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Re: Re: Re: negative Lookahead

by wog (Curate)
on Nov 12, 2001 at 08:15 UTC ( [id://124737]=note: print w/replies, xml ) Need Help??


in reply to Re: Re: negative Lookahead
in thread negative Lookahead or

What appears to be happening with the second string is this:

You match HIT. Then you go looking matching [^<]*?. The *? first tries to match the minimal number of characters, so it gets 0. And, the negative look-ahead does not try to do backtracking to make that *? match more so it can make the negative look-ahead match. (update: because, if it did that, then the look-ahead would be doing more then just looking ahead in the string.) Thus, it doesn't find a match and the negative lookahead succeeds, so the positive lookahead succeeds.

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Re: Re: Re: Re: negative Lookahead
by eddmund (Initiate) on Nov 12, 2001 at 21:00 UTC
    OK, thanks. Considering switching to greedy instead of non-greedy results in the regex 
        $string =~ /
        (                                    
            HIT                                
            (?=                                
                [^<]*                        
                (?!                            
                    <a[^>]*>                
                )                
                .*?<\/a>                    
            )                                
            .*?<\/a>                        
        )
    /x;

    [download]
    but still gives the same results. If the $string 
    '<a href="#1">--HIT<a name="MUST NOT MATCH">--</a> <a href="#2">2</a>'

    [download]
    does properly not match and 
    '<a href="#1">--HIT-<a name="MUST NOT MATCH">--</a> <a href="#2">2</a>
+'

    [download]
    does "falsely" match I don't understand why the problem lies in the fact that a negative lookahead does not backtrack.
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