Instead of acting on items when the logfile is being read, they act on individual lines in the output. In fact, there are several differences, resulting from the different times at which the aliases are processed. Otherwise you might as well go straight on to the next section . There are lots of implementations of regular expressions. If you're familiar with regular expressions from Perl, or even from GNU grep -E , you will not find all the same features here.
Regular expressions match if they match just part of the string.