Logging logstash是否支持正则表达式中的If-Then-Else条件？

我尝试使用follow regex:a？b？1c | d。这是我们的榜样

但我犯了个错误

/opt/logstash-1.4.2/bin/logstash -e 'input { stdin { } }  filter { grok { match => [ "message", "(a)?b(?(1)c|d)" ] } } output { stdout { codec => rubydebug }}'
+---------------------------------------------------------+
| An unexpected error occurred. This is probably a bug.   |
| You can find help with this problem in a few places:    |
|                                                         |
| * chat: #logstash IRC channel on freenode irc.          |
|     IRC via the web: http://goo.gl/TI4Ro                |
| * email: logstash-users@googlegroups.com                |
| * bug system: https://logstash.jira.com/                |
|                                                         |
+---------------------------------------------------------+
The error reported is: 
  undefined group option: /(a)?b(?(1)c|d)/

Logstash使用。他们的文档说，与Perl5.8.0相比，这是一个缺乏的特性。所以，不