This

ls -l /var/log | awk '{print $9}' | grep "^[a-z]*.log."

outputs this:

alternatives.log.1

alternatives.log.10.gz

alternatives.log.2.gz

alternatives.log.3.gz

alternatives.log.4.gz

alternatives.log.5.gz

alternatives.log.6.gz

alternatives.log.7.gz

alternatives.log.8.gz

alternatives.log.9.gz

apport.log.1

apport.log.2.gz

apport.log.3.gz

but this:

ls -l /var/log | awk '{print $9}' | grep "^[a-z]+.log."

outputs nothing.

Why? I just changed * to +. Isn't it similar? Operator + just needs at least one match, and * zero or more.