Second off, use XML::Simple instead . . .

use strict;
use XML::Simple;

local $/;  # just so i can slurp __DATA__

my $old_xml = XMLin(<DATA>,forcearray=>1);
my $new_xml;

foreach my $heading (@{ $old_xml->{'heading'} }) {
    my $level = delete $heading->{'level'};
    $new_xml->{"heading$level"} = $heading;
}

print XMLout($new_xml);

__DATA__
<xml>
<heading level="2">Introduction to Arguments</heading>
<heading level="3">
<index primary-key="procedures" secondary-key="definition, rest argume
+nts in"/>
<index primary-key="rest arguments" secondary-key="specifying, in proc
+edure definition"/>
Specifying Rest Arguments in a Procedure Definition
</heading>
</xml>
[download]

This works, but i only tested it on the XML that i provided, your milleage may vary. ;)

output:

<opt>
  <heading2>Introduction to Arguments</heading2>
  <heading3>
Specifying Rest Arguments in a Procedure Definition
<index primary-key="procedures" secondary-key="definition, rest argume
+nts in" />    <index primary-key="rest arguments" secondary-key="spec
+ifying, in procedure definition" />
  </heading3>
</opt>
[download]

jeffa

    A flute with no holes is not a flute . . .
a doughnut with no holes is a danish.
                                - Basho,
                                  famous philosopher

When you say that it isn't well formed XML, are you referring only to the unquoted 2 in the tag, or something else? Anyway, this is a fantastic solution, and I will implement it right away...talk about service ;-P
OK, so certainly you've illuminated the best way to do it, but....just for my own learning....What's failing with the regex? Anybody? Anybody? Beuller?
Thanks jeffa, you're a life saver!

Yes, the unquoted 2 and 3 level attributes need to be quoted, else XML::Simple complains:

not well-formed (invalid token) at line . . .
[download]

Also, i really don't think this will do what you think:

<heading>
level="3", blah blah blah
</heading>
[download]

You really should change that to:

<heading level="3">blah blah blah</heading>
[download]

You are welcome for the solution! . . . i would like to see a regex that solved the problem as well, but i imagine it would be an unruly beast . . . anybody? beuller? japhy?

Hmmmm, on second thought - this is something probably best not done!

Your logic goes astray because you try to parse XML with regexps.

You shouldn't.

Please read On XML parsing for a (n incomplete) list of reasons not to.

Actually the problem seems even worse than that: it looks like you are calling XML something that is not XML: level=2 is not an attribute value in XML, you need to quote the value.

I can't really test much right now due to Antonio keeping me busy, but something like this using XML::Twig would certainly work:

#!/usr/perl -w
use strict;

use XML::Twig;

my $t= XML::Twig->new( twig_roots => { heading => \&heading },
                       twig_print_outside_roots => 1,
                       pretty_print => 'indented'
                     );
$t->parse( \*DATA);

sub heading
  { my( $t, $heading)= @_;
    if( defined $heading->att( 'level'))
      { $heading->set_gi( 'heading' .  $heading->att( 'level'));
        $heading->del_att( 'level');
      }
    elsif( my $pcdata= $heading->first_child( '#PCDATA'))
      { my $text= $pcdata->pcdata;
        if( $text=~ s/^\s*level\s*=\s*(\d+),//s)
          { $pcdata->set_pcdata( $text);
            $heading->set_gi( 'heading' . $1);
          }
      }
    $heading->print;
  }

__DATA__
<doc>
<heading>
  <index/>
  <index/>
  level=3,
  Specifying Rest Arguments in a Procedure Definition</heading>
<heading level="2">
    Introduction to Arguments</heading>
</doc>
[download]

And finally, if you really want to use regexps, you can have a look at XML::Regexp or at XML::Parser::Lite.