Can you use your IPhone to record a concert?

Can you use your IPhone to record a concert? I've seen numerous people on YouTube capture the whole performance of a concert while sitting in and watching it. I mean who's to tell you out of the thousands of people in the audience that you can't record? How could they possibly even notice it.

No, you can't. Unless the people giving the concert say so beforehand.
Yes, thousands do it. They are breaking the law.
If they distribute the video on YouTube or other website, they can be found and fined.
If they are noticed during the concert, they can be evicted, no refunds.
Seen it happen.

No, it is illegal.

It will depend if it's allowed or not by the performers or venue's administrator.

In theory you are not supposed to do that, and some venues even have posted signs forbidding it, and you risk being kicked out.

But the truth is that enforcing the no-recording order in a very crowded event could be problematic.