To regulate in this sense is to manage the equipping (arming) of the militia so that it may function to serve it’s purpose in proper manner. An unarmed militia is unable to function. And before you opine on the National Guard, remember the National Guard did not exist under Federal law until 1903, referred to as the “organized militia” under federal law. The “unorganized militia” also still exists under federal law. The unorganized militia receives no equipment, facilities or training from the government, existing as it always has from colonial times. The citizen, providing his own arms and equipment, called up on a volunteer basis in times of need.
Now while we haven’t been involved in a situation were our standing armed forces and organized reserves have required activation of the unorganized militia in recent memory, it still exists and is the militia embodied in the 2d amendment.