Regular Expression - technical definition

In programming, a set of symbols used to search for occurrences of text or to search and replace text. The simplest regular expressions are DOS/Windows wildcards; for example, *.html refers to all file names with HTML extensions. However, regular expression functions are available in many programming languages that allow for complex pattern matching and text manipulation. For example, replacing specific text within a sentence when the sentence begins with a certain word can be performed with a regular expression. See expression.