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I need help regarding using negative lookahead. I am using Notepad++ and I want to delete all lines except the lines that contain <title>(.*)</title>
I tried a couple of things but that didnt work.
^.*(?!<title>).*</title>
^.*(?!<title>.*</title>)
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A negative look-ahead, on the other hand, is when you want to find an expression A that does not have an expression B (i.e., the pattern) after it. Its syntax is: A(?!B) . In a way, it is the opposite of a positive look-ahead.
Lookbehind, which is used to match a phrase that is preceded by a user specified text. Positive lookbehind is syntaxed like (? <=a)something which can be used along with any regex parameter. The above phrase matches any "something" word that is preceded by an "a" word.
Lookahead assertions are part of JavaScript's original regular expression support and are thus supported in all browsers.
You probably cant perform standard negative lookaheads using grep, but usually you should be able to get equivalent behaviour using the "inverse" switch '-v'. Using that you can construct a regex for the complement of what you want to match and then pipe it through 2 greps.
You are close:
^(?!.*<title>.*</title>).*
By this regex ^.*(?!<title>.*</title>), the regex engine will just find some position that it cannot find <title>.*</title> (end of line is one such valid position).
You need to make sure that, from the start of the line, there is no way you can find <title>.*</title> anywhere in the line. That is what my regex does.
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