What is the difference between atomic and non-capturing groups?

Question

1. What is an atomic group, ((?>expr)) and what is it used for? In https://www.regular-expressions.info/atomic.html, the only example is when expr is alternation, such as the regex a(?>bc|b)c matches abcc but not abc. Are there examples with expr not being alternation?
2. Are atomic and non-capturing groups, ((?:expr)) the same thing?

Answer 1

Atomic groups (and the possessive modifier) are useful to avoid catastrophic backtracking - which can be exploited by malicious users to trigger denial of service attacks by gobbling up a server's memory.

Non-capturing groups are just that -- non-capturing. The regex engine can backtrack into a non-capturing group; not into an atomic group.