pre_match

/*
 * Return the <i><b>pre</b>-match</i> (in the regular expression sense) of the last scan.
 *
 *   s = StringScanner.new('test string')
 *   s.scan(/\w+/)           # -> "test"
 *   s.scan(/\s+/)           # -> " "
 *   s.pre_match             # -> "test"
 *   s.post_match            # -> "string"
 */
static VALUE
strscan_pre_match(VALUE self)
{
    struct strscanner *p;

    GET_SCANNER(self, p);
    if (! MATCHED_P(p)) return Qnil;

    return extract_range(p, 0, p->prev + p->regs.beg[0]);
}