Abstract
This paper studies machine-readable notations for describing subsets of Unicode or ISO/IEC 10646. Unicode regular expressions can describe any subset, but they have performance problems for huge subsets and cannot directly capture subsets defined in terms of other subsets. Meanwhile, the upcoming second edition of ISO/IEC 19757-7 Character Repertoire Description Language (CREPDL) overcomes these problems by providing references to well-known subsets and external CREPDL scripts.