So, the scenario is that you want to do some specific validation or matching, but part of what you're validating or matching will decide how the rest is validated or matched. For example, in my scenario I have a field in the database that will store a report destination in the URI format that can be either for e-mail, FTP, or a file (mailto://, ftp://, or file://). You could create three regular expressions and evaluate each, but that can be klunky especially if you're doing on the fly validation...