VIIN, Section 8.32.29 (ECMA-367, 2nd Edition)

     An Integer is valid if and only if it satisfies 
     the following conditions:

        1 It contains no breaks.

        2 Neither the first nor the last Digit of the 
          Digit_sequence is an underscore "_".

        3 If there is no Integer_base (decimal integer),
          every Digit is either one of the decimal
          digits 0 to 9 (zero to nine) or an underscore.

        4 If there is an Integer_base of the form 0b or 0B 
          (binary integer), every Digit is either 0, 1 or
          an underscore.

        5 If there is an Integer_base of the form 0c or 0C
          (octal integer), every Digit is either one of the
          octal digits 0 to 7 or an underscore.