function isValidDouble(value as string) as boolean
    dim as byte comma, exponent, sign
    for i as integer = 0 to len(value)-1
        select case as const value[i]
            case  48,49,50,51,52,53,54,55,56,57 ' 0 - 9
                ' do nothing
            case 101: 'e
                exponent += 1
                if (exponent > 1 or i = 0) then
                    return false
                end if
            case 46: ' .
                comma += 1
                if (comma > 1 or i = 0) then
                    return false
                end if
            case 45: ' -
                sign += 1
                if (sign > 1 or i > 0) then
                    return false
                end if
            case else
                return false
        end select
    next
    return true
end function