gistfile1.txt

input { stdin { type => "test"}}

filter {
  grok {
    type => "test"
    pattern => [ "%{NUMBER:number}" ]
    add_field => [ "x", "%{number}" ]
  }
}

output { stdout { debug => true debug_format => "json"}}

123
{"@source":"stdin://max.local/","@fields":{"x":["%{number}"]},"@timestamp":"2012-11-02T00:44:09.173Z","@source_host":"max.local","@source_path":"/","@message":"123","@type":"test"}