how do you regain it???

You gain 1 sp every 3 minutes (I thinks that's it at least it maybe 2 minutes)
In addition to this there are medical supplies that restore SP

^not any more

Those were mostly for Beta testing purposes

the medical supplies are still there, you just cant buy them anymore, they must be scav'ed

Here is the most debated thing on the forum-Stamina. In REAL TIME, you gain 1 Sp every 3 minutes. So once you run out of stamina points, your basically done for the day. That sounds pretty harsh, no? Well, first off, it limits how much scavenging you can do. If you scavenge everything you see, you won't be playing a lot. Second, to off-set this, there are training locations at most mission-givers, if you have made good progress so far, you will have reached the collage safe-house that you were told of at your apartment. These training locations let you use the down-time when you are not playing by training your stats. Once again, this is in REAL TIME, you gain 1 point in whatever stat you were training for every ten hours you put into training. Don't worry, you will continue to train even when you are logged out, and you can cancel training any time and still receive the time you put into it. If you have any problems with the SP system (which I can bet you do), try looking around the forums for a debate on it. There's tons of them.