testing castability of VARCHAR data to INET/CIDR

Hello,

I have a VARCHAR column containing mostly ip addresses, with an
occasional piece of junk, and would like to transfer this data to an
INET column. The UPDATE (SET inet_col = CAST(vc_col AS INET)) aborts
as soon as it hits an invalid datum. I'm looking for a way to add
something like WHERE IS_CASTABLE(vc_col, INET) to the update.
Is there anything ready to use? I don't see anything directly usable
in src/backend/utils/adt/network.c.

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