Cannot use regex_match in select

I'm trying to do:

{{ list_of_strings | select('regex_match', something + 'somethingelse') }}

To avoid needing another task doing just:

echo '{{ list_of_strings }}' | grep '{{ something + 'somethingelse' }}'

But I'm told:

no test named 'regex_match'

Is that because regex_match is defined as a filter and not as a test ?

I couldn't find a way to use:

{{ list_of_strings | select('in', something + 'somethingelse') }}

To work either, is this expected to work ?

Start by tracing how select resolves named tests, then inspect the registration and behavior of regex_match and in. Reproduce the two template examples with a minimal input and determine whether the expected result is supported; done means the behavior is fixed or clearly documented.