Re: Search for line containing stringB, not preceded by a line containing stringA

On Wednesday, February 24, 2016 at 12:00:22 PM UTC-6, Paul wrote:
> I have a text file and I would like to compose a regular expression
> that matches lines containing stringB but not preceded by a line
> containing string A. I thought one of these might do it:
>
> \(stringA.*\n\)\@<!.*stringB
> \(stringA.*\n\)\@<!\(.*stringB\)
>
> The \@<! was *intended* to ensure a non-match with stringA in the
> preceding line. However, they highlight the same lines that are
> highlighted by the opposite regular expressions:
>
> \(stringA.*\n\)\@<=.*stringB
> \(stringA.*\n\)\@<=\(.*stringB\)
>
> With a little pondering, it is obvious why. The first .* matches any
> set of characters, so even if stringA exists in the preceding line,
> you can always find some portion of the line that doesn't match
> \(stringA.*\n\).
>
> Is there a way to achieve the search described in above?

Move the newline outside of your negative look-behind, then start the match using \zs after the newline if you only want to match the 2nd line.

Make it work on the 1st line as well by temporarily inserting a blank line at the top, or checking it manually, if needed.

Here's what I mean, if my description isn't clear: \(stringA.*\)\@<!\n\zs.*stringB

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