flex lexer - Bison, simple calculator program -

i want make parser simple calculator cant uderstand whu error simple input. flex file looks this

%{ #include "exp.tab.h" #include <string.h> %}  blanks          [ \t\n]+   %%  {blanks}        { /* ignore */ } [0-9]+           {yylval= strtol(yytext, null, 10); return(numb);}  %% 

bison file looks this:

%{ #include <stdio.h> %}   %token numb   %left '+'   %%  exp: numb                { $$ = $1;           } | exp '+' exp        { $$ = $1 + $3;      }  %% int yyerror(char *s) { printf("yyerror : %s\n",s); }  int main(void) { yyparse(); } 

for input

123 + 12 

i error message.why happening?

your lexer missing rule match/return '+' token. try adding @ end:

.    { return *yytext; }  /* other single character returns */ 

the default lexer action if text doesn't match rule print , skip it. error parser because gets 123 12 input, causes syntax error.