do while validation problem

Hey guys having problems with my do while loop. I am trying to do some validation which works but however i am trying to ask the input to repeat itself if the input is invalid. But if it is valid break out of the do while and proceed with the rest of the code. I have no idea what i can put in the while(); part to do this.

Code:

---

printf("\nPlease Enter your age in dd/mm/yyyy format:");

  fgets(input, MAXDATE, stdin);

  do
  {
 
  if ( fgets ( input, sizeof input, stdin ) != NULL )
  {
    printf("is an invalid date!");
  }

    if ( sscanf ( input, "%d/%d/%d", &m, &d, &y ) == 3 )
    {
      if ( is_valid ( m, d, y ) )
      {
        printf ( "%d/%d/%d is a valid date\n", m, d, y );
      }
    }
      else
      {
        printf ( "Invalid date\n" );
      }
  }
  while(valid ==1 || input !=1);


 }


int is_valid ( int m, int d, int y )
{
  static const int  mday[] = {
    0,31,28,31,30,31,30,31,31,30,31,30,31
  };

  if ( m <= 0 || m > 12 )
    return 0;
  else if ( d <= 0 || d > mday[m] )
    return 0;
  else if ( y < 1900 || y > 3000 ) /* Arbitrary choices */
    return 0;

  return 1;
}

---

> while(valid ==1 || input !=1);
1. valid isn't even declared, or assigned in the loop
2. input is an array, so not comparable to 1

Try

Code:

---

while ( fgets ( input, sizeof input, stdin ) != NULL ) {
    if ( sscanf ( input, "%d/%d/%d", &m, &d, &y ) == 3 ) {
        if ( is_valid ( m, d, y ) ) {
            printf ( "%d/%d/%d is a valid date\n", m, d, y );
            break;
        } else {
            // range error
        }
    } else {
        // input format error
    }
}

---

would it be a do while loop still?

Sure you can put it into a do while loop, but it's extra effort IMO.
A do while loop has to run at least once, whereas a normal while loop can exit without doing anything.