Add bidi support and address UAX31/UTS55 requirements

spec/formatting.md

@@ -768,7 +768,16 @@ That is, the text can can consist of a mixture of left-to-right and right-to-lef
 The display of bidirectional text is defined by the
 [Unicode Bidirectional Algorithm](http://www.unicode.org/reports/tr9/) [UAX9].
 
-The directionality of the message as a whole is provided by the _formatting context_.
+The directionality of the formatted _message_ as a whole is provided by the _formatting context_.
+
+> [!NOTE]
+> Keep in mind the difference between the formatted output of a _message_,
+> which is the topic of this section,
+> and the syntax of _message_ prior to formatting.
+> The processing of a _message_ depends on the logical sequence of Unicode code points,
+> not on the presentation of the _message_.
+> Affordances to allow users appropriate control over the appearance of the
+> _message_'s syntax have been provided.
 
 When a _message_ is formatted, _placeholders_ are replaced
 with their formatted representation.

spec/message.abnf

@@ -52,13 +52,13 @@ match = %s".match"
 
 ; Names and identifiers
 ; identifier matches https://www.w3.org/TR/REC-xml-names/#NT-QName
-; name matches https://www.w3.org/TR/REC-xml-names/#NT-NCName but excludes U+FFFD
+; name matches https://www.w3.org/TR/REC-xml-names/#NT-NCName but excludes U+FFFD and U+061C
 identifier = [namespace ":"] name
 namespace  = name
-name       = name-start *name-char
+name       = [bidi] name-start *name-char [bidi]
 name-start = ALPHA / "_"
            / %xC0-D6 / %xD8-F6 / %xF8-2FF
-           / %x370-37D / %x37F-1FFF / %x200C-200D
+           / %x370-37D / %x37F-61B / %x61D-1FFF / %x200C-200D
            / %x2070-218F / %x2C00-2FEF / %x3001-D7FF
            / %xF900-FDCF / %xFDF0-FFFC / %x10000-EFFFF
 name-char  = name-start / DIGIT / "-" / "."

spec/syntax.md

@@ -733,7 +733,13 @@ A **_<dfn>name</dfn>_** is a character sequence used in an _identifier_
 or as the name for a _variable_
 or the value of an _unquoted literal_.
 
-_Variable_ names are prefixed with `$`.
+A _name_ can be preceded or followed by bidirectional marks or isolating controls
+to aid in presenting names that contain right-to-left or neutral characters.
+These characters are **not** part of the _name_ and MUST be treated as if they were not present
+when matching _name_ or _identifier_ strings or _unquoted literal_ values.
+Implementations MAY remove these characters from a _message_.
+
+_Variable_ _names_ are prefixed with `$`.
 
 Valid content for _names_ is based on <cite>Namespaces in XML 1.0</cite>'s 
 [NCName](https://www.w3.org/TR/xml-names/#NT-NCName).
@@ -773,10 +779,10 @@ option     = identifier owsp "=" owsp (literal / variable)
 
 identifier = [namespace ":"] name
 namespace  = name
-name       = name-start *name-char
+name       = [bidi] name-start *name-char [bidi]
 name-start = ALPHA / "_"
            / %xC0-D6 / %xD8-F6 / %xF8-2FF
-           / %x370-37D / %x37F-1FFF / %x200C-200D
+           / %x370-37D / %x37F-61B / %x61D-1FFF / %x200C-200D
            / %x2070-218F / %x2C00-2FEF / %x3001-D7FF
            / %xF900-FDCF / %xFDF0-FFFC / %x10000-EFFFF
 name-char  = name-start / DIGIT / "-" / "."
@@ -834,14 +840,14 @@ _option values_, and _keys_)
 _Messages_ that contain right-to-left (aka RTL) characters SHOULD use one of the 
 following mechanisms to make messages display intelligibly in plain-text editors:
 
-1. Use paired isolating bidi controls `U+2066 LEFT-TO-RIGHT ISOLATE`
-   and `U+2069 POP DIRECTIONAL ISOLATE` as permitted by the ABNF around
+1. Use paired isolating bidi controls `U+2066 LEFT-TO-RIGHT ISOLATE` ("LRI")
+   and `U+2069 POP DIRECTIONAL ISOLATE` ("PDI") as permitted by the ABNF around
    parts of any _message_ containing RTL characters:
    - _inside_ of _placeholder_ markers `{` and `}` 
    - _outside_ _quoted-pattern_ markers `{{` and `}}`
-   - _identifiers_
-   - _literals_ (This is especially important for individual _keys_ in a _variant_)
-   - _option_ values
+   - _outside_ of _literals_, paying particular attention to _keys_ in a _variant_
+   - _outside_ of _variable_, _function_, _markup_, or _attribute_ _names_/_identifiers_,
+     including the identifying sigil (e.g. `<LRI>$var</PDI>` or `<LRI>:ns:name</PDI>`)
 2. Use the 'local-effect' bidi marks
    `U+061C ARABIC LETTER MARK`, `U+200E LEFT-TO-RIGHT MARK` or
    `U+200F RIGHT-TO-LEFT MARK` as permitted by the ABNF before or after _identifiers_,