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

I don't have a great deal of Perl experience, so a bit of patience with this question please if it seems basic.

I recently used Perl to automate marking up some data files with HTML for inclusion in a website. This, naturally, proved much faster than coding HTML by hand! However, there was one scenario that I couldn't figure out, and wound up doing manually--I needed to remove six-line blocks of data, but only if a particular pattern existed in the fourth line.

For future reference, how do you do this in Perl? Thanks very much for your reply.

jens

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Re: mutiple-line regexes?
by spurperl (Priest) on Aug 19, 2002 at 06:06 UTC
    Shortly, the /m and /s pattern modifiers should help dealing with multi-line mathches. 
           m   Treat string as multiple lines.  That is, change "^"
           and "$" from matching the start or end of the string
           to matching the start or end of any line anywhere
           within the string.

       s   Treat string as single line.  That is, change "." to
           match any character whatsoever, even a newline, which
           normally it would not match.

    [download]
    Read "perldoc perlre" for more information, or post a specific problem.
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Re: mutiple-line regexes?
by jens (Pilgrim) on Aug 19, 2002 at 06:55 UTC
    Here's some simplified code to give you an idea of what I wanted to accomplish:

    Example HTML code (produced by saving as HTML from OpenOffice Calc) simplified for sake of argument:

    <tag1>
<tag2>
<tag3>
<tag4><MYTAG>is there stuff here?</MYTAG></tag4>
</tag3>
</tag2></tag1>

    [download]

    If the contents between MYTAG were *blank*, then I wanted to delete the entire six lines.



    --jens
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[d/l]

      does your data always look like pseudo-HTML? tagged? maybe you can have better results with HTML::TokeParser

      here's some untested code:

      my $p = HTML::TokeParser->new($html) || die "Can't tokenize: $!";
# get each <tag1> alone.
while (my $token = $p->get_tag('tag1')) {
    # store the original text
    $origtext = $token->[3];

    # get data between <MYTAG></MYTAG>
    my $myTag = $p->get_tag('MYTAG');
    my $text = $p->get_text('/MYTAG');
    
    if ($text ne '') {
        
        # tag is not empty.. so $origtext retains
        # the data we want..

        # ... do whatever with $origtext and move on
    }
}

      [download]
      He who asks will be a fool for five minutes, but he who doesn't ask will remain a fool for life.

      Chady | http://chady.net/
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[d/l]

      Instead of using a multine regex, you could consider looping over the lines in turn and removing every six lines where there's a match on the fourth.

      I think the code is pretty self-explanatory (@lines contains your file):

      my $i = 3;                             # from line 4 ...
while ($i < @lines - 2) {              # ... until last but 3
    if ($lines[$i] =~ m!<MYTAG></MYTAG>!) {
        splice(@lines, $i - 3, 6);     # kill 6 lines from $i - 3
    } else {
        $i++;
    }
}

      [download]

      — Arien

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