That's very easy task for all Java's users. I think, that it task is easely than other task's.

yes, Strings are always immutable (unchangable)

But now you do...

Wish I knew the replace method......

Please use spoiler flag next time.

The description is confusing.
It is not clear what is the expected result and I suspect most of people will spend most of time trying to figure out not how to implement it, but what should they do.
I did it by trial and error.

There should be annotation like:
Return the number of ADULT PAIRS of rabbits at the BEGINNING of the mth month.

Thanks a lot!

I googled, and may be understood the patterns problem you mentioned. But this version is same as:
replace(char oldChar, char newChar) right?

In my opinion, they are worth it. They become much easier to read once you know a few concepts, and they are very powerful even when you are no expert at regex. Downside is that they are not maintainable for someone who hasn't spent at least a few days practicing them. I recommend this resource: https://www.regular-expressions.info/, it is much more in depth than the many online regex-tutorials, that are just way too shallow in my opinion (you can find the book of the website's author if you search it)