Sscanf get $ variables

Let's say I have char ** argv and that for every argv[i] I'll have a random string but some of them have a env inside, for example:

argv[2] = "asdad$HOMEsada";

If my home directory is /home/user, how can I get argv[2]= "asdad/home/usersada";

IMPORTANT: I have to use sscanf and "%[_a-zA-Z0-9]" format.

I have implemented some code that works but doesn't use those options, here it is:

    for (int j = 0; argv[j]; j++)
    {
        for (int i = 0; argv[j][i]; i++)
        {
                if (argv[j][i] == '$')
            {
                char principio[i + 1];
                strncpy(principio, argv[j], i);
                principio[i] = '\0';
                argv[j] = strdup(strcat(principio, getenv(argv[j] + i + 1)));
                break;
            }

Thanks!