concat_opt_exact_str(OptExact* to, UChar* s, UChar* end, OnigEncoding enc)
{
  int i, j, len;
  UChar *p;

  for (i = to->len, p = s; p < end && i < OPT_EXACT_MAXLEN; ) {
    len = enclen(enc, p);
    if (i + len > OPT_EXACT_MAXLEN) break;
    for (j = 0; j < len && p < end; j++)
      to->s[i++] = *p++;
  }

  to->len = i;
}