Overview
ISO/IEC 19757-2:2008 defines RELAX NG, a regular-grammar-based schema language for XML as part of the Document Schema Definition Language (DSDL) family. The standard specifies how a RELAX NG schema describes patterns for the structure and content of XML documents using a regular tree grammar, and establishes the formal requirements for when an XML document “matches” a RELAX NG schema. The document includes full and compact syntaxes, transformation rules that simplify schemas, formal semantics, and conformance criteria for validators.
Key topics and requirements
- Schema language specification: Formal definition of RELAX NG patterns for elements, attributes, text, data, lists, interleaving, and grouping.
- Regular tree grammar: Use of regular-grammar constructs to express valid XML tree structures and content models.
- Syntax and simplification: Full XML-based syntax and the RELAX NG Compact Syntax (compact, human-friendly form), plus rules for simplifying schemas into a canonical simple form.
- Semantics and validation: Formal inference rules and semantics that define when an XML document is valid against a RELAX NG schema.
- Datatype support: Integration points for datatype libraries (built-in and external) and mechanisms for data/value validation.
- Restrictions and conformance: Constraints on schema constructs, prohibited patterns, and formal conformance requirements for RELAX NG validators and translators.
- Normative annexes: Includes a RELAX NG schema for RELAX NG itself, examples, and the compact syntax specification to aid implementers.
Applications and who uses it
RELAX NG (ISO/IEC 19757-2:2008) is widely used for:
- XML schema authors who need an expressive, easy-to-read schema language for complex document grammars.
- Tool and validator developers building XML validation engines that implement RELAX NG conformance rules.
- Document architects and integrators designing document exchange formats, technical documentation, or content models where flexible content ordering (interleave) and concise schemas are beneficial.
- that require formal validation for XML-based data interchange, publishing, or archiving.
