Gmail – Search for a sender on Gmail

This is a result of Gmail ignoring periods (".") in email addresses, a function. You may need to build a filter targeting all the email addresses from whom you receive the offending messages:

from:spam@linked.in OR spam@registry.in OR ...

A pain, I know, but given Google is unlikely to change how the period is interpreted, it may be your only option.

From the "About Gmail Search" documentation:

Gmail doesn't recognize special search characters like square brackets, parentheses, currency symbols, the ampersand, the pound sign, and asterisks.

As you appear to have already discovered, and my quick tests appear to confirm, this seems to include the exclamation mark character.

I would assume that the regular expression Google are using to match search queries with possible results simply excludes all special characters and operates only on "word characters" (you can somewhat confirm this by the fact that searches for the underscore character, _, do work - the underscore is treated as a word character in a regular expression).