How do you reinstall nano on Mac OS X?

1. Download the nano source (I recommend the tar.xz one) (nano-editor.org/download.php)
2. Go to the folder it was downloaded to (By default it downloads to ~/Downloads but if you changed the directory where files download then just go there).
3. Extract the nano source you downloaded.
4. Open your terminal
5. Type "cd " without the "" then drag and drop the folder you extracted and click enter.
6. run "./configure" without the "".
7. run "make" without the "".
8. run "sudo make install" without the "".]

You have now successfully installed nano!

Update: The answer below is no longer accurate. As of Apr 4, 2017, the formulae were moved to homebrew/core. See https://github.com/Homebrew/homebrew-dupes/commits/master/README.md

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The reason you can't find it in homebrew is because homebrew's main repo doesn't include programs that come with the OS. You have to add an additional repo:

brew tap homebrew/dupes
brew install nano

https://github.com/Homebrew/homebrew-dupes