Limited entry in C

#include <stdio.h>
#include <stdlib.h>
#include <string.h>  /// We include the string lib to use the function strlen()

int main(int argc, char *argv[])
{
     int loop = 0, i = 0, len = 0;
     char enter[50] = "";

     do /// With the do while loop the program will execute at least once because loop is equal to 0
     {
        printf("Enter something: ");
        scanf("%s", enter);  /// We ask the user to enter something

        len = strlen(enter);  /// len takes the value of the string enter

        for(i=0;i<len;i++)  /// With this for loop we check if the characters are authorized refering to ASCII
        {
         if(enter[i] > 47 && enter[i] < 58) /// If it is a number the program stop
         {
              printf("Characters authorized %d\n", i);
         }
         else if(enter[i] > 64 && enter[i] < 91)  /// If it is a small letter the program stop
         {
              printf("Characters authorized %d\n", i);
         }
         else if(enter[i] > 96 && enter[i] < 123)
         {
              printf("Characters authorized %d\n", i); /// If it is a capital letter the program stop
         }
         else
         {
             printf("Unauthorized character. \n"); /// If it is another character, loop is now equal to 1 and the loop repeat
             loop = 1;
         }
        }
        printf("\n\n", len);
     }while(loop);

    return 0;
}