digits   [0-9]
ltr      [a-zA-Z]
alphanum [a-zA-Z0-9]
%%
(-|\+)*{digits}+      printf("found number: '%s'\n",yytext);
{ltr}(_|{alphanum})*  printf("found identifier: '%s'\n",yytext);
\.                    printf("found character: {%s}\n",yytext);
.                     { /* ignore others */ }
%%
int main(int argc, char **argv)
{
  yylex();
}