/*
 * call-seq: [](n)
 *
 * Return the n-th subgroup in the most recent match.
 *
 *   s = StringScanner.new("Fri Dec 12 1975 14:39")
 *   s.scan(/(\w+) (\w+) (\d+) /)       # -> "Fri Dec 12 "
 *   s[0]                               # -> "Fri Dec 12 "
 *   s[1]                               # -> "Fri"
 *   s[2]                               # -> "Dec"
 *   s[3]                               # -> "12"
 *   s.post_match                       # -> "1975 14:39"
 *   s.pre_match                        # -> ""
 */
static VALUE
strscan_aref(self, idx)
    VALUE self, idx;
{
    struct strscanner *p;
    long i;

    GET_SCANNER(self, p);
    if (! MATCHED_P(p))        return Qnil;
    
    i = NUM2LONG(idx);
    if (i < 0)
        i += p->regs.num_regs;
    if (i < 0)                 return Qnil;
    if (i >= p->regs.num_regs) return Qnil;
    if (p->regs.beg[i] == -1)  return Qnil;

    return extract_range(p, p->prev + p->regs.beg[i],
                            p->prev + p->regs.end[i]);
}