This class was extremely difficult I would recogmend not taking it with him. This class is also technically skippable. If you must take this class take it with a diffrent professor.
He is a great guy, funny, and lectures well. But his exams can be astronomically difficult.

Regan doesn’t know the material, and doesn’t want you to know it either. 17 is supposed to be an introduction to atomic phenomena and some elements of quantum mechanics. He refuses to make a class that is in itself supposed to be easy actually digestible, by including as much content as possible, and presenting it as superficially as he can. He mentioned geometric algebra (cause I cannot use the word “taught”), only to not use it again in the remainder of the course. He usually starts using some physical concepts under the umbrella of “I’ll make it clear later” and then forgets to define it properly until everyone is utterly confused. He spent an entire lecture talking about fermion and boson statistics, which required attention to the effects of spin-coupling. He used the word “spin” about 30 times that entire lecture, without defining spin once. The first question he was asked after the class was “What is spin?” He went out of his way to give the most rigid, abstract, confusing, and mind-boggling presentation of one feature (the turning of a playing card), instead of defining it for the means of a lower division class. The student was visibly confused. They were speechless. Regan smirked and moved on. He didn’t even ask the student if it made sense. He just moved on. That happened to me too. He just doesn’t know the material profoundly enough to actually answer questions, or teach for that matter. And he enjoys confusing his students. Otherwise, he would not smirk so visibly when someone doesn’t know something.
His homework is unreasonably difficult. He always taught the concepts needed for 80% of an assignment the day it was due. Since his lectures lacked any conceptual insight and were nothing but regurgitations of formulas, the assignments felt demoralizingly difficult. And the grading was harsh and lazy. His homework problems had a lot of subsections, and if you had one typo for one, your score for that problem started from 75%. It didn’t matter if everything but your answer for one subsection out of say 5 was perfect, you still got a 75% for the problem.
His exams require literal memorization of problems, instead of conceptual understanding. Preparing for exams was a witch hunt of “What could Regan decide to test us on?” He didn’t teach what a blade in geometric algebra was. He briefly described it in some typed lecture notes on his website. We got that on our exam. And he shamed us publicly for not knowing it in the lecture notes.
Regan is too concerned with trash talking other physicists to actually do his job. He builds his personality on the hate he has for the textbook, the constants in physics, or different historical concepts. No human being should build a personality on what they hate. That is not just counterproductive, but harmful. As a physics major, I want to be encouraged by my professor’s love for physics, not their hate for what they perceive to be “stupid.”
Don’t take his class. Just don’t. As a physics major in college you deserve a professor that is knowledgeable, passionate, and sincere. Regan is none of those things.

This class for most of the quarter was quite difficult and had me questioning my major. There were two midterms, a final, and a bunch of homework. The homework was very doable and Regan would almost always extend the deadline. The midterms were difficult but overall I would say they were fair. The final was also fair in my opinion. Some tips for future people taking this class: 1) Don't freak out if you start off the quarter not knowing wtf is going on in the class. The first lecture petrified me and I didn't think it would get better from there, but it does. 2) KNOW THE HOMEWORK. It seems that Regan really liked pulling questions from the homework to put on the midterms/final. If you really know the homework and the ideas that were covered then you will do fine on the exams. 3) Don't freak out if you do poorly on the first (or maybe second) midterms. Since Regan curves, as long as you stay near the average you should do fine. I was below average by a lot on the first midterm, around average on the second midterm, but I crushed the final, so I ended up with an A-. For me, Regan's lectures weren't that helpful, but they were entertaining. I found myself mostly self-studying for the course. Overall, study the homework/textbook really hard and don't freak out and you should do fine.

Weakest bivector enjoyer absolutely clears strongest cross product fan

This is probably the strangest class I have ever taken at UCLA. I read a review from a while ago, and that student mentioned how Regan does not know the material and that he doesn't want you to know it either. How can this be? He's mostly in charge of the curriculum, right? Well, the homework is rarely anything like the lectures. Going to office hours usually does not help with the homework because he doesn't know how to solve the problems half the time, and the other half of the time, he just refuses to explain how to do it. Unless a student starts working out the solution, you're only getting an answer if you go to TA office hours. The lectures themselves are mostly fascinating. He has a very unique perspective on physics, and despite my opinion of him as a lecturer, I can say "I think Regan is low key right." The exams are interesting. He holds true to his promise that if you do the homework and actually understand the process (or just memorize the problems), you should get at least some kind of B. That said, the exams are brutal when you're taking them. You can usually get a lot of points from partial credit, so finish each problem to the best of your ability. Just write formulas if you don't know how to even start. About half of the points on the exams come from questions very similar (or identical) to former exams he distributes. The other half come from questions that vary in difficulty from "yeah I could probably figure this out if I studied a bit more or wrote that one equation on my index card" to "how tf do I even start this" (looking at you, examons). As long as you can get most of the first 50 points and then get partial credit, you should have some kind of a B. The curve is quite generous for people who end up with some flavor of A as well.

Ultimately, I cannot really say I actually learned everything I was supposed to. This is kind of a crash course in quantum mech, stat mech, and thermo, which is an odd combination, especially considering each of these classes are their own upper division (quantum mech is actually THREE separate courses). It very much is a speedy overview, and the class is more about learning the exam questions (study the homework questions and the practice exams) than actually mastering content. Your TAs are a great resource since the textbook (Serway) is not that great. Aside from Regan's notes, the hw, and the practice exams, there are not many resources, and as a textbook enthusiast, I found that difficult to get used to. I do recommend Seway for the Bohr model stuff.

One of the most interesting and important courses in UCLA Physics undergrad in my opinion, as it gives you a taste of real physics after the intro courses. The breadth of this course intrigued me the most (covers a lot of ground), and despite its difficulty, it is nevertheless a highly enjoyable course. Homework and exams are challenging, but helps strengthen your knowledge of physics and builds the key fundamentals. Prof Regan is very enthusiastic and engaging in his lectures, although it is pretty fast-paced and hard to follow live without pre-reading the notes beforehand. Homework is interesting although it can be very time-consuming. Mastery of lecture notes is important, as it contains a lot of useful information as well. Great course and would definitely recommend!

Notorious for a reason. Ask around the department and they can provide you a thorough synopsis. His class is the only one I've ever happily and excitedly attended.
Iconic and difficult professor, worth it for the experience IMO

Modern physics was a cool experience! Regan definitely added his flavor to the course, which made the first 3 weeks feel a little rocky because the material taught didn't run very parallel with the textbook, but by week 9 it all came together and wrapped up super nicely. Side-note: He has a mad passion for physics, which made each lecture that much more enjoyable. The lectures are spent motivating and deriving some of the main ideas/equations that are relevant to whatever is being discussed that day. Something that i thought was awesome is Regan provided typed-up lecture notes on BruinLearn(also recorded lectures), so throughout the quarter I felt free to sit and tune-into a lecture, without feeling the need to write down everything as he lectures. Also, having a thorough understanding of the homework problems is important, as he tends to pick similar problems for the exams, so with that said, re-working old problem-sets is a great way to prepare for exams. Doing a lot problems/exercises in general is the other secret!! - solve as many problems as possible(solve them honestly!), it'll help a ton(this is probably general advice for physics/math, but it helped a lot in this course). Another thing with hw, he almost always extends the deadline, which was super nice. The exams are curved, so staying near/above the average is what really counts(obviously you want to work and shoot for the stars, but keep this in mind if things don't go so well). I was around avg on the first midterm(54%), nicely above avg on the second midterm, and did well on final. SO even if that first midterm doesn't go so well, just learn from it, figure out what went wrong and keep at it. Tbh, this class does require a lot of work, but the effort is worth it! Goodluck !!

This class took away any doubt I had about majoring in physics. Regan started off at a hundred miles an hour with geometric algebra and stat mech but backed off quite a bit after the first midterm. The first 3-4 weeks were absolutely terrifying, especially since his lectures moved incredibly quickly. Don't try to take notes during lecture unless you already have some understanding of what he's talking about - he complained a lot about the textbook but ended up following it pretty closely, so try to at least skim the section before lecture. The class slowed down a lot after midterm 1 when we got into the background and very basics of quantum, which was by far the easier part of the class.
He assigned a lot of homework but was very forgiving with deadlines and his exams were very fair. Learn the homework well and you should be fine for the exams, many of the problems were similar or identical to ones we had already seen. Also, don't expect to get much homework/content help from office hours - Regan would always start off trying to answer questions but would quickly get distracted and go off on tangents about his own interests. This ended up being my favorite part of the class and I would highly recommend going, he brought up so many little things that made me even more interested in physics. He has a thing against the cross product as well as hbar and other "fundamental" constants like c and the Boltzmann constant and prefers to describe everything in terms of frequencies but he won't force that on you in exams. In summary - Regan is a little crazy and makes you feel like you have no idea what's happening but taking it with him is absolutely worth the extra effort, at least it was for me.

The professor drops a lot of formulas throughout the class. While he often does not give explanations as to where they come from, he will explain them in his office hours. Reused questions from the homework for midterms and questions from the midterms for the final. Going to class is essential.