Apparently different and not 100% compartible libraries are used on the Frontend, partictlarly in the section "Administration">"General">"Regular expression" and on the Server, in my example in LLD filter.

Synopsis:

We are creating Regular expression to use with LLD filter

^Ethernet\d{3}/1/ [Result is FALSE]

This regular expression verifies, and when we are testing it with the following string, it is works in Frontend as expected:

Ethernet119/1/1      Match, so result is false
Ethernet1/19         No match, so result is true

But when we are adding this expression to use with LLD filter, it doesn't work. The entries, which this expression suppose to filter-out are passing through. Re-test expression in "Regular expressions" - works, re-created LLD rule - doesn't work. The solution were to modify the regular expression. When we changed it to

^Ethernet[0-9]{3}/1/ [Result is FALSE]

it begins to produce predictable results in both, "Regular expression builder" and in LLD filter.