#print
When you want to match one of a few specific characters,
and not anything, the pattern is similar to that used in
file names. That is,
  [abc]
matches the letters 'a', 'b', or 'c', but nothing
else.  Try this sequence and note what happens:

ed lines
1,$p
1s/[123]/9/
2s/[123]/9/
1,$p
w
q
ready
#create Ref
599
948
#create lines
519
348
#user
#cmp lines Ref
#log
#next
33.2b 5