Re: backreferneces in search pattern
From: Bob Walton (see_at_sig.invalid)
Date: 11/12/04
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Date: Thu, 11 Nov 2004 22:49:11 -0500
Marek Stepanek wrote:
...
> I try to set up a Perl-Filter (for BBEdit on Macintosh). I want to match and
> print out all gifs needed for a rollover in an HTML-File. The filter should
Unless you control the layout of the HTML, you would be much better off
to parse your HTML using a HTML parser module (HTML::Parser, perhaps).
Correctly parsing HTML is much harder than it appears at first glance.
> match
>
> onmouseover="document.podium.src='../../pix/grafix/hili_podium.gif';
>
> but also beasts like the following :
>
> onmouseover="document.podium.src='../../pix/logos/theembassy1.gif';
> document.bios.src='../../pix/logos/theembassy2.gif';
> document.addresses.src='../../pix/logos/theembassy3.gif';
> document.events.src='../../pix/logos/theembassy4.gif';
> document.priwat.src='../../pix/logos/theembassy5.gif';
> document.yrpodium.src='../../pix/logos/theembassy6.gif';
> document.logo.src='../../pix/logos/hili_pilogo.gif'"
>
>
> the result should print from the above examples :
>
> '../../pix/grafix/hili_podium.gif',
> '../../pix/logos/theembassy1.gif',
> '../../pix/logos/theembassy2.gif',
> '../../pix/logos/theembassy3.gif',
> '../../pix/logos/theembassy4.gif',
> '../../pix/logos/theembassy5.gif',
> '../../pix/logos/theembassy6.gif',
> '../../pix/logos/hili_pilogo.gif',
>
>
> I am labouring since a long while already on this filter. Could somebody
> help me out ? I am beginner and want learn Perl very much.
>
> This filter here produces the following error :
>
>
> Modification of a read-only value attempted <> line 1.
>
>
> (mind line breaks from my email client)
>
>
> #!/usr/bin/perl -w
>
You are missing:
use strict;
use warnings;
Let Perl help you all it can. I see you did use the -w switch -- but
these days it is better to
use warnings;
because of the additional control over the warnings it affords. See
below for what I am talking about.
>
> while (<>) {
> ($1, $2, $3) =
-------^^--^^--^^
Those are your readonly variables that you are attempting to modify. See
perldoc perlvar
particularly the section about $<digits> . Note that the $1, $2, etc
variables are assigned by simply placing capturing sets of parentheses
in a regular expression. Judging from your next couple of lines of
code, you probably want:
my @results =
on this line, and then remove the "my @results=($1,$2,$3);" line.
> m!onmouseover="[^']+'([^']+)'(?:(?:(?:;\s+[^']+)'([^']+)')+)?(?:(?:[^']+)'([
> ^']+)')?"!g;
> my @results = ($1, $2, $3);
> foreach $i (0..$#results) {
> print "'", $i, "',\n";
> }
> }
>
> My question is not about the grep search pattern, but how to put the
> backreferences into an array to print it out.
>
> one other try gives :
>
> Use of uninitialized value in print <> line 2.
Note that that is a warning, and is not fatal. Your program should
continue to execute normally.
>
>
> #!/usr/bin/perl -w
>
> while (<>) {
> while (
> s!onmouseover="[^']+'([^']+)'(?:(?:(?:;\s+[^']+)'([^']+)')+)?(?:(?:[^']+)'(
> [^']+)')?"!!g ) {print "'", $1, "',\n"; print "'", $2, "',\n"; print "'",
> $3, "',\n";}
> }
The uninitialized value might have come from a capturing parentheses
pair that matched an empty string because it was inside the scope of a
"?" metacharacter and the "no occurrences" branch was taken. Prior to
the match, all the $1, $2 etc variables are set to undef, and are only
modified upon their corresponding set of parens actually being used in
the final match. This particular warning can be much less than totally
useful in a situation like this, which is why it can be suppressed in a
block with a "no warnings qw(uninitialized);" statement at its start.
The means something like:
while(<>){
no warnings qw(uninitialized);
s!onmouseover="...
}
But you can only do that if you
use warnings;
rather than the -w switch.
> marek
-- Bob Walton Email: http://bwalton.com/cgi-bin/emailbob.pl
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