v-extension = v-prefix ":" v-name

v-prefix = v-label *("." v-label)

v-label = alnum-int / alnum-int *(alnum-int / "-") alnum-int

alnum-int = ALPHA / DIGIT / NON-ASCII
  ; see RFC 6350, Section 3.3

v-name = 1*(WSP / "!" / %x23-2e / %x30-7d / NON-ASCII)
  ; any characters except CTLs, DQUOTE, SOLIDUS, and TILDE