Crispin Roven is a Wired Digital engineer. Regular expressions are used to recognize patterns within textual data. Their use has become so widespread that they appear in configuration files, mail filters, text editors, and any number of programming languages. Any application that acts on text may very well harness their power.
Regular expressions evaluate text data and return an answer of true or false.
That is, either the expression correctly describes the data, or it doesn't. What data the expression evaluates and what transpires after a successful match depends entirely on the application. We might substitute new text in the place of the text matched by a regular expression. We might save the matched text in a variable for later use. We might execute a new program when we see a correct match.