RegEx is fun
🙂
Take a look at the IP Address examples for TCL on this page:
http://wiki.tcl.tk/989
Here's a quick and simple one you could use: ^[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.6$
It doesn't do any checks to make sure your octets are 0-255, but that doesn't seem necessary in your case since it can't be configured out-of-bounds anyway.
-Drew