[1] RFC 5234 - Augmented BNF for Syntax Specifications: ABNF ( (7/4/2014, 6:02:25 AM 版)) <https://tools.ietf.org/html/rfc5234#page-14>
[2] Character set "$rfc6532:VCHAR" ( (9/16/2014, 8:07:12 PM 版)) <http://chars.suikawiki.org/set/%24rfc6532%3AVCHAR>
Google search: VCHAR