Label Generation Rules for Japanese

Overview

This document specifies a reference set of Label Generation Rules for Japanese using a limited repertoire as appropriate for a second level domain.

Repertoire

All references converge on the 26 basic ASCII Latin letters (a to z), 85 Hiragana, 88 Katakana, 6,538 ideographs (of which 6,536 are CJK unified ideographs). Each code point is tagged with its script category. Note that there are 3 code points that have more than one of these categories.

The core of the Japanese LGR repertoire is made of the J0 set of 6,536 CJK unified ideographs specified by JIS X 0208:1997, "Japanese Industrial standard, 7-bit and 8-bit double byte coded KANJI sets for information interchange". [110]

Unlike many other non-Latin 2nd level reference LGRs, the Japanese LGR includes the basic ASCII Latin set (a to z) because it is common practice in Japanese text to mix Kanji, Hiragana, Katakana and ASCII. Therefore this does not create confusability or additional security risks in the context of a second level LGR for the Japanese language. It is also supported by current IDNA practice, see [700].

Excluded code points

None.

Extended code points

None.

References

In the listing of the repertoire by code point, references starting from [0] refer to the version of the Unicode Standard in which the corresponding code point was initially encoded. Other references (starting from [100]) document usage of code points. For more details, see the Table of References below.

Variants

There are two variant pairs: U+30FC (ー) : U+4E00 (一) and U+30FD (ヽ) : U+4E36 (丶) that correspond to code points that have confusable glyphs which may be rendered without distinction. These are single stroke characters. In addition, they resemble punctuation characters. All variant mappings have been assigned the variant type "blocked" (see [RFC7940]).

Character Classes

The LGR defines the following named character classes:

• han — all code points tagged as sc:Hani (Han ideograph)
• hiragana — all code points tagged as sc:Hira (Hiragana)
• katakana — all code points tagged as sc:Kana (Katakana)
• japanese-code-point — the union of the above

The tags are assigned based on the Unicode script extension property, which allows multiple script values per code point.

Rules

Common rules:

• Hyphen Restrictions — restrictions on the allowable placement of hyphens (no leading/ending hyphen and no hyphen in positions 3 and 4). These restrictions are described in section 4.2.3.1 of RFC5891 [120]. They are implemented here as context rule on U+002D (-) (-) HYPHEN-MINUS.

• Leading Combining Marks — restrictions on the allowable placement of combining marks (no leading combining mark). This rule is described in section 4.2.3.2 of RFC5891 [120].

Japanese specific rules:

• Japanese in Label — restricts the occurrence of U+30FB (・) KATAKANA MIDDLE DOT to labels containing at least one code point from any of these scripts: Han, Hiragana, and Katakana. This rule is described in Appendix A.7 of RFC5892 [122]

• Preceded by Katakana — constrains U+30FD (ヽ) KATAKANA ITERATION MARK to follow a Katakana character. This constraint matches modern Japanese practice, see also the Unicode standard [123]. Note: because of its resemblance to a punctuation character, a conservative approach motivates some restrictions on the occurrence of this code point.

• Preceded by Japanese — constrains U+30FC (ー) KATAKANA-HIRAGANA PROLONGED SOUND MARK to always follow another code point: either a Han, Hiragana, or a Katakana code point, or itself. This constraint matches modern Japanese practice, see also the Unicode standard [123]. Note: because of its resemblance to a punctuation character, a conservative approach motivates some restrictions on the occurrence of this code point.

Actions

Actions included are the default actions for LGRs as well as those needed to invalidate labels with misplaced combining marks.

Methodology and Contributors

This reference LGR for Japanese for the 2nd Level has been developed by Michel Suignard and Asmus Freytag, verified in expert reviews by Lu Qin and Wil Tan, and based on multiple open public consultations.

References

General references for the language:

• Wikipedia: "Japanese writing system",
  https://en.wikipedia.org/wiki/Japanese_writing_system

• Omniglot: Japanese
  http://www.omniglot.com/writing/japanese.htm

Other references cited in this document:

[RFC7940]

Davies, K. and A. Freytag, "Representing Label Generation Rules in XML", RFC7940,
https://tools.ietf.org/html/rfc7940

In the listing of the repertoire by code point, references starting from [0] refer to the version of the Unicode Standard in which the corresponding code point was initially encoded. Other references (starting from [100]) document usage of code points. For more details, see the Table of References below.