/in/foo.cc: In function 'int main()':
/in/foo.cc:21:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i = 1; i <= str.length()-2; i++)
                     ~~^~~~~~~~~~~~~~~~~
/in/foo.cc:34:28: warning: suggest parentheses around '&&' within '||' [-Wparentheses]
         if (!(isdigit(pre) && isdigit(nxt) || isalpha(pre) && isalpha(nxt)))
               ~~~~~~~~~~~~~^~~~~~~~~~~~~~~