            trigger_error('%Attr.IDPrefixLocal cannot be used unless '.
                '%Attr.IDPrefix is set', E_USER_WARNING);
        }

        if (!$this->selector) {
            $id_accumulator =& $context->get('IDAccumulator');
            if (isset($id_accumulator->ids[$id])) return false;
        }

        // we purposely avoid using regex, hopefully this is faster

        if (ctype_alpha($id)) {
            $result = true;
        } else {
            if (!ctype_alpha(@$id[0])) return false;
            $trim = trim( // primitive style of regexps, I suppose
                $id,
                'A..Za..z0..9:-._'
              );
            $result = ($trim === '');
        }

        $regexp = $config->get('Attr.IDBlacklistRegexp');
        if ($regexp && preg_match($regexp, $id)) {
            return false;
        }

        if (!$this->selector && $result) $id_accumulator->add($id);

        // if no change was made to the ID, return the result
        // else, return the new id if stripping whitespace made it
        //     valid, or return false.
        return $result ? $id : false;

    }
The software does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users.