Implement code more introducer (#529)

* Add .keywords to ABNF

* Consider all messages starting with . as complex

* Rename text-start as simple-start-char, drop text, fix whitespace

Co-authored-by: Richard Gibson <richard.gibson@gmail.com>

* Rename reserved-declaration as reserved-syntax

* Rename & group reserved syntax rules together

* Fix typo

* Rename reserved-keyword as reserved-statement

* Apply suggestions from code review

* Drop \. escapes

* Add reserved-keyword to reserved-statement

* Apply suggestions from code review

Co-authored-by: Richard Gibson <richard.gibson@gmail.com>

---------

Co-authored-by: Richard Gibson <richard.gibson@gmail.com>

Author: eemeli

spec/message.abnf

@@ -1,43 +1,46 @@
-message = pattern / complex-message
+message = simple-message / complex-message
 
-complex-message = "{{" [s] *(declaration [s]) body [s] "}}"
+simple-message = [simple-start pattern]
+simple-start = simple-start-char / text-escape / expression
+pattern = *(text-char / text-escape / expression)
 
-declaration = input-declaration / local-declaration
+complex-message = *(declaration [s]) complex-body
+declaration = input-declaration / local-declaration / reserved-statement
 input-declaration = input [s] variable-expression
 local-declaration = local s variable [s] "=" [s] expression
 
-body = quoted-pattern
-     / (selectors 1*([s] variant))
-
+complex-body = quoted-pattern
+             / ((selectors / reserved-statement) 1*([s] variant))
 quoted-pattern = "{{" pattern "}}"
-pattern = *(text / expression)
+
 selectors = match 1*([s] expression)
-variant = when 1*(s key) [s] quoted-pattern
+variant = key *(s key) [s] quoted-pattern
 key = literal / "*"
 
 expression = literal-expression / variable-expression / function-expression
 literal-expression = "{" [s] literal [s annotation] [s] "}"
 variable-expression = "{" [s] variable [s annotation] [s] "}"
 function-expression = "{" [s] annotation [s] "}"
-annotation = (function *(s option)) / reserved / private-use
+annotation = (function *(s option))
+           / reserved-annotation
+           / private-use-annotation
 
 literal = quoted / unquoted
 variable = "$" name
 function = (":" / "+" / "-") identifier
 option = identifier [s] "=" [s] (literal / variable)
 
-; reserved keywords are always lowercase
-input = %s"input"
-local = %s"local"
-match = %s"match"
-when  = %s"when"
+input = %s".input"
+local = %s".local"
+match = %s".match"
 
-text = 1*(text-char / text-escape)
-text-char = %x0-5B         ; omit \
-          / %x5D-7A        ; omit {
-          / %x7C           ; omit }
-          / %x7E-D7FF      ; omit surrogates
-          / %xE000-10FFFF
+simple-start-char = %x0-2D         ; omit .
+                  / %x2F-5B        ; omit \
+                  / %x5D-7A        ; omit {
+                  / %x7C           ; omit }
+                  / %x7E-D7FF      ; omit surrogates
+                  / %xE000-10FFFF
+text-char = simple-start-char / "."
 
 quoted      = "|" *(quoted-char / quoted-escape) "|"
 quoted-char = %x0-5B         ; omit \
@@ -49,16 +52,19 @@ unquoted       = unquoted-start *(name-char / ":")
 unquoted-start = name-start / DIGIT / "."
                / %xB7 / %x300-36F / %x203F-2040
 
-; reserve sigils for private-use by implementations
-private-use    = private-start reserved-body
-private-start  = "^" / "&"
+; Reserve additional .keywords for use by future versions of this specification.
+reserved-statement = reserved-keyword [s reserved-body] 1*([s] expression)
+; Note that the following expression is a simplification,
+; as this rule MUST NOT be considered to match existing keywords
+; (`.input`, `.local`, and `.match`).
+reserved-keyword = "." name
 
-; reserve additional sigils for use by 
-; future versions of this specification
-reserved       = reserved-start reserved-body
-reserved-start = "!" / "@" / "#" / "%" / "*" / "<" / ">" / "/" / "?" / "~"
-reserved-body  = *( [s] 1*(reserved-char / reserved-escape / quoted))
+; Reserve additional sigils for use by future versions of this specification.
+reserved-annotation = reserved-annotation-start reserved-body
+reserved-annotation-start = "!" / "@" / "#" / "%" / "*"
+                          / "<" / ">" / "/" / "?" / "~"
 
+reserved-body  = *([s] 1*(reserved-char / reserved-escape / quoted))
 reserved-char  = %x00-08        ; omit HTAB and LF
                / %x0B-0C        ; omit CR
                / %x0E-19        ; omit SP
@@ -67,6 +73,10 @@ reserved-char  = %x00-08        ; omit HTAB and LF
                / %x7E-D7FF      ; omit surrogates
                / %xE000-10FFFF
 
+; Reserve sigils for private-use by implementations.
+private-use-annotation = private-start reserved-body
+private-start = "^" / "&"
+
 ; identifier matches https://www.w3.org/TR/REC-xml-names/#NT-QName
 ; name matches https://www.w3.org/TR/REC-xml-names/#NT-NCName
 identifier = [namespace ":"] name