The term regular expression is used in the context of compiler construction and scanners. To develop a technique for designing scanners, it is necessary to precisely define the pattern that describes the set of lexemes to a symbol.
A common notation for specifying patterns is regular expressions. Regular expressions describe the set of strings corresponding to each pattern in the input text. Identifiers associated with regular expressions can thus be used as names for sets of strings and thus represent the class of strings associated with a symbol.