I think you've almost answered your own question there.
NOTE: egrep is the same as grep -E
What you have there:
ls | egrep [a-z]{5}
That will match any file that has a 5 letter name with all lowercase characters. But it will also match any files that have more than 5 lowercase characters.
If you only want matches that exactly match the expression, then you can either surround the expression with ^ and $
e.g.
ls | egrep ^[a-z]{5}$
The above will match all files that contain exactly 5 lowercase letters.
OR
Another way of doing exactly the same thing is to use greps -x option.
e.g.
ls | egrep -x [a-z]{5}
Above will match all files that contain exactly 5 lowercase letters.
Either of those will do what you want.
But if you want to include any valid uppercase, or lowercase alphabetic, or numeric digits, you could use a character class.
e.g.
ls | egrep -x [[:alnum:]]{5}
The above matches all files that contain exactly 5 characters (uppercase, lowercase, or numeric characters for the environments current LOCALE setting)
Another way to do it would be:
ls | egrep -x .{5}
Which would match all 5 letter file-names, regardless of which characters were used.
