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In computing, regular expressions provide a concise and flexible means for identifying strings of text of interest, such as particular characters, words, or patterns of characters. A regular expression (often shortened to regex or regexp) 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.
Regular expressions can be defined in terms of formal language theory. Regular expressions consist of constants and operators that denote sets of strings and operations over these sets, respectively.

The following examples illustrate a few specifications that could be expressed in a regular expression:
the sequence of characters “car” in any context, such as “car”, “cartoon”, or “bicarbonate”
the word “car” when it appears as an isolated word
the word “car” when preceded by the word “blue” or “red”
a dollar sign immediately followed by one or more digits, and then optionally a period and
Uses of regular expressions
Regular expressions are useful in the production of syntax highlighting systems, data validation, and many other tasks.
While regular expressions would be useful on search engines such as Google, Live Search or Yahoo! Search, processing them across the entire database could consume excessive computer resources depending on the complexity and design of the regex.

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