If someone can give the precise part number for the regulators on board, I can check precisely how much they can take.
see rurwin's comment above – there are no regulators on board – just a couple of resetable fuses
Ummm. Not necessarily. Assuming that you are running off of four 1.5V batteries, it MAY work — for a while.
The problem is that voltage regulators work kind of like variable resistors. They adjust their resistance so that the output voltage is the right amount. However, there is a limit to how low the resistance can go. If you are trying to output exactly 5.0V, then if the input is too close to 5.0V (say, 5.2V, for example). then it just plain won't work. If you really want to use a simple linear regulator, then you probably want five batteries (or use four alkalines, and throw them out when they stop working for this application, well before they are truly dead).
There are things called "low-drop-out" (LDO) voltage regulators, but you have to be careful on the capacitance on it, or it can become unstable and turn into a nice little oscillator.
What you "REALLY" want (if you can afford it) is a DC-to-DC converter. A quick search of Newark turned up this (link to PDF data sheet):
Note that this part runs around $26, but can take 4.5V to 9V and output a smooth 5V the entire time.