BNF Mathematical Operations

<digit> ::= 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9

<positive int> ::= <digit> | <digit><positive int>
<positive float> ::= <positive int> | <positive int> . <positive int>
<valid number> ::= <positive float> | -<positive float>

<whitespace> ::= " " <whitespace> | ""
<arithmetic operator> ::= + | - | ÷ | x | *
<user input> ::= <valid number><arithmetic operator><valid number>|<valid number><whitespace><arithmetic operator><whitespace><valid number>
<computer output> ::= <valid number>