Make fields optional in a Logstash Grok pattern

Wrap an optional segment in a group followed by ?. Include the segment's separator in that group so the pattern still matches when the field is absent.

For example, these lines differ only by user ID:

2024-01-15 10:30:00 INFO User logged in
2024-01-15 10:30:01 INFO User 42 logged in

Keep the space optional too

filter {
  grok {
    match => {
      "message" => '^%{TIMESTAMP_ISO8601:timestamp} %{LOGLEVEL:level} User( %{INT:user_id:int})? logged in$'
    }
  }
}

( %{INT:user_id:int})? consumes either a space followed by an integer or nothing at all. The captured ID is numeric. Under Grok's default empty-capture behavior, a missing optional capture does not create a field. This assumes the event did not already contain that field; an unmatched group does not clear an earlier value.

Add other optional segments carefully

This example permits an optional username and optional duration:

grok {
  match => {
    "message" => '^%{IP:ip}( - %{WORD:user})? "%{WORD:method} %{URIPATH:path}" %{INT:status:int}( %{NUMBER:duration:float})?$'
  }
}

The URL here is a path without a query string, and WORD deliberately limits the username format. Use patterns matching your actual data if usernames contain punctuation or request targets include query parameters.

Check the captured field before using it

if [user_id] {
  mutate { add_field => { "user_kind" => "identified" } }
} else {
  mutate { add_field => { "user_kind" => "anonymous" } }
}

A Logstash condition such as if [user_id] also treats false and null as false; it is not a universal existence test. It works for these numeric IDs. Keep a default label in a separate field instead of mixing the string anonymous into an integer field and causing Elasticsearch mapping conflicts.

Prefer separate patterns when formats diverge

grok {
  match => { "message" => [
    '^%{TIMESTAMP_ISO8601:timestamp} %{LOGLEVEL:level} User %{INT:user_id:int} %{GREEDYDATA:msg}$',
    '^%{TIMESTAMP_ISO8601:timestamp} %{LOGLEVEL:level} User %{GREEDYDATA:msg}$'
  ] }
}

Grok normally stops after the first successful pattern, so put the more specific format first. (...)? is usually sufficient; (... )?? changes preference to a lazy optional match and does not solve an incorrectly placed separator. Test both present and absent cases, plus malformed lines that should fail, with the Grok plugin.