logstash: pulling json from a message into the event itself

gistfile1.txt

% ruby bin/logstash agent -e 'filter { grok { pattern => "^.*?\{%{GREEDYDATA:jsoninsides}\}" } mutate { replace => [ "jsoninsides", "{%{jsoninsides}}" ] } json { jsoninsides => "@fields" } mutate { remove => "jsoninsides" } } output { stdout { debug => true } }'

input (on stdin)

hello world { "foo": "bar", "super": 123.456 } blah blah blah

logstash.conf

input { stdin { type => "stdin" } }
filter { 
  grok { pattern => "^.*?\{%{GREEDYDATA:jsoninsides}\}" } 
  mutate { replace => [ "jsoninsides", "{%{jsoninsides}}" ] } 
  json { jsoninsides => "@fields" } 
  mutate { remove => "jsoninsides" } 
} 
output { stdout { debug => true } }

output

{
  "@source"      => "stdin://pork/",
  "@type"        => "stdin",
  "@tags"        => [],
  "@fields"      => {
    "foo"   => "bar",
    "super" => 123.456
  },
  "@timestamp"   => "2012-06-14T18:41:09.165430Z",
  "@source_host" => "pork",
  "@source_path" => "/",
  "@message"     => "hello world { \"foo\": \"bar\", \"super\": 123.456 } blah blah blah"
}