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Trying a few things with negative and positive look-behind, can't really get exactly what I want. Went through this SO question as well: Regex match a substring if that substring is not preceded by a specific string?

What I want is just a "yes this entire string should be considered" and a "no, ignore this entire string", because of these substring matches. The post above will help me match the substring, but if the negative words precede the substring, it's still a match, you can see my tests here: https://regex101.com/r/aqn1gO/2

What I'm trying to do is have a regex match for the substring i need, but ignore cases where it's not a request, but more a question. The examples are:

  1. i need you to do this
  2. hey i need this by tomorrow
  3. hey do i need this in the deliverable?
  4. hey should i need to do this?
  5. how are you doing today?

Where 1. and 2. should match, but 3., 4., and 5. should not, even though there's an i need in there.

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One approach might be then to negate those with do or should followed by any chars and a must positive lookahead with i need: 

^(?=.*i need.*)(?!.*should.*i need.*|.*do.*i need.*).*$

Demo

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Demo 2 is close but I require the substring i\s+need to be present in the string to take action, except for when it is preceded by specific keywords like (do|should) (this list might grow). So any other string like "hey how's it going today?" should not trigger a match. I've updated the question with this example to make it more clear.

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