Basic Arduino questions

In the versions of the Arduino with a voltage regulator I am wondering is it only from the plug in port that is regulated or is it regulated from the pin input labeled power as well? I have 12v in 1 spot and 5v in another but I have 0 of thoes round adapters. I guess I could solder input wires on in place of that plug but I prefer not to. So basically if I put 12v in the power input pin will it be converted to the correct voltage or go boom

Both the DC socket and V-in are the input for the 5volt regulator.
The only difference is that the DC socket is reverse voltage protected and V-in is not.

12volt on the DC socket or V-in is nearing the limit of the input voltage of the 5volt regulator, and is only ok if you don’t draw a lot of current from the I/O pins and/or the 5volt and 3.3volt pins.

Better/safer to use a 7.5volt or 9volt regulated supply on the DC socket.
Another option is a 5volt cellphone/tablet charger, connected to the USB socket.