Our last puzzle involves a search of the Unix file system for historical artifacts. Many of the original Unix commands were two letters long; the text editor was ed, the copy program was cp, and so on. Let us find all such commands. To solve the puzzle, you need to know that the very oldest Unix programs reside in the /bin directory. There are other old Unix command in /usr/bin. Be sure to also provide a count of how many such commands there are. Note: when you look in the /usr/bin, you may find some 2-character commands that are not old. To see if a command dates from the early days of Unix, check its man page. But you need not to implement this in your Regex.