http://qs321.pair.com?node_id=124731


in reply to negative Lookahead or

First, this code produces that following output for me: 

string: <a href="#1">HIT<b>MUST MATCH</b></a> <a href="#2">2</a>
match:  HIT<b>MUST MATCH</b></a>

string: <a href="#1">HIT<a name="MUST NOT MATCH">-</a> <a href="#2">2<
+/a>
NO match!

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This seems like the result you desire.

However, I would advise you do not use regexes to parse HTML, because it is very, very hard to make the regex do it correctly (especially since there can be >s or <s in comments or quoted attribute values.) It would be better HTML::Parser or HTML::TokeParser. Searches of this site should give you many examples of their use.

Replies are listed 'Best First'.
Re: Re: negative Lookahead
by eddmund (Initiate) on Nov 12, 2001 at 08:05 UTC
    thanks for your suggestion, wog. But I would like to know what I did wrong in my regex. When I tried to post my code, the 2 strings have been too long so I cut it down a little bit, including dummy characters after "HIT". Add a "-" or use the original definition 
    my @strings = (
 '<a href="#1">--HIT-<b>MUST MATCH</b></a> <a href="#2">2</a>',
 '<a href="#1">--HIT-<a name="MUST NOT MATCH">--</a> <a href="#2">2</a
+>'
);

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    and the second string does not match anymore. Any help is greatly appreciated.
[reply]
[d/l]
      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.

[reply]
[d/l]
[select]
        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.
[reply]
[d/l]
[select]

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