usr/lib/learn/editor/L38.2a

#print
Sometimes you want to recognize a string of characters
only if they appear at the beginning of the line.
The character '^' is used for this purpose.  The
command
  s/^ab/xy/
will change "ab" to "xy" only if "ab" begins the line.
Try the following and watch what it does.

ed file
1,2p
1,2s/^ab/xy/
w
q
ready
#create Ref
xycd
  abcd
#create file
abcd
  abcd
#user
#cmp file Ref
#log
#next
38.2b  5