public boolean valid() {
        if (checksum.equals("")) return true;  // Empty checksum is ok (e.g. when running 'vespa-get-config')

        Matcher m = hexChecksumPattern.matcher(checksum);
        return m.matches();
    }
Checksums of config definition payload or config payload,
md5 and xxhash64 are the supported types at the moment.

@author hmusum
config/src/main/java/com/yahoo/vespa/config/PayloadChecksum.java