Isn't that more or less the same as what it already does? That is by telling frak which words you want to match you're implying you don't want to match anything else.

Of course, you could generate two patterns; one with the words you do want to match and the other with words you don't. Then simply handle the logic in your program.

frak seems to generate a regex that precisely only matches the input list of strings, no generalizations.

So this seems not to be the tool to auto-create a .gitignore file...