From the GNU sed documentation:
If no -e, -f, --expression, or --file options are given on the command-line, then the first non-option argument on the command line is taken to be the script to be executed.
Your two sed commands each has one non-option argument, which gets treated as the script. It would be better practice to always explicitly put a -e in front of the script. Then you can write the command this way, as just one command instead of a pipeline:
sed -r -e 's/^[[:alnum:]]*.[[:alnum:]]*.?[a-z]*.[[:blank:]]+[0-9]+[[:blank:]]+IN[[:blank:]]+[A-Z]+[[:blank:]]+//g' \
-e 's/\.*.$//' test
It looks like you are attempting to craft the first regex to validate each column, checking that the first column looks like a domain ending with a dot ([[:alnum:]]*.[[:alnum:]]*.?[a-z]*.), the second column looks like an integer ([0-9]+), the third column is IN, and the fourth columns is a record type ([A-Z]+).
The regex for the first column probably doesn't work the way you expect. Each . means "match any character"; it does not mean "match a dot character". To match a dot character, you would write \. instead.
If you just want to extract the last column without validation, and suppressing the trailing dot, you could just write instead:
sed -e 's/.*[ \t]\([^ \t]*\)\.$/\1/' test
[^ \t]*\.$ should match the last column ("all non-space characters followed by a dot at the end of the line"). The parentheses capture everything except the trailing dot. \1 is a backreference referring to the first and only captured group.
I've opted to use [ \t] instead of [[:blank:]] because the latter is an extended regular expression, which is a non-standard GNU extension, and the -r option makes your command less portable.
