负前瞻正则表达式python

Question

I have a list of strings, I want to match a keyword if is not preceded by an "and". However, I would like to include the keyword if the "and" is preceded by another word. Taking into account that the position of the "and" is not the same in all the strings.

In my example below I would like to match strings that have the word "book", only if the word "book" is not after "and" unless the "and" is precede by a "tall" (the first and the third item in my list). My attempt was using

^((?!((?<!tall)\band\b)).)*book

but it is not working. I'm not sure if I can include a negative look behind inside a negative lookahead

1. is a tall and wide book

2. is a computer and a book of instructions

3. book with a pretty cover

4.computer with nice headphones

Answer 1

I would phrase this using an alternation with both a positive and negative lookbehind:

((?<!\band )|(?<=\btall and ))book

Demo

The logic here is to match the word book if either of the following two conditions are true:

• book is not preceded by the word and at all
• book is preceded by and , which in turn is also preceded by tall

I didn't bother to include any Python code, but from what I gleaned in your question, the meat and potatoes is figuring out the regex, rather than the Python code.

Answer 2

I figure out that my approach was not entirely wrong. It works when I take out the first boundary.

^((?!((?<!tall) and\b)).)*book

I don't know yet why it doesn't work when I include that boundary. But it works. Thank you