Logstash configuration error when using if test with regular expression

error_message.txt

# /opt/logstash/bin/logstash -t -f logstash.conf
SyntaxError: (eval):164: syntax error, unexpected tRPAREN
        if (((event["[syslog_program]"] =~ /^postfix//)))
                                                      ^
        eval at org/jruby/RubyKernel.java:1121
  initialize at /opt/logstash/lib/logstash/pipeline.rb:28
     execute at /opt/logstash/lib/logstash/agent.rb:112
         run at /opt/logstash/lib/logstash/runner.rb:168
        call at org/jruby/RubyProc.java:271
  initialize at /opt/logstash/vendor/bundle/jruby/1.9/gems/stud-0.0.17/lib/stud/task.rb:12

logstash.conf

        if [syslog_program] =~ "^postfix/" {
            grok {
                # Convert combined postfix program name into separate parts
                match   => [ "syslog_program", "^postfix/%{GREEDYDATA:postfix_program}" ]
                add_tag => [ "grokked" ]
            }
            if "grokked" in [tags] {
                mutate {
                    # Set syslog_program to "postfix" if tagged and grokked
                    replace    => [ "syslog_program", "postfix" ]
                    remove_tag => [ "grokked" ]
                }
            }
        }