= ALPHA / DIGIT ; obsolete, usage is deprecated or, in the form of a syntax diagram, as: The leading scheme () is case-insensitive. is the namespace identifier, and may include letters, digits, and -. The NID is followed by the namespace-specific string , the interpretation of which depends on the specified namespace. The NSS may contain ASCII letters and digits, and many punctuation and special characters. Disallowed ASCII and Unicode characters may be included if percent-encoded. In 2017, the syntax for URNs was updated: The slash character (/) is now allowed in the NSS to represent names containing slashes from non-URN identifier systems. The q-component was added to enable passing of parameters to named resources. The r-component was added to enable passing of parameters to resolvers. However, the updated specification notes that it should not be used until its semantics are defined via further standardization. Namespaces In order to ensure the global uniqueness of URN namespaces, their identifiers (NIDs) are required to be registered with the IANA. Registered namespaces may be "formal" or "informal". An exception to the registration requirement was formerly made for "experimental namespaces", since rescinded by RFC 8141. Formal Approximately sixty formal URN namespace identifiers have been registered. These are namespaces where Internet users are expected to benefit from their publication, and are subject to several restrictions. They must: Not be an already-registered NID Not start with Be more than two letters long Not start with , where XY is any combination of two ASCII letters Not start with (see "Experimental namespaces", below) Informal Informal namespaces are registered with IANA and assigned a number sequence (chosen by IANA on a first-come-first-served basis) as an identifier, in the format Informal namespaces are fully fledged URN namespaces and can be registered in global registration services. Experimental An exception to the registration requirement was formerly made for "experimental namespaces". However, following the deprecation of the "X-" notation for new identifier names, RFC 8141 did away with experimental URN namespaces, indicating a preference for use of the urn:example namespace where appropriate. Examples See also Archival Resource Key (ARK) .arpa – urn.arpa is for dynamic discovery Extensible resource identifier (XRI) Handle System Info URI scheme Life Science Identifiers (LSID) The Magnet URI scheme, which uses URNs Persistent Uniform Resource Locator (PURL) Tag URI scheme Digital Object Identifier (DOI) EPC Identification Keys. Maritime Resource Names (MRN) References Citations Sources External links Official