[1] RFC 5234 - Augmented BNF for Syntax Specifications: ABNF ( (2014-07-04 15:02:25 +09:00 版)) <https://tools.ietf.org/html/rfc5234#page-14>

[2] Character set "$rfc6532:VCHAR" ( (2014-09-17 05:07:12 +09:00 版)) <http://chars.suikawiki.org/set/%24rfc6532%3AVCHAR>