See http://download.java.net/jdk6/docs/api/java/util/UUID.html#toString():

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The UUID string representation is as described by this BNF :

  UUID = "-" "-"
                            "-"
                            "-"
                           
  time_low = 4*
  time_mid = 2*
  time_high_and_version = 2*
  variant_and_sequence = 2*
  node = 6*
  hexOctet =
  hexDigit =
        "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9"
        | "a" | "b" | "c" | "d" | "e" | "f"
        | "A" | "B" | "C" | "D" | "E" | "F"
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Note the missing info. The Javadoc comment is

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     * <p>The UUID string representation is as described by this BNF :
     * <pre>
     * UUID = <time_low> "-" <time_mid> "-"
     * <time_high_and_version> "-"
     * <variant_and_sequence> "-"
     * <node>
     * time_low = 4*<hexOctet>
     * time_mid = 2*<hexOctet>
     * time_high_and_version = 2*<hexOctet>
     * variant_and_sequence = 2*<hexOctet>
     * node = 6*<hexOctet>
     * hexOctet = <hexDigit><hexDigit>
     * hexDigit =
     * "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9"
     * | "a" | "b" | "c" | "d" | "e" | "f"
     * | "A" | "B" | "C" | "D" | "E" | "F"
     * </pre>
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