Battery gauge – Microcontroller 3.3V using a 4.8V NiMh battery source
Since you want to use a regulator, you can't assume Vbat=Vcc. So you need to use an external pin to measure battery voltage.

The most simple method I can think of is a voltage divider tied to an ADC pin (if you want full voltage level) or an analog comparator pin (if you only need 1.1v threshold; you can use the internal reference as the other input to the internal comparator)

However you need to use resistor values high enough for an acceptable current draw, and low enough for adc input impedance and noise. A cap across input pin may help with noise and for storing enough charge for the adc s/h capacitor noise. 

And if you need very low average power draw, you can tie the ground side of the resistor divider to a digital pin or (better) tie the upper side to a p-mosfet whose gate is controlled by a digital pin. This way you can enable the divider only while you're actually measuring it.

In any case I don't think you need an external comparator unless you need advanced features (super low offset voltage, for example) the AVR peripherals don't provide.