[Freeswitch-users] Help on Explaination of the configurations parrametters

Hi, I am new to FS and trying to setup a SBC using the example 2 in the FS wiki website. The link is http://wiki.freeswitch.org/wiki/SBC_FreeSWITCH_Configuration_Example_2. There are parameters in the example that I have to replaces for it to work with my own scenarios. However I have problem understanding the following lines. Is there anyone able to explain what does this mean <condition field="${sip_redirect_contact_0}" expression="(((192.168.)|(172.24.)|(10.10.))\d+\.\d+(:\d+)(;dtg=\w+)?)"> The bits I am not sure is )\d+\.\d+(:\d+)(;dtg=\w+)?). What does this do? Thanks in advance. Al