how to use filename-regex option in int-ftp:outbound-gateway?

firstly thanks for attention
i defined ftp outbound adapter with ls command and Recursive mode in my spring integration project, i want to filter and get files in specified sub-Directories, server directory structure is:
root
----------a\
---------------in\
---------------------a.op
----------b\
---------------in\
---------------------b.op
i want to get a.op and b.op files, i set filename-regex option to ([a-z]|[in]|.*\.op) but not worked correctly and only first level directory filtered, and my adapter code is:

 <int-ftp:outbound-gateway id="gatewayLS"
                              session-factory="ftpSessionFactory"
                              request-channel="inbound"
                              command="ls"
                              filename-regex="([a-z]|[in]|.*\.op)"
                              command-options="-R"
                              expression="payload"
                              reply-channel="toSplitter"/>

how to solved it? thanks.