expand the given string if it contains a range

Objective of this program is to expand the given string if it
contains a range, for example a-f, 0-5. 

Following are considered valid range expressions: 
1. lower case letters between a and z.  Ex: a-f, d-i
2. Upper case letters between A and Z.  Ex: A-F, D-I
3. Numbers between 0 and 9. Ex: 1-5
4. All the above 3 in the reverse order. Ex: f-a, F-A, 3-0

In case of invalid sequences in the input string, sequence
should be copied as is. 

--------------------------------------
Sample Input       Sample Output
--------------------------------------
a-fuy0-3ZA-F       abcdefuy0123ZABCDEF
a-f6-3F-9          abcdef6543F-9
-ABCD-             -ABCD-
-------------------------------------



Program:
#include<stdio.h>
int main()
{
       int l,m,n,f,g;
       char a[1000];
       int i;
       int j;
       scanf("%s",a);
       int valid,validr;
       i=0;
       while(a[i]!='\0')
      {
              printf("%c",a[i]);
              i++;
             while((a[i]!='-')&&(a[i]!='\0'))
             {
                    printf("%c",a[i]);
                    i++;
             }
             if(a[i]!='\0')
            {
                   f=i-1;g=i+1;
                   valid=0;validr=0;
                   if(a[f]>='A' && ((a[g]>='A')&&(a[g]>=a[f])))
                   valid=1;
                   else if((a[f]>='a' && ((a[g]>='a')&&(a[g]>=a[f]))))
                   valid=1;
                   else if(a[f]>='0' && ((a[g]>='0')&&(a[g]>=a[f])))
                   valid=1;

                   else if((a[g]>='A' && ((a[f]>='A')&&(a[f]<=a[g]))))
                   validr=1;
                   else if((a[g]>='a' && ((a[f]>='a')&&(a[f]<=a[g]))))
                   validr=1;
                   else if((a[g]>='0' && ((a[f]<='9')&&(a[f]>=a[g]))))
                   validr=1;
                   if(valid)
                   {
                          for(j=a[i-1]+1;j<a[i+1];j++)
                          printf("%c",j);
                          i++;
                  }
                  else if(validr)
                 {
                         for(j=a[i-1]-1;j>a[i+1];j--)
                         printf("%c",j);
                         i++;
                 }
 
           }
           else break;
       }
       return 0;
}