grok过滤器（regex）提取方括号内的字符串

我的应用程序日志条目如下：

2015-06-24 14:03:16.7288  Sent request message [649b85fa-bfa0-4cb4-8c38-1aeacd1cbf74] <Request>sometext</Request>

2015-06-24 14:38:05.2460  Received response message [649b85fa-bfa0-4cb4-8c38-1aeacd1cbf74] <Response>sometext</Response>

我正在使用logstash grok过滤器来提取xml内容和带有方括号的客户端令牌。

grok {  
    match => ["message", "(?<content>(<Request(.)*?</Request>))"]   
    match => ["message", "(?<clienttoken>(Sent request message \[(.)*?\]))"]
    add_tag => "Request"
    break_on_match => false
    tag_on_failure => [ ]
}

grok {  
    match => ["message", "(?<content>(<Response(.)*?</Response>))"] 
    match => ["message", "(?<clienttoken>(Received response message \[(.)*?\]))"]
    add_tag => "Response"
    break_on_match => false
    tag_on_failure => [ ]
}

现在结果如下所示

对于第一条日志行：

Content =  <Request>sometext</Request>
clienttoken = Sent request message [649b85fa-bfa0-4cb4-8c38-1aeacd1cbf74]

对于第二条日志行：

Content = <Response>sometext</Response>
clienttoken = Received response message [649b85fa-bfa0-4cb4-8c38-1aeacd1cbf74]

但我希望结果是这样的：

Content = <Request>sometext</Request>
clienttoken = 649b85fa-bfa0-4cb4-8c38-1aeacd1cbf74

请让我知道如何仅提取方括号内的字符串，而不提取模式中所有匹配的字符串。