use strict;
use warnings;

my %VALID_ELEMENTS = map { $_ => 1 } qw( b uc );

my %state;
my @to_close;

sub output_text {
   my ($text) = @_;

   $text =~ s/([[:print:]])/$1$1/g
      if $state{b};

   $text = uc($text)
      if $state{uc};

   print($text);
}

sub open_tag {
   /\G ( \w+ ) /xgc
      or die("Expecting tag name\n");

   my $ele = lc($1);

   $VALID_ELEMENTS{$ele}
      or die("Unknown element name $ele\n");

   /\G > /xgc
      or die("Expecting closing bracket\n");

   !$state{$ele}
      or die("Attempting to open previously opened element\n");

   $state{$ele} = 1;
   push @to_close, $ele;
}

sub close_tag {
   /\G (\w+) /xgc
      or die("Expecting tag name\n");

   my $ele = lc($1);

   $VALID_ELEMENTS{$ele}
      or die("Unknown element name $ele\n");

   /\G > /xgc
      or die("Expecting closing bracket\n");

   $state{$ele}
      or die("Attempting to close unopened element\n");

   my $to_close = $to_close[$#to_close];

   $to_close eq $ele
      or die("Missing closing tag for element $to_close\n");

   $state{$ele} = 0;
   pop @to_close;
}

sub process {
   # The following "for" aliases $_ to $text and
   # provides a target for the upcoming "redo" stmts.
   for ($_[0]) {
      /\G <\/       /xsgc && do { close_tag();       redo };
      /\G <         /xsgc && do { open_tag();        redo };
      /\G ( [^<]+ ) /xsgc && do { output_text("$1"); redo };

      # Falls through at the end of the string.
   }
}

process("This is a nifty <b><uc>uppercase</uc> test</b> to see what <uc>this</uc> thing can do.\n");