How do you use grep to find terms that are n characters long?

Question

I'm using grep to filter the Mac OS X dictionary words file (by default located at /usr/share/dict/words).

I want to use grep to retrieve all words four characters long. How do I do this?

My best guess for how to do this was:

grep [:alpha:]{4} words

But that returns zero results.

Answer 1

Trying to remember how much of regex language grep supports... does

grep '^....$' words

work for you? Note that since you are searching a dictionary file, I'm not sure you need to restrict yourself to letters.