pubid

Pubid, short for public identifier, is a string used to identify a public resource, most commonly a document type definition (DTD) or other formal resource in SGML and XML. It functions as a symbolic name that can be mapped to a concrete location by the parser or a catalog.

In practice, pubid appears in DOCTYPE declarations as the public identifier portion of a two-part identifier. A typical DOCTYPE has the form: <!DOCTYPE root-element PUBLIC "public-id" "system-id">, where the public-id is the formal public identifier and the system-id provides the location of the DTD. When a public identifier is supplied, a system identifier is usually required as well, and the processor may use catalogs or mappings to resolve the public-id to a specific system resource.

Public identifiers follow a formal syntax developed for SGML and adopted by XML; they often begin with a prefix like -// and end with //EN (for English). They are not URLs themselves, but they can be associated with a URL via a catalog or resolver. This mechanism allows authors to reference DTDs and other public resources in a stable, standardized way, even if the actual location changes over time.

Examples include public identifiers for well-known standards such as XHTML and MathML, paired with a corresponding system identifier that locates the DTD. In modern XML usage, public identifiers are less central than system identifiers or URIs, but they remain part of the historical and interoperability framework for SGML/XML documents.

See also: Doctype declaration, DTD, public identifier, system identifier, XML catalogs.