In computer (software) technology, a wildcard character can be used to substitute for any other character or characters in a string. The asterisk (*) usually substitutes as a wildcard character for any zero or more characters, and the question mark (?) usually substitutes as a wildcard character for any one character, as in the CP/M, DOS, Microsoft Windows and POSIX (Unix) shells. (In Unix this is referred to as glob expansion.) In SQL, the wildcard characters are percent (%) for zero or more characters, and underscore (_) for one character. In many regular expression implementations, the period (.) is the wildcard character for a single character.