grep approach

To create a copy of the file without lines matching "cat" or "rat", one can use grep in reverse (-v) and with the whole-word option (-w).

grep -vwE "(cat|rat)" sourcefile > destinationfile

The whole-word option makes sure it won't match cats or grateful for example. Output redirection of your shell is used (>) to write it to a new file. We need the -E option to enable the extended regular expressions for the (one|other) syntax.

sed approach

Alternatively, to remove the lines in-place one can use sed -i:

sed -i "/(cat|rat)/d" filename

The  sets word boundaries and the d operation deletes the line matching the expression between the forward slashes. cat and rat are both being matched by the (one|other) syntax we apparently need to escape with backslashes.

Tip: use sed without the -i operator to test the output of the command before overwriting the file.

(Based on Sed - Delete a line containing a specific string)