I haven't tested this, but I notice that Ubuntu has a patch for this package which adds libregexp-java as a build dependency in order to fix a FTBFS issue. Hm... might be worth checking out :)
For details, see http://patches.ubuntu.com/a/activemq/activemq_5.6.0+dfsg1-4+deb8u1ubuntu1.patch -- mvh / best regards Hans Joachim Desserud http://desserud.org
