Is the proposition $P$ equivalent to the axiom of regularity

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My understanding of Axiomatic Set theory is not too deep so apologies if its not very clear. This question is assuming we are considering ZFC.

Background to question

Consider the proposition $P$. For sets $x,y$ we say define recursively $ P(x,y)$ is true $ \iff (x \in y)\lor (\exists a \in y (P(x,a)))$

Alternatively $P(x,y)$ is true if $x\in y$ or $\exists$ z at the end of a terminating chain of inclusions of $y$ such that $x \in z$.

It seems intuitively true to me that $P(x,x)\to\bot$ but I'm not sure how to formalize it.

Assuming $P(x,x)$ to be true we see that evidentially $x\not \in x$ so we must have that $\exists$ some $z \in a \in b \in \cdots\in x$ such that $x \in z$. But this just means you can follow a chain of inclusions of x and find x in itself again.

It seems analogous to a case of sets $(a \in b) \land (b\in a)$ but two such sets can never exist as it contradicts the axiom of regularity.

Question

First off how do I properly formalize this proof? And is the proposition P equivalent to the axiom of regularity (as it seems to only be decided by that)?