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Specify private reserved sigils ^ and & #404

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Merged
merged 3 commits into from
Jul 3, 2023
Merged

Specify private reserved sigils ^ and & #404

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9d0d157
Specify private reserved sigils ^ and &
eemeli Jun 30, 2023
26c7a2a
Apply suggestions from code review
eemeli Jul 2, 2023
bfaf458
Add / as a reserved-start char
eemeli Jul 3, 2023
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8 changes: 5 additions & 3 deletions spec/message.abnf
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Original file line number Diff line number Diff line change
Expand Up @@ -42,9 +42,11 @@ unquoted = unquoted-start *name-char
unquoted-start = name-start / DIGIT / "."
/ %xB7 / %x300-36F / %x203F-2040

; reserve additional sigils for future use
reserved = reserved-start reserved-body
reserved-start = "!" / "@" / "#" / "%" / "^" / "&" / "*" / "<" / ">" / "?" / "~"
; reserve additional sigils for use by future versions of this
; specification or for private use by implementations
reserved = ( reserved-start / private-start ) reserved-body
reserved-start = "!" / "@" / "#" / "%" / "*" / "<" / ">" / "/" / "?" / "~"
private-start = "^" / "&"
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We should discuss as a group which characters to reserve for private use, although these seem as good as any.

reserved-body = *( [s] 1*(reserved-char / reserved-escape / literal))
reserved-char = %x00-08 ; omit HTAB and LF
/ %x0B-0C ; omit CR
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34 changes: 30 additions & 4 deletions spec/syntax.md
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Expand Up @@ -412,14 +412,40 @@ option = name [s] "=" [s] (literal / variable)

#### Reserved

**_Reserved_** sequences start with a reserved character
and are intended for future standardization.
A reserved sequence MAY be empty or contain arbitrary text.
A reserved sequence does not include any trailing whitespace.
**_Reserved_** annotations start with a reserved character
and are intended for future standardization
as well as private implementation use.
A _reserved_ _annotation_ MAY be empty or contain arbitrary text.
This allows maximum flexibility in future standardization,
as future definitions are expected to define additional semantics and constraints
on the contents of these _annotations_.
A _reserved_ _annotation_ does not include trailing whitespace.

Implementations MAY define their own meaning and semantics for
_reserved_ annotations that start with
the U+0026 AMPERSAND `&` or U+005E CIRCUMFLEX ACCENT `^` characters.
Implementations MUST NOT assign meaning or semantics to
an _annotation_ starting with `reserved-start`:
these are reserved for future standardization.
Implementations MUST NOT remove or alter the contents of a _reserved_ _annotation_.

While a reserved sequence is technically "well-formed",
unrecognized reserved sequences have no meaning and MAY result in errors during formatting.

```abnf
reserved = ( reserved-start / private-start ) reserved-body
reserved-start = "!" / "@" / "#" / "%" / "*" / "<" / ">" / "/" / "?" / "~"
private-start = "^" / "&"
reserved-body = *( [s] 1*(reserved-char / reserved-escape / literal))
reserved-char = %x00-08 ; omit HTAB and LF
/ %x0B-0C ; omit CR
/ %x0E-19 ; omit SP
/ %x21-5B ; omit \
/ %x5D-7A ; omit { | }
/ %x7E-D7FF ; omit surrogates
/ %xE000-10FFFF
```

## Tokens

The grammar defines the following tokens for the purpose of the lexical analysis.
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