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Cold, not when you breathe normally but if you have a free flowing regulator it is very cold.
Regulators work on a diaphragm principal. It is opened when you take a breath due to a vacuum that is created, but when you do not it is in place. This is a very simplistic view of how it works. Also remember that there is a first and second stage in a regulator. Pressure is reduced by the first stage down to (I believe, I am too lazy to look at my regs right now) 300 psi from the 3,000 or more PSI in the tank.