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

I might have simplified this too much, but:

$html = '<!--% xxxx UNO xxxx %-->   <!--% xxxx DOS xxxx %-->';
$html =~ s/<!--%(.*?) DOS (.*?)%-->/GONE/s;
print $html;

what I want to produce is leave the first comment tag, while
replacing the second with GONE (and yes I do need to grab the xxxx tex
+t)

the 'xxxx' in both tags may be anything, and may span several lines
I may not always be looking for 'DOS', this was just an example.

my problem is that it chews up the first one, starting with
<!--% from UNO's tag, and ending with --> from DOS's tag..

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Replies are listed 'Best First'.
Re: UNO DOS
by lhoward (Vicar) on Sep 13, 2000 at 04:13 UTC
    You might be better off using HTML::Parser to parse out HTML tags, then apply your regular expression on a tag-by-tag basis. It will be very difficult to get your regular expression to work properly considering the variety and complexity that can occur in an HTML document.
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      I would agree, unless this is the ONLY thing he wants to do.
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Re: UNO DOS
by jreades (Friar) on Sep 13, 2000 at 03:29 UTC

    Your problem is the .*? -- although you rightly tried to limit the number of don't care characters using '?' your match still grabs the smallest number of don't care characters between '<!--' and 'DOS'... which just happens to include another '-->' and '<!--', the pieces it isn't supposed to grab.

    You'll need a few baselines to come up with a workable regexp:

    • Can xxxx ever include '<!--' or '-->'? (We'd better hope not)
    • Can xxxx contain only word-like characters (\w)?
    • Or can it include space characters as well (\s)?

    These would help you optimize your regexp...

    But the key point is that you need to limit your regexp to a single comment group containing 'DOS'.

    I'd suggest using:

    $html = '<!--% xxxx UNO xxxx %-->   <!--% xxxx DOS xxxx %-->';
$html =~ s/<!--%([^->]+?) DOS ([^->]+)%-->/GONE/s;
print $html;

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    It's a little ugly, and notice that it assumes that your 'xxxx' can't contain '->', which may or may not be the case.

    YMMV

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[d/l]
      Good call on the .* There is a node around here called Death to Dot Star! which explores this further. But your regex still needs work. The bracket elements are not a group, they are individual. meaning that it would catch any html item, not just -- >, because it matches the > alone. Ok?
      How about: 
      $html =~ s/<!--%(?!%-->)DOS(?!%-->)%-->/GONE/s;

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      I'm not sure about that regex, I've never used a zero-width negative look-ahead assertion, but I think that's the right direction.
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[d/l]

        I've been sweating buckets about this one ever since I left the office... <visions of minus 30XP dancing in my head > which is, of course, just the time to realize that you screwed up the regexp. :^P

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Re: UNO DOS
by Anonymous Monk on Sep 13, 2000 at 03:41 UTC 
    Yeah that's part of the problem.. the xxxx can contain HTML
code, which may contain regular comments, <!-- --->
however, xxxx will never contain <!--% and %-->, those are
only used as braces..
and to mirod, another thing is that there could be any number
of these tags before/after the tag we're intending to grab..

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[d/l]
RE: UNO DOS, HTML
by runrig (Abbot) on Sep 13, 2000 at 04:02 UTC
    My attempt:

    $html =~ s/(<!--%(.*?)%-->)/($2=~m|DOS|)? 'GONE' : $1/esg;

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[d/l]
RE: UNO DOS, HTML
by mirod (Canon) on Sep 13, 2000 at 03:14 UTC

    Just grab the first comment;

    $html = '<!--% xxxx UNO xxxx %-->   <!--% xxxx DOS xxxx %-->';
$html =~ s/(<!--.*?-->\s*)<!--%(.*?) DOS (.*?)%-->/$1GONE/s;
print $html;

    [download]

    Or make sure there is a comment beforehand:

    $html = '<!--% xxxx UNO xxxx %-->   <!--% xxxx DOS xxxx %-->';
$html =~ s/-->\s*<!--%(.*?) DOS (.*?)%-->/--> GONE/s;
print $html;

    [download]

    There is probably a cleaner way to do this without capturing the first comment at all, or by using a g modifier, skipping the first comment and replacing the second.

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[d/l]
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Re: UNO DOS
by Anonymous Monk on Sep 13, 2000 at 04:35 UTC
    well, sounds like I have to parse out each tag individually..
    yucky
    I guess I'll try to figure out some slob fix right now and rework the design of the templates..
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      Instead of reworking "the design of the templates" why not use one of the many text/html templating modules already in place?
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      My answer DOES parse each tag individually, it uses a regex inside a regex, and seems to work.
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        yeah I got it.. I was trying to avoid that, but I guess I can't.. (?)
        thanks tho
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