Lexical Structure - The Java Language Specification - page 24

HexDigits:
HexDigit
HexDigit HexDigitsAndUnderscores opt HexDigit
HexDigit: one of
0 1 2 3 4 5 6 7 8 9 a b c d e f A B C D E F
HexDigitsAndUnderscores:
HexDigitOrUnderscore
HexDigitsAndUnderscores HexDigitOrUnderscore
HexDigitOrUnderscore:
HexDigit
_
The HexDigit production above comes from § 3.3 .
An octal numeral consists of an ASCII digit 0 followed by one or more of the ASCII digits
0 through 7 interspersed with underscores, and can represent a positive, zero, or negative
integer.
OctalNumeral:
0 OctalDigits
0 Underscores OctalDigits
OctalDigits:
OctalDigit
OctalDigit OctalDigitsAndUnderscores opt OctalDigit
OctalDigit: one of
0 1 2 3 4 5 6 7
OctalDigitsAndUnderscores:
OctalDigitOrUnderscore
OctalDigitsAndUnderscores OctalDigitOrUnderscore
OctalDigitOrUnderscore:
OctalDigit
_
Note that octal numerals always consist of two or more digits; 0 is always considered
to be a decimal numeral - not that it matters much in practice, for the numerals 0 , 00 ,
and 0x0 all represent exactly the same integer value.
A binary numeral consists of the leading ASCII characters 0b or 0B followed by one or
more of the ASCII digits 0 or 1 interspersed with underscores, and can represent a positive,
zero, or negative integer.