浮点数

FloatValue ::
    IntegerPartFractionalPart
    IntegerPartExponentPart
    IntegerPartFractionalPartExponentPart
FractionalPart
    .Digit list
ExponentPart
    ExponentIndicator Sign opt Digit list
ExponentIndicator :: end of
    e    E
Sign :: one of 
    +    -

A Float number includes either a decimal point (ex.1.0) or an exponent (ex.1e50) or both (ex.6.0221413e23).