Regular Expression

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. Regular expressions (abbreviated as regex or regexp, with plural forms regexes, regexps, or regexen) are 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" 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 exactly two more digits 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. For example, Perl, Ruby and Tcl have a powerful regular expression engine built directly into their syntax. Several utilities provided by Unix distributions—including the editor ed and the filter grep—were the first to popularize the concept of regular expression A regular expression is a pattern describing a certain amount of text. Their name comes from the mathematical theory on which they are based.