> eg to accept say "skip=????" (ie a four digit number
> such as 1475) but reject "skip=???" (ie a three
> digit number such as 755).

-www.example.com/*skip=*[0-9]*[0-9]*[0-9]
+www.example.com/*skip=*[0-9]*[0-9]*[0-9]*[0-9]

Should do the trick.