Simple flex

flextest.l

%{
#include <stdio.h>
int line_num = 1;
%}

%%

"("     { printf("a opening parenthesis\n"); }
")"     { printf("a closing parenthesis\n"); }
[0-9]+  { printf("a number: %s\n", yytext); }
[-*+/]  { printf("an operator: %s\n", yytext); }
[\t ]+  { }
.       { printf("line %d: error: unexpected: %s\n", line_num, yytext); }
\n      { line_num++; }
%%

int yywrap(void){return 1;}
int main(int argc, char *argv[]) {
  yylex();
}