Sometimes it is desirable to specify that
the characters to be changed are at the beginning
of the line. This uses the '^' character. For example,
will change "ab" to "xy" ONLY if "ab" begins the line.
would just change the first "ab" anywhere in the line.
In this directory there is a file "text"; whenever the
letters "cat" begin a line, change them to "mouse; but leave
other instances of "cat" unaltered. Do this on each line (it only
takes one command, as you know). Then rewrite the file and
let the cat out of the bag
let the cat out of the bag