formal language that can be interpreted by a regular expression processor, which is a program that either serves as a parser generator or examines text and identifies parts that match the provided specification. Historically, the concept of regular expressions is associated with Kleene's formalism of regular sets, introduced in the 1950s.

Here are examples of 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, "$100" or "$245.99").

These examples are simple. Specifications of great complexity can be conveyed by regular expressions.

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, integrate regular expressions into the syntax of the core language itself. Other programming languages like .NET languages, Java, and Python instead provide regular expressions through standard libraries. For yet other languages, such as Object Pascal and C and C++, non-core libraries are available (however, version C++11 provides regular expressions in its Standard Libraries).

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". Thus \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 the first is not a word character and the second is a word character. For instance, 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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