Hello if unregulated power supply (transformer, bridge and main caps) is 30 cm away from the circuit to feed, is it better to put the voltage regulator close to the circuit (long input and short output) or close to the power supply (short input and long output)?
If your DC currents are high and the regulator is at the power supply then you will either need to use heavy cables and connectors or you may experience voltage drops along the wires that make the supply voltage too low.
However, 30cm is by no means long for power cables.
If the supply is only used on the target board it is generally better to do the regulation on the board itself so your wiring and connector resistances are a bit less critical. It also alleviates you of the necessity of having a separate PCB to house the bridge rectifier and capacitors which can also be located on the target board.
If the power supply is shared across multiple boards, sensors, indicators etc, it may be more appropriate to have a central power regulator rather than add additional connectors to the main board. However, when you do that you have to pay special attention to ground and power loops that may introduce noise issues.