Gyak

#include <stdio.h>
#include <stdlib.h>

char* unpack(char *input) {
    int i=0;
    int j=0;
    int ch;
    char* output = malloc(1000*sizeof(char));
    while(input[i]!=0) {
        if(input[i]=='[') {
            for(ch = input[i+1]; ch<=input[i+3]; ch++) {
                output[j]=ch;
                j++;
            }
            i=i+5;
        }
        else {
            output[j] = input[i];
            i++;
            j++;
        }
    }
    return output;
}

int main() {
    char input[]="hello [a-c] bello [d-k] aa";
    char input2[]="Wtf???";
    char* output = unpack(input2);
    printf("%s\n%s\n", input2, output);
    free(output);
}