I am in a good paying job outside of aviation and getting ready to leave to a regional.
Instead of resigning, I am thinking of taking a Leave Of Absence, 3 months is the maximum allowed, I could maybe extend it to 4 with vacation time.
This is just in case something happens in training or I discover that this really isn't for me (but highly doubt it).
Is this option legally possible? Do I have to disclose this to the airline?
Appreciate the insight.
Thanks!

Probably a good idea, don't give up your day job to go to a regional. Go for a few months, get it out of your system and then go back to your life. If you enjoy flying, join a local flying club and fly on nice vfr weekend days. if you are a CFI you can trade BFR's and IPC's for free or reduced charge of aircraft.

Well, I may not be able to "get it out of my system"
I have delayed this switch for a while and I got to the point where it is now or never. As for your idea of joining a club, that's how I built most of my time over the years (in my own aircraft) but it doesn't seem enough...

While I completely recommend against leaving your current career for a regional, if you MUST do it, taking a leave of absence is a good idea if it is a available option. Training is generally around 2 months long and another month or two on the line enjoying life on reserve should give you a taste of regional life, which should be enough to help you make a decision. Also, if for some reason training goes south, you have something to fall back on.

Two other pieces of advice.
1. If you like where you live, pick an airline that is based there. Commuting sucks.
2. Avoid airlines that have any kind of "term commitment". You shouldn't have to promise to work anywhere for any certain period of time, especially if it turns out to be a really crappy airline, which most regionals are.

In conclusion, flying is a great hobby.... And should be kept that way.

I think it is a very wise idea. Don't pay much attention to the naysayers, finding out for yourself is the only way to be satisfied one way or the other.

You are under no obligation whatsoever to inform the airline (or any other employer) of your situation (Leave of Absence.) I would also suggest that informing any airline that you're on an LOA from another job, would probably significantly hurt your chances of getting hired.

Never a bad idea to have a back-up plan in place...

I am in your situation and will basically be doing the same thing. I have 18 weeks of vacation to burn and I plan on using as much as needed to make sure I get through training. I've been out of 121 flying for 13 years and, although I'm fully commited to returning, I have no delusions that it will be a cake walk to get back in.

Yes, I like where I live and don't plan to move. The problem is then, only Envoy has a large base here and I really don't know if it is an option right now.

Thanks for the words on encouragement!
I have been thinking about this long and hard and the LOA is the best option I could find, assuming it is approved.

One thing to consider is that ALL the major airlines now have contracts in place which allow them to use 76 seat regional aircraft. Almost all of ASA/XJT, Eagle are 50 seats or fewer. This applies to many of the other carriers too.

There is going to be a big shake-up coming as the transition to 76 seat aircraft occurs and as some of these regionals go out of business. XJT is losing about 50 aircraft by year's end. You are getting in during a time of much needed optimism in this industry, but also great upheaval. UAL lost 200 + million last Q while Delta made 200 million. There could be a substantial shrinkage in the regional airlines as the 50 seaters are retired. I think this summer is going to be chaos even more than usual.

Just some things to consider before jumping in right now.