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Formal Public Identifier
A Formal Public Identifier (FPI) is a short piece of text with a particular structure that may be used to uniquely identify a product, specification or document. FPIs were introduced as part of Standard Generalized Markup Language (SGML), and serve particular purposes in formats historically derived from SGML (HTML and XML). Some of their most common uses are as part of document type declarations (DOCTYPEs) and document type definitions (DTDs) in SGML, XML and historically HTML, but they are also used in the vCard and iCalendar file formats to identify the software product which generated the file.
More recently, Uniform Resource Identifiers (URIs) and universally unique identifiers (UUIDs) are usually used to uniquely identify objects. FPIs have become a legacy system.
An FPI consists of an owner identifier, followed by a double slash (//), followed by a text identifier. For example, the identifier "-//W3C//DTD HTML 4.01//EN" can be broken down into two parts: the owner identifier which indicates the issuer of the FPI, and the text identifier which indicates the particular document or object the FPI identifies. In the example, the owner identifier is "-//W3C" and the text identifier is "DTD HTML 4.01//EN".
The text identifier itself consists of multiple constituent parts. Sequences of whitespace are treated as equivalent to a single space.
There are three types of owner identifier, distinguished by their first three characters, which are ISO for an ISO owner identifier, -// for an unregistered owner identifier or +// for a registered owner identifier.
An ISO owner identifier is either an ISO publication number such as ISO 8879:1986, or an ISO-IR registration number given as e.g. ISO Registration Number 111 for ISO-IR-111. The latter type is only permitted for CHARSET FPIs (see below). In either case, it is distinguished by beginning with the characters ISO, and does not require any prefix before those characters.
The year was formerly separated from the standard number by a hyphen (-, e.g. ISO 8879-1986), which use is now deprecated. The hyphen is now, instead, used to separate the part number from the standard number (replacing earlier use of a single slash (/) for that purpose); the year follows any part number if present, and is separated by a colon (:).
An unregistered owner identifier begins with -//. Owners which use unregistered identifiers include the W3C (-//W3C), the Internet Engineering Task Force (-//IETF), the United States Department of Defense (-//USA-DOD), the European Parliament (-//EP) and others. Since it is not registered, it is not guaranteed to be unique (another owner may choose the same owner identifier), which weakens the uniqueness guarantee of the FPI as a whole, although it is still guaranteed to be distinct both from all other FPIs with the same owner, and also from all FPIs with registered owners.
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Formal Public Identifier
A Formal Public Identifier (FPI) is a short piece of text with a particular structure that may be used to uniquely identify a product, specification or document. FPIs were introduced as part of Standard Generalized Markup Language (SGML), and serve particular purposes in formats historically derived from SGML (HTML and XML). Some of their most common uses are as part of document type declarations (DOCTYPEs) and document type definitions (DTDs) in SGML, XML and historically HTML, but they are also used in the vCard and iCalendar file formats to identify the software product which generated the file.
More recently, Uniform Resource Identifiers (URIs) and universally unique identifiers (UUIDs) are usually used to uniquely identify objects. FPIs have become a legacy system.
An FPI consists of an owner identifier, followed by a double slash (//), followed by a text identifier. For example, the identifier "-//W3C//DTD HTML 4.01//EN" can be broken down into two parts: the owner identifier which indicates the issuer of the FPI, and the text identifier which indicates the particular document or object the FPI identifies. In the example, the owner identifier is "-//W3C" and the text identifier is "DTD HTML 4.01//EN".
The text identifier itself consists of multiple constituent parts. Sequences of whitespace are treated as equivalent to a single space.
There are three types of owner identifier, distinguished by their first three characters, which are ISO for an ISO owner identifier, -// for an unregistered owner identifier or +// for a registered owner identifier.
An ISO owner identifier is either an ISO publication number such as ISO 8879:1986, or an ISO-IR registration number given as e.g. ISO Registration Number 111 for ISO-IR-111. The latter type is only permitted for CHARSET FPIs (see below). In either case, it is distinguished by beginning with the characters ISO, and does not require any prefix before those characters.
The year was formerly separated from the standard number by a hyphen (-, e.g. ISO 8879-1986), which use is now deprecated. The hyphen is now, instead, used to separate the part number from the standard number (replacing earlier use of a single slash (/) for that purpose); the year follows any part number if present, and is separated by a colon (:).
An unregistered owner identifier begins with -//. Owners which use unregistered identifiers include the W3C (-//W3C), the Internet Engineering Task Force (-//IETF), the United States Department of Defense (-//USA-DOD), the European Parliament (-//EP) and others. Since it is not registered, it is not guaranteed to be unique (another owner may choose the same owner identifier), which weakens the uniqueness guarantee of the FPI as a whole, although it is still guaranteed to be distinct both from all other FPIs with the same owner, and also from all FPIs with registered owners.