4.3BSD-UWisc/lib/learn/editor/L33.1a

#print
When you want to match one of a few specific characters, and not
just any character, the pattern is similar to the pattern used
in filenames.  The patterns used with the editor, by
the way, are called "regular expressions".  The string
   [abc]
matches 'a', 'b' or 'c'.  So if the current line
is
   now is the time for all good men
and the command is
   s/t[ijk]/xx/
what word is changed?  Type "answer WORD" where WORD is the
word changed by that command.
#copyin
#user
#uncopyin
#match time
#log
#next
33.1b 10