It has it's faults but is also a bit more specific by looking for:
1-3 digits
followed by . or -
followed by 1-3 digits
followed by . or -
followed by 1-3 digits
followed by any number of any character
followed by a dot
followed by 2 or 3 characters a-z in caps or lower case
then end of string
My reasoning is that many generic names only have the first 3 sets of digits
of the IP and often leave off the 4th set. The regex above also looks for a
TLD of ONLY 2 or 3 digits so it cant accidentally pick up the .arpa record
from the PTR (which that particular server has a nasty habit of doing (It's
not an exim server)).
I am assuming that the regex format would be the same for Exim but someone
else may like to confirm or deny that !!