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Recipe 20.21 Program: hrefsub

hrefsub makes substitutions in HTML files, so changes apply only to text in <A HREF="..." > tags. For instance, if you had the scooby.html file from the previous recipe, and you've moved shergold.html to be cards.html, you need but say:

% hrefsub shergold.html cards.html scooby.html
<BODY><H1>Welcome to Scooby World!</H1>
I have <A HREF="pictures.html">pictures</A> of the crazy dog
himself.  Here's one!<P>
<IMG SRC="scooby.jpg" ALT="Good doggy!"><P
<BLINK>He's my hero!</BLINK>  I would like to meet him some day,
and get my picture taken with him.<P>
P.S. I am deathly ill.  <a href="cards.html">Please send

The HTML::Filter manual page has a BUGS section that says:

Comments in declarations are removed from the declarations and then inserted as separate comments after the declaration. If you turn on strict_comment( ), then comments with embedded "-\|-" are split into multiple comments.

This version of hrefsub (shown in Example 20-13) always lowercases the a and the attribute names within this tag when substitution occurs. If $foo is a multiword string, then the text given to MyFilter->text may be broken such that these words do not come together; i.e., the substitution does not work. There should probably be a new option to HTML::Parser to make it not return text until the whole segment has been seen. Also, some people may not be happy with having their 8-bit Latin-1 characters replaced by ugly entities, so htmlsub does that, too.

Example 20-13. hrefsub
  #!/usr/bin/perl -w
  # hrefsub - make substitutions in <A HREF="..."> fields of HTML files
  # from Gisle Aas <>
  sub usage { die "Usage: $0 <from> <to> <file>...\n" }
  my $from = shift or usage;
  my $to   = shift or usage;
  usage unless @ARGV;
  # The HTML::Filter subclass to do the substitution.
  package MyFilter;
  use HTML::Filter;
  use HTML::Entities qw(encode_entities);
  sub start {
     my($self, $tag, $attr, $attrseq, $orig) = @_;
     if ($tag eq 'a' && exists $attr->{href}) {
             if ($attr->{href} =~ s/\Q$from/$to/g) {
                 # must reconstruct the start tag based on $tag and $attr.
                 # wish we instead were told the extent of the 'href' value
                 # in $orig.
                 my $tmp = "<$tag";
                 for (@$attrseq) {
                     my $encoded = encode_entities($attr->{$_});
                     $tmp .= qq( $_="$encoded ");
                 $tmp .= ">";
  # Now use the class.
  package main;
  foreach (@ARGV) {
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