This is a generic (item-independent) checker for regular expressions.
As it's also the first generic checker in the standard distribution, it
also serves to document how to create one for other types.
This checker is the default for items of type REGEX. It can be subclassed
if an individual item needs additional checks, but for most items, this is
all that is necessary.
This checker is normally instantiated by Types/REGEX (see makeChecker).