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

On Wednesday, February 24, 2016 at 3:01:24 PM UTC-5, Ben Fritz wrote:
> 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

Thanks, Ben. The problem isn't whether the 1st and/or the 2nd of the two lines are matched. It's whether I can even locate lines containing stringB *only* when the preceding line does not contain stringA. Currently, all expressions locate cases of stringB regardless of whether the preceding contains stringA. The text file I use is:

asldkfjals stringA asldkjf
asldkfjals stringB asldkjf

asldkfjals stringX asldkjf
asldkfjals stringB asldkjf

asldkfjals stringA asldkjf
asldkfjals stringB asldkjf

asldkfjals stringX asldkjf
asldkfjals stringB asldkjf

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