%incremental lexer 'Try a valid expression like "a = b*2": ' %whites /([ \t]+)/ 
%token NUM =/([0-9]+(?:\.[0-9]+)?)/ 
%token VAR =/([A-Za-z][A-Za-z0-9_]*)/ 
%right '=' 
%left '-' '+' 
%left '*' '/' 
%left NEG 

%%

input:
    | input line 
;
line:
      '\n'
    | exp '\n'
    | error '\n' 
;
exp:
      NUM
    | VAR
    | VAR '=' exp
    | exp '+' exp
    | exp '-' exp
    | exp '*' exp
    | exp '/' exp
    | '-' exp %prec NEG
    | '(' exp ')'  
;

%%