Formal syntax of query expressions

QueryExpression   := ZettelList? QueryDirective* SearchExpression ActionExpression?
ZettelList        := (ZID (SPACE+ ZID)*).
ZID               := '0'+ ('1' .. '9'') DIGIT*
                   | ('1' .. '9') DIGIT*.
QueryDirective    := ContextDirective
                   | IdentDirective
                   | ItemsDirective
                   | UnlinkedDirective.
ContextDirective  := "CONTEXT" (SPACE+ ContextDetail)*.
ContextDetail     := "BACKWARD"
                   | "FORWARD"
                   | "COST" SPACE+ PosInt
                   | "MAX" SPACE+ PosInt.
IdentDirective    := IDENT.
ItemsDirective    := ITEMS.
UnlinkedDirective := UNLINKED (SPACE+ PHRASE SPACE+ Word)*.
SearchExpression  := SearchTerm (SPACE+ SearchTerm)*.
SearchTerm        := SearchOperator? SearchValue
                   | SearchKey SearchOperator SearchValue?
                   | SearchKey ExistOperator
                   | "OR"
                   | "RANDOM"
                   | "PICK" SPACE+ PosInt
                   | "ORDER" SPACE+ ("REVERSE" SPACE+)? SearchKey
                   | "OFFSET" SPACE+ PosInt
                   | "LIMIT" SPACE+ PosInt.
SearchValue       := Word.
SearchKey         := MetadataKey.
SearchOperator    := '!'
                   | ('!')? ('~' | ':' | '[' | '}').
ExistOperator     := '?'
                   | '!' '?'.
PosInt            := '0'
                   | ('1' .. '9') DIGIT*.
ActionExpression  := '|' (Word (SPACE+ Word)*)?
Word              := NO-SPACE NO-SPACE*