QRegExp - Question on defining a regular expression

I am using QRegExp to validate user input. I would like the user to input a number from 0 to 255. (Eventually this will be used to validate input of an IP address like 192.168.0.123)

At this point, I have a working expression that allows the user to enter 1-3 digits, each in the range 0-9:
[0-9]{1,1}[0-9]{0,1}[0-9]{0,1}

This works, but allows the user to enter values from 0 to 999. How best to restrict the input to 0 to 255?

If the first digit entered is greater than 2, then we can only allow one more digit.
If the first digit is equal to 2, then if the user enters a total of three digits, the second and third digits cannot exceed 5.

Any guidance is appreciated.