My program always comes back as true, instead of requiring only 1 decimal point. I can't figure out why. Help?

9. (Useful utility) a. Write a C++ function named isvalidreal) that checks for a valid dou ble-precision number. This kind of number can have an optional + or - sign, at most one decimal point (which can be the first character), and at least one digit between 0 and 9. The function should return a Boolean value of true if the entered number is a real number; other- wise, it should return a Boolean value of false