Thank you... i wasnt asking for "xor" actualy... i didnt know the operator "^" :D

Could someone explain that?

asprintf() is a Linux function, not in standard C or POSIX. you must either declare it manually, or #define _GNU_SOURCE before you #include <stdio.h>

The tests free it. It's always the case when you must return a heap allocated array / string (you cannot free it yourself: where would you? After return, the statement is not executed, before return then you would return a pointer to something not allocated anymore).

Why is the memory pointed to by r not released at the end of the function

Looks like you may need to consider illegal leading and trailing characters.

Very nice

That's right, it's supported by most modern compilers like gcc and clang but not vc++ for example.