DATA "PI", "0123", "-0123", "12.30", "-12.30", "123!", "0"
DATA "0.0", ".123", "-.123", "12E3", "12E-3", "12+3", "end"


while n$ <> "end"
    read n$
    print n$, IsNumber(n$)
wend
end

function IsNumber(string$)
    on error goto [NotNumber]
    string$ = trim$(string$)
    'check for float overflow
    n = val(string$)

    'assume it is number and try to prove wrong
    IsNumber = 1
    for i = 1 to len(string$)
        select case mid$(string$, i, 1)
            case "0", "1", "2", "3", "4", "5", "6", "7", "8", "9"
                HasNumeric = 1 'to check if there are any digits
            case "e", "E"
                '"e" must not occur more than once
                'must not occur before digits

                if HasE > 0 or HasNumeric = 0 then
                    IsNumber = 0
                    exit for
                end if
                HasE = i 'store position of "e"
                HasNumeric = 0 'needs numbers after "e"
            case "-", "+"
                'must be either first character or immediately after "e"
                '(HasE = 0 if no occurrences yet)
                if HasE <> i-1 then
                    IsNumber = 0
                    exit for
                end if
            case "."
                'must not have previous points and must not come after "e"
                if HasE <> 0 or HasPoint <> 0 then
                    IsNumber = 0
                    exit for
                end if
                HasPoint = 1
            case else
                'no other characters allowed
                IsNumber = 0
                exit for
        end select
    next i
    'must have digits
    if HasNumeric = 0 then IsNumber = 0
    [NotNumber]
end function