How do I power this laser pointer with a 5V power supply?

I have a laser pointer that uses 3 - 1.5V watch batteries. I assume that means it needs exactly 4.5V to work? What do I need to do to power the laser pointer off of my electronics projects 5V power supply?

You should probably be fine to power the laser at 5 V. It does not need exactly 5V to work.

Most electronics have a voltage tolerance, and 5V over a 4.5 V requirement is fine. A slight overvoltage is fine. Undervoltage is not.

You should add a resistor inline with the supply, though to limit the current so you don't fry the electronics. A 5 V supply and a 1 kOhm resistor will yield a mA. Read up on the datasheet of the laser to see the needed current and adjust it appropriately. (A smaller resistor will give more current. A higher resistance will give less current).

You could also build a voltage divider to divide the voltage down to 4.5.

Watch batteries are cheap, if you buy them online. A battery that might cost $5.00 at a convenience store might cost 10 cents if you buy it online.