Grok Plugin: load custom patterns in Oniguruma format from file

[who]

According to the documentation, we can use Oniguruma regex pattern formats in grok plugin (i.e. (?<field_name>the pattern here)).
Also, there's the patterns_dir configuration option that instructs grok filter to load patterns from files.

However, the format of patterns in files should be NAME PATTERN (presumably in each line). But I need to save patterns in files like below:

Patterns.txt file:

(?<field_name>the pattern 1 here)
(?<field_name>the pattern 2 here))

And then use these patterns in a way that do the same this as this configuration:

grok {
       match => {
         "message" => [
                  "(?<field_name>the pattern 1 here)",
                  "(?<field_name>the pattern 2 here)"
        ]
    }
}

So that the message field is being tested against multiple Oniguruma patterns, that are loaded from an external pattern file.

How would it be possible?