(*PART A*)
begin  end
case <stmt> endcase
case <some_variable> of: <line> endcase
 case <some_variable> of: <line> endcase
 case <some_variable> of: {<string> (,|_)}* : {<one-statement>(;|_)} ; [<end>] endcase
case <some_variable> of: {<string> (,|_)}* : {<one-statement>(;|_)} ; else [{<one-statement>(;|_)};] endcase

<stmt>          ::= <String-Case>
<String-Case>   ::= case <some_variable> of: <line>
<line>          ::=     {<string> (,|_)}* : {<one-statement>(;|_)} ; [<end>]
                | <line> ,{<string> (,|_)}* : {<one-statement>(;|_)} ; [<end>]

<end>           ::= [<end>] endcase |
                else [{<one-statement>(;|_)};] endcase

(* define variables , strings, and digits*)
<string>        ::= ""({<letter>|<digit>})
<letter>            ::= "a" | "b" | "c" | "d" | "e" | "f" | "g" | "h" | "i" | "j" | "k" | "l" | "m" | "n" |                         "o" | "p" | "q" | "r" | "s" | "t" | "u" | "v" | "w" | "x" | "y" | "z" | "A" |                       "B" | "C" | "D" | "E" | "F" | "G" | "H" | "I" | "J" | "K" | "L" | "M" | "N" | "O" |                         "P" | "Q" | "R" | "S" | "T" | "U" | "V" | "W" | "X" | "Y" | "Z" ;
<digit>             ::= "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9" ;
 (* <some_variable> and <one-statement> defined //somewhere else *)

(*PART B*)
begin String-Case end
begin
String-Case = "case" some_variable "of:" line+  ;
line        =   {string ,(","|"_")} : {one-statement,(";"|"_")} ";" ?end? ;
end     =  endcase |
            "else" [{<one-statement>(";"|"_")}";"] endcase

(* define variables , strings, and digits*)
string      = "",({letter|digit})
letter      = "a" | "b" | "c" | "d" | "e" | "f" | "g" | "h" | "i" | "j" | "k" | "l" | "m" | "n" | "o" | "p" | "q" | "r" | "s" | "t" | "u" | "v" | "w" | "x" | "y" | "z" | "A" | "B" | "C" | "D" | "E" | "F" | "G" | "H" | "I" | "J" | "K" | "L" | "M" | "N" | "O" | "P" | "Q" | "R" | "S" | "T" | "U" | "V" | "W" | "X" | "Y" | "Z" ;
digit       = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9" ;
(c)

 <S> => "case" <some_variable> "of:" <line>