6.1 Ruby (ルビ)

Synopsis

Ruby attaches a small reading (furigana) to a base run of text. It is the most common construct in the corpus.

ruby = [ BAR base ] RUBY-OPEN reading RUBY-CLOSE   ; ［｜］ base 《 reading 》

The reading is written in double angle brackets 《 … 》 immediately after the base. The base is identified in one of two ways:

Explicit base — a ｜ (U+FF5C) marks where the base begins; the base is the run from ｜ up to 《:
```
｜青梅《おうめ》
｜後手々々《ごてごて》
```
Implicit base — with no ｜, the base is the maximal run of same-class characters immediately preceding 《 (see Parameters):
```
青空《あおぞら》
```

base — the run the reading applies to.
- For the explicit form, the base is exactly the text between ｜ and 《.
- For the implicit form, the base is the maximal preceding run of one character class — a processor MUST take a trailing run of kanji (CJK ideographs); it MAY extend the implicit base over other single classes (kana, Latin) as the official guide allows, but MUST stop at a class boundary or at any preceding notation construct. When the intended base spans classes, the author uses the explicit ｜ form; this is why ｜ exists.
reading — the furigana text inside 《 … 》; it is plain text and MUST NOT itself contain a ruby (see Error conditions).

A ruby span yields a ruby node whose span covers the whole construct (from ｜ or the implicit base start, through the closing 》), carrying the base and the reading.
Reference rendering (§8) is HTML5 <ruby> with <rp> fallback parentheses:
```
<ruby>青梅<rp>(</rp><rt>おうめ</rt><rp>)</rp></ruby>
```
Serialization (§7) reconstructs the source form, including the ｜ when the base was explicit, so the round-trip is byte-exact.
An adjacent 《《…》》 is two ruby openers — a nested ruby error, not a single construct. The double-angle quotation notation (§6.15) is the distinct ≪…≫.

Empty reading with explicit base — ｜青梅《》 raises empty-ruby-reading (error) and degrades to plain text. A bare 《》 with no ｜ is literal text and is not flagged.
Nested ruby — a reading body that opens another ruby (｜漢《か《ん》じ》) raises nested-ruby (error); the outer ruby is parsed best-effort and the label points at the inner 《.
Unclosed 《 — a reading with no closing 》 raises unclosed-bracket; the region degrades to plain text.

ruby_explicit, ruby_implicit, empty-ruby-reading, nested-ruby, mixed_ruby_bouten (under conformance/vectors/).