Fluentd: Error: Invalid Conditional Pattern for, apparently, valid regex

[robert-7]

fluentd or td-agent version

td-agent-2.3.1-0.el6

Environment information, e.g. OS

CentOS 6

Your configuration

<source>

  @type tail
  path /var/log/minimal.log
  pos_file /var/log/td-agent/pos/minimal-log.pos
  tag minimal
  format /(?(?=request:)request:(?<request>.*))/
</source>

<match **>
  type bufferize
  buffer_type file
  buffer_path /var/log/td-agent/*.buffer
  <config>
    type secure_forward
    secure true
    shared_key ---------------
    self_hostname ${hostname}
    ca_cert_path /etc/td-agent/ssl/certs/ca_cert.pem
    <server>
      host fake.hostname.com
    </server>
  </config>
</match>

Your problem explanation. If you have an error logs, write it together.

When I try to start td-agent, I get an error saying my conditional pattern is invalid.

# /etc/init.d/td-agent start
Starting td-agent: 2016-08-15 12:19:07 -0400 [error]: fluent/supervisor.rb:359:rescue in main_process: config error file="/etc/td-agent/td-agent.conf" error="Invalid regexp '(?(?=request:)request:(?<request>.*))': invalid conditional pattern: /(?(?=request:)request:(?<request>.*))/"
                                                           [FAILED]

This is also happens when I use Fluentular:
goo.gl/URE310

However, when I use regex101 to verify my statement, it works just fine.
https://regex101.com/r/lT9vW6/4

Could someone please point out my mistake or what's expected differently?

[repeatedly]

Ruby's regexp is not based on PCRE.
Your pattern is PCRE specific syntax, right?

[tagomoris]

It uses PCRE extended regular expression syntax.

[robert-7]

I'm not sure what the differences between the syntaxes are in this case, but it was resolved with this small change:
format /((?=request:)request:(?<request>.*))/

[Crystark]

Just so it's clear and in a same post to compare characters (also added a non capturing version for the global if):

Original non working
format /(?(?=request:)request:(?<request>.*))/
@robert-7 's version
format /((?=request:)request:(?<request>.*))/
non capturing global if
format /(?:(?=request:)request:(?<request>.*))/