This defines a range of characters matched in regular expressions:

zarrelli:~$ ls | grep ^[c-m]
comment.sh
copy.txt
c-style-counter.sh
c-style.sh
curly.sh
external-data
external-script-return.sh
external-script-return-whatever.sh
external-script.sh
menu.sh

In this case, we matched all the filenames starting with a character in the range between c and m.