It appears from the test case that this behavior was added on purpose. @44px do you have a specific use case where you need this behavior?

I think it makes sense to prevent empty tag/fields keys and tag values, though I'm less sure about field values. It's imaginable that someone would want an empty string as a field value, but I imagine it is almost always unwanted.

I also think this is kind of a roundabout way to fix the issue. It prevents a non matching pattern from clearing out fields/tags by preventing empty values, but I think it would be better if we also directly fixed the core issue: when the pattern doesn't match it shouldn't modify the key/value at all (pending response from @44px).

Nope, I don't rely on this behavior. I added this because it seems more correct to emit empty string instead of original tag/field value when result_key specified and pattern doesn't match.

For example, having config like:

[[processors.regex]]
  [[processors.regex.fields]]
    key = "request"
    pattern = "^/api(?P<method>/[\\w/]+)\\S*"
    replacement = "${method}"
    result_key = "method"

And given request="/favicon.ico" it should emit method="", not method="/favicon.ico".

when the pattern doesn't match it shouldn't modify the key/value at all

You're right, it seems like more correct behavior. Plugin should not alter existing or emit new tag/field (depending on result_key) if there is no match. Here is my fix with new behavior: #4396

Fixed in #4396