size_t length = regerror(retval, &s->accept_controller_regex, NULL, 0);
char *buffer = xmalloc(length);
regerror(retval, &s->accept_controller_regex, buffer, length);
size_t length = regerror(retval, &s->accept_controller_regex, NULL, 0);
char *buffer = xmalloc(length);
regerror(retval, &s->accept_controller_regex, buffer, length);