Logstash split field solution.

gistfile1.txt

Problem:  We have a log line that includes a perl class that we want to log the class 
and method in their respected fields.  An example class and method in perl:

Animal::Dog::bark

In this example, "bark" is the method.  "Animal::Dog" is the class.

After some searching and hacking, I found a solution that works with Logstash 1.4.2

Assume the input is "Animal::Dog::bark".

For completeness, I'm going to just add my entire configuration file I used for testing.  
Comments are included and should explain what's going on.

input {
 stdin { }
}

filter {
    grok {
        match => { "message" => "%{GREEDYDATA:api_class}" }
     }
    mutate {
        # split the field on ::
        split => ["api_class" , "::"]
        # save the last element of the array as the api_method.
        add_field => ["api_method", "%{[api_class][-1]}" ]
    }
    ruby {
        # Go directly to the array and remove the last element.
        code => "event['api_class'].pop()"
    }
    mutate {
        # Join together whats left as the class name.
        join  => ["api_class", "::"]
    }
}

output {
  stdout { codec => rubydebug }
}

I tried to use mutate's remove_field to remove the last element of the array but it didn't work.  
There are tickets created and possibly even a fix in the new version, however, this should continue 
to work as long as the ruby filter is around.

The output:

{
       "message" => "Animal::Dog::bark",
      "@version" => "1",
    "@timestamp" => "2014-12-09T13:38:58.178Z",
          "host" => "host.example.com",
     "api_class" => "Animal::Dog",
    "api_method" => "bark"
}

Hi. I came here from a stackoverflow answer: https://stackoverflow.com/a/46031524
I appreciate the Ruby code to understand some useful array operations :-)

I think your use case could be solved with a simpler grok pattern though, the trick is to leverage the end anchor "$" in the regexp

grok { match => { "message" => "^%{GREEDYDATA:api_class}::%{WORD:api_method}$" } }

I totally forgot about this gist! I'm glad it's helping others and inspiring better solutions.