C89 Switch case using a set

Question

I'm trying to write a fuction which checks whether an input is a valid double value. since I recently learned about the switch/case/default feature I wanted to use it to solve the problem

switch(carrier[i]){
        case ("+" || "-") :  //case 1
            if(kvcase == closed){
                printf("Error! Invaled input\n");
            }
            else /*save sign and close case*/
                break;

        case '.' : 
            if(deccase == closed){
                printf("Error! Invaled input\n");
            }
            else /*save comma and close case*/
                break;

        case '[0-9]' :  //case 3
            break;

        case ' ':
            printf("staring with whitespace\n");
            break;

        default:
        printf("empty\n");
            break;


    }

To answer the question please igonre the resr of the code. The Question is only about the proper way to state a case.

1) is case 1 valid or do I have to seperate it into two different cases

2) is case 3 valied? I'm trying to use a set of numbers. so if char carrier[i] is a number case is matched. I know of isdigit() function, but I don't want to just get around the problem, unless it's impossible for switch case to work with a set.

Extra Info: - carrier ist of type char* and has been given a value - I'm compiling with gcc and the code needs to be in c89 standard, whatever that is.

Answer 1

The Question is only about the proper way to state a case.
the code needs to be in c89 standard

Only 1 constant per case in standard C. No use of // comments in C89.

switch(carrier[i]) {
  case `+`: /* Fall through */  /*Comment not required, but good style */
  case `=`: 
    /* case 1 code here */
    break;
  case '.' : 
    if(deccase == closed) ...
    break;
  case '0' : /* Fall through */
  case '1' : /* Fall through */
  case '2' : /* Fall through */
  case '3' : /* Fall through */
  case '4' : /* Fall through */
  case '5' : /* Fall through */
  case '6' : /* Fall through */
  case '7' : /* Fall through */
  case '8' : /* Fall through */
  case '9' : 
    /* case 3 */
    ...
    break;
  case ' ':
    printf("staring with whitespace\n");
    break;
  default:
    printf("empty\n");
    break;
}