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Inverting match with grep, unless another term exists (lemmy.world)

submitted 9 months ago by [email protected] to c/[email protected]

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I have a list containing a set of tags, and would like to exclude one tag, unless another tag exists in the line.

Say I have the following list, and want to exclude B, unless A is present.

[A,B]
[A,C]
[B,C]
[A]
[B]

I can reverse grep for B:

> grep --invert-match "B"
[A,C]
[A]

How can I find the previous list, but also the item containing [A,B]?

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[–] [email protected] 7 points 9 months ago* (last edited 9 months ago) (2 children)

grep -E '(^[^B]*$|A)'

EDIT: Whoops, I meant to make this a top-level comment.

EDIT 2: On one client it looked like a nested comment and on this other client it looks top level and now I'm a confused old man.

[–] [email protected] 1 points 9 months ago (1 children)

You don't need parentheses here.

[–] [email protected] 1 points 9 months ago

~~How dare you?~~

Thanks!

[–] [email protected] 1 points 9 months ago

Haha, on Lemmy.World it looks like a top level. Thank you, either way :)

[–] [email protected] 7 points 9 months ago (1 children)

I like to use awk instead of grep wherever possible, especially for weird logic like this.

awk '!/B/ || /A.*B/' is one way to skin that cat. If you don't care what order A and B are in on the lines containing both, then awk '!/B/ || (/A/ && /B/)' will work.

[–] [email protected] 1 points 9 months ago

Thank you! This was exactly was I was looking for. :)