[1] 
>A single Unicode code point is denoted by
"U+" followed by four to six hexadecimal digits, while a range of
Unicode code points is denoted by two hexadecimal numbers separated
by "..", with no prefixes.

;; [2] [CITE@en[RFC 3490 - Internationalizing Domain Names in Applications (IDNA)]] 
<http://tools.ietf.org/html/rfc3490#section-2>、
[3] [CITE@en[RFC 3492 - Punycode: A Bootstring encoding of Unicode for Internationalized Domain Names in Applications (IDNA)]] 
<http://tools.ietf.org/html/rfc3492#section-2>



[4] [CITE@en[RFC 5890 - Internationalized Domain Names for Applications (IDNA): Definitions and Document Framework]]
( ([TIME[2011-02-05 07:34:19 +09:00]] 版))
<http://tools.ietf.org/html/rfc5890#section-2.1>

[5] [CITE@en[Unicode Mail List Archive: Re: Origin of the U+nnnn notation]]
( ([[Kenneth Whistler (kenw@sybase.com)]] 著, [TIME[2005-11-09 16:55:17 +09:00]] 版))
<http://unicode.org/mail-arch/unicode-ml/y2005-m11/0060.html>