What regular expression can I use to match an IP address?

try this:

grep -E '[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}' /etc/hosts

which matches all expressions from 0.0.0.0 to 999.999.999.999

with

grep -Eo '[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}' /etc/hosts

you will get IP addresses only

note:
on solaris probably egrep will do the job.

How's this:

perl -MRegexp::Common=net -ne '/($RE{net}{IPv4})/ and print "$1\n"' /etc/hosts

The

-w / --word-regexp 

flag to grep makes it only match on word boundaries, meaning that your match must either be surrounded by whitespace or begin / end at the beginning / end of the line!