ddi-urn              := [Uu][Rr][Nn] : [Dd][Dd][Ii] :
                          <agency-identifier> :
                          <resource-identifier> :
                          <version-identifier>
agency-identifier    := <top-level-domain> \.
                          <ddi-authority-id>
                          (\. <ddi-sub-authority-id>)*
agency-identifier    := .{1,255}

top-level-domain     := <dns-label>
ddi-authority-id     := <dns-label>

ddi-sub-authority-id := <dns-label>
dns-label            := [A-Za-z0-9]([-A-Za-z0-9]*[A-Za-z0-9])?

dns-label            := .{1,63}

resource-identifier  := <restricted-string>
                        (/ <restricted-string>)*

version-identifier   := <restricted-string>
                        (/ <restricted-string>)*

restricted-string    := [A-Za-z0-9-._~!$&'()*+,;=@]+