Shell Programming and Scripting

echo 'This cat is a cat with a tail longer of all cat basdf' | sed 's/\(.*\)cat\(.*\)/\1\2/'

while(<DATA>){
	s/cat(?!.*cat.*)/---/;
	print;
}
__DATA__
This cat is a cat with a tail longer of all cat.
This cat is a cat with a tail longer than any others.
This is a cat.

__OUTPUT__
This cat is a cat with a tail longer of all ---.
This cat is a --- with a tail longer than any others.
This is a ---.