I often use this to check website access logs by IP address. The problem is that it only includes IPV4 and not IPV6.
Any idea what regex I can use so that it includes (or runs a separate) command for IPV6?
cat access.log | sed -e 's/^\([[:digit:]\.]*\).*"\(.*\)"$/\1 \2/' | sort -n | uniq -c | sort -nr | head -50
Matching IP addresses via regular expressions can be tricky - yours matches lots of things that aren't valid IPv4 addresses, like 100000.55, for example.
There's a perl module, Regexp::Common that provides well tested regular expressions for matching all sorts of things, including both IPv4 and IPv6 addresses. If you install it (The Ubuntu package is libregexp-common-perl), you can replace the sed part of that pipeline with
perl -MRegexp::Common=net -lne '/^($RE{net}{IPv4}|$RE{net}{IPv6}).*"(.*)"$/ && print "$1 $2"'
to match both address families.