in reply to Help in regex

This looks very much like an HTML parsing issue and the conventional answer is "use an appropriate module". In this case I would recommend HTML::TreeBuilder. Consider the following:

use strict;
use warnings;
use HTML::TreeBuilder;

my $str = <<DOC;
<s1>King of England
JAMES, by the Grace of God, King of England, Scotland, France and Irel
+and,
Defender of the Faith... ... This first selection was written in Engla
+nd to
establish the colony of Virginia
<sb><s1>Rights of Landowners We, greatly commending, and graciously ac
+cepting
of, their Desires for the Furtherance</sb>
DOC

my $root = HTML::TreeBuilder->new ();

$root->ignore_unknown (0);
$root->parse ($str);
$root->eof ();

my @s1Nodes = $root->look_down ('_tag', 's1');

for my $node (@s1Nodes) {
     if ($node->look_up ('_tag', 'sb')) {
          # nested in a sb element - convert to h1
          $node->{_tag} = 'h1';
     } else {
          $node->{_tag} = 'section1';
     }
}

print $root->as_HTML ();

[download]

Prints:

<html><head></head><body></body><section1>King of England JAMES, by th
+e Grace of God, King of England, Scotland, France and Ireland, Defend
+er of the Faith... ... This first selection was written in England to
+ establish the colony of Virginia <sb><h1>Rights of Landowners We, gr
+eatly commending, and graciously accepting of, their Desires for the 
+Furtherance</h1></sb> </section1></html>

[download]
DWIM is Perl's answer to Gödel

In Section Seekers of Perl Wisdom