an unspecified data structure that is used to
represent a regular expression, and which permits efficient matches
against the regular expression to be obtained.
a sequence of zero or more characters is matched by
a regular expression when the characters in the sequence
correspond to a sequence of characters defined by the pattern.
a set of one or more characters which
share the same primary sort key: that is the sort key weighting that
depends only upon character shape, and not accents, case, or
locale specific tailorings.