Regex to find A and not B on a line

Question

I'm looking for a regex to search my python program to find all lines where foo, but not bar, is passed into a method as a keyword argument. I'm playing around with lookahead and lookbehind assertions, but not having much luck.

Any help?

Thanks

Answer 1

Having the ^ after the lookaheads in these scenarios always seems to work better for me. Reading it makes more sense to me, too.

(?!.*bar)^.*foo

this has a foo          # pass
so does this has a foo  # pass
i can haz foo           # pass
but i haz foo and bar!  # fail

Answer 2

If you have a string foo that you want to find and another string bar that must not be present, you can use this:

^(?!.*bar).*foo

Creating a regular expression that exactly meets all your requirements is very difficult as Python code is not a regular language, but hopefully you should be able to use this as a starting point to get something good enough for your needs.