Logstash mutate replace filter fails

Elastic Stack Logstash
5

I finally got it to work, the main problem was the if [source] =~ "file.log" statement was not grabbing the file thus it couldn't find "Z0" in the field "zone" that's why the mutate wasn't working. I solved it by removing that if statement so now the filter will be applied to the log files that filebeat is grabbing which are not of type syslog. I also fixed my syntax thanks to you guys because the syntax that i had wasn't correct.

Here is my final code:

filter {
    grok {
      match => { "message" => "%{DAY:day} %{MONTH:month} %{MONTHDAY:date} %{TIME:time} %{YEAR:year} \[%{GREEDYDATA:zone}\]\[%{GREEDYDATA:module}\]\[%{GREEDYDATA:severity}\]: %{GREEDYDATA:message_content}" }
      add_field => [ "received_at", "%{@timestamp}" ]
      add_field => [ "received_from", "%{host}" ]
         }
}
filter {
  if [zone] == "Z0" {
    mutate {
      replace => { "zone" => "Zone0 - BlockA" }
           }
                    }
}