Can PoE switches be used as regular switches?

The answer is YES. But, The premise is that POE switches must comply with the IEEE802.3af, IEEE802.3at, or IEE802.3bt protocol standards.

POE switches that support IEEE802.3af/at will undergo a testing process before power supply

When the POE switch starts working, it outputs a very small voltage at the port until it detects that the cable terminal is connected to a power receiving device PD that supports the IEEE802.3af/IEEE802.3at/IEEE802.3bt standard (a new standard based on Power over Ethernet POE). Afterwards, POE switches may classify PD devices and evaluate the power loss required by PD devices. Then, the POE switch starts supplying power to the PD devices from low voltage until it provides 48V DC power. If the PD device is disconnected from the network, the POE switch will quickly stop supplying power to it and repeat the detection process to check whether the cable terminal is connected to the PD device.

Therefore, PoE switches can be used as regular switches without harming non PoE device