Overview
ISO/IEC 19757-7:2020 defines the Character Repertoire Description Language (CREPDL) as part of the DSDL family for XML validation. A CREPDL schema describes a character repertoire so that a stream of UCS code points (Unicode/ISO‑10646) or grapheme clusters can be validated against that schema. The 2020 second edition updates the first edition by adding support for grapheme cluster validation and referencing the Unicode Ideographic Variation Database (IVDB).
Key topics and technical requirements
- Repertoire descriptions: CREPDL schemas express which code points or code point sequences (grapheme clusters) are permitted, excluded, or unknown.
- Kernel and hull: Each basic construct uses a pair of regular expressions - the kernel (guaranteed included) and the hull (not guaranteed excluded) - enabling exact or fuzzy repertoire specification.
- Modes: Option to validate individual characters/code points or multi-code-point grapheme clusters (e.g., letter + combining mark, ideograph + variation selector).
- Compound constructs and set operators: Repertoires can be combined using union, intersection, and difference to build complex sets.
- Syntax and integration: CREPDL syntax ties into DSDL technologies: RELAX NG schemas, NVDL scripts and regular-expression-based constructs for machine-readable schemas.
- Semantics and validation: The standard defines three-valued logic for membership - in, not-in, unknown - and specifies processor behavior for validating streams of UCS code points (excluding surrogate code points).
- Registries: Support for referencing external registries (for example, Unicode IVDB) for named repertoires.
Practical applications and users
CREPDL is valuable for:
- Schema designers and XML developers who must constrain allowed characters in XML documents or feeds.
- Internationalization (i18n) engineers and locale specialists enforcing language-specific character sets, normalization, and grapheme rules.
- Security & data validation teams controlling input character repertoires to mitigate homoglyph or injection risks.
- Font vendors, text rendering and NLP engineers needing to express allowed character sequences (variation selectors, combining marks).
- Standards bodies and registries creating or publishing named repertoires and collections.
CREPDL is typically referenced from schemas, forms, and stylesheets to centralize character validation rules and improve interoperability.
Related standards
Keywords: CREPDL, ISO/IEC 19757-7:2020, character repertoire, DSDL, grapheme clusters, kernel hull, Unicode validation, RELAX NG, NVDL.
Open preview (PDF)Information technology — Document Schema Definition Languages (DSDL) — Part 7: Character Repertoire Description Language (CREPDL)