Logstash filter issue

I try to parse the log.
if the log "didn't" have 'specific' like as follow logstash prints ,then will be drop the event This is what logstash prints:

"message" => "...",
"host" => "",
"@version" => "1",
"@timestamp" => "2016-04-12T02:28:18.233Z",
"type" => "snmptrap",
"1_3_6_1_2_1_1_3_0" => "21 days, 17:08:57.94",
"1_3_6_1_6_3_1_1_4_1_0" => "specific::userdefined",

I can only specify the content of the field (prefix is specific) rather than its field name (1_3_6_1_6_3_1_1_4_1_0) ,so how to parser the log to filter and to drop the event?
What do I need to do?
If anyone knows how to do with it , please help. Thanks.