gistfile1.txt

%skip   space     \s
%token  bracket_  \(
%token _bracket   \)
%token  comma     ,
%token  number    (0|[1-9]\d*)(\.\d+)?([eE][\+\-]?\d+)?
%token  plus      \+
%token  minus     \-|−
%token  pow       \*\*
%token  times     \*|×
%token  percent   %
%token  div       /|÷
%token  ID        [A-Z_0-9]+
%token  id        [a-z_0-9]+

expression:
    primary()
    ( ( ::plus:: #addition | ::minus:: #substraction ) expression() )?

primary:
    term()
    ( (
        ::times:: #multiplication |
        ::div:: #division |
        ::pow:: #pow |
        ::percent:: #modulo
        )
    expression() )?

term:
    ( ::bracket_:: expression() ::_bracket::)
  | number()
  | function()
  | constant() #constant
  | ( ::minus:: #negative | ::plus:: ) term()

number:
    <number>

constant:
    <ID>

#function:
    <id> ::bracket_::
    ( expression() ( ::comma:: expression() )* )?
    ::_bracket::