eddmund has asked for the wisdom of the Perl Monks concerning the following question:

Hi, I have a problem that sounds very simple: if you found a hit within a webpage's source, how can you find out if this hit is part of a link, or more generally, within a tag? I tried the following code with finds the first string correctly, but also the second one that should not match. 
#!/usr/bin/perl -w
use strict;

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>'
); 
 
foreach my $string (@strings) {
"" =~ /()/;        # set $1 to false
$string =~ /
(                        # the whole match should be catches as $1
    HIT                    # start at the first found "HIT"
    (?=                    # begin of positive Lookahead
        [^<]*?            # zero or more characters other than "<" 
        (?!                # start a negative Lookahead
            <a[^>]*?>    # an opening a tag is forbidden
        )                
        .*?<\/a>        # any characters until the closing a tag
    )                    # end of positive Lookahead
    .*?                    # catch all chars after HIT,
    <\/a>                # non-greedy until the first closing a tag
)
/x;

print "\nstring:\t$string\n";
if ($1) { print "match:\t$1\n" }
else { print "NO match!\n" }
}

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I think the problem is related to my use of the negative Lookahead. Could anybody point me to the right direction? Thanks, rob.

Replies are listed 'Best First'.
Re: negative Lookahead or
by wog (Curate) on Nov 12, 2001 at 07:39 UTC
    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!

    [download]

    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.

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      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
+>'
);

      [download]
      and the second string does not match anymore. Any help is greatly appreciated.
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[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.

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