use strict;
use warnings;

use Text::Balanced qw( extract_bracketed );
use HTML::Entities qw( encode_entities );

my $default_allowed_tags = do {
	my @tags = qw(
		A ABBR ACRONYM B BIG CITE CODE DFN EM I KBD Q SAMP
		SMALL SPAN STRONG SUB SUP TT VAR
	);
	qr/${\( join "|", map quotemeta, @tags )}/;
};

sub parse_markup
{
	my ($text, $tags) = @_;
	$tags ||= $default_allowed_tags;
	
	my ($before, $match) = ($text =~ m{
		\A             # start of string
		(.*?)          # leading text ($before)
		(              # either...
			\<\!--      #    the start of a comment
			|           # or...
			$tags\<     #    a tag
		)
	}xsm) or do {
		my @return = split /\|/, $text;
		$return[0] = encode_entities($return[0]);
		return @return;
	};
	
	# strip $before from $text
	substr($text, 0, length($before)) = '';
	
	# If the first thing that needed to be handled was a comment
	if ($match eq '<!--') {
		# Strip it out
		$text =~ s/\<\!--(.+?)--\>//g;
		
		# Handle the rest via recursion
		return join "", $before, parse_markup($text, $tags);
	}
			
	chop(my $found_tag = lc $match);
	substr($text, 0, length($found_tag)) = '';
	my ($got, $remainder) = extract_bracketed($text, q/<"'>/);
	$got = substr($got, 1, length($got) - 2);
	my ($markup, @attrs) = parse_markup($got, $tags);
	my ($more_markup, @more_attrs) = parse_markup($remainder, $tags);
	defined($_) or $_='' for $markup, $more_markup;
	
	join("",
		$before,
		(@attrs ? "<$found_tag @attrs>" : "<$found_tag>"),
		$markup,
		"</$found_tag>",
		$more_markup,
	), @more_attrs;
}

print for parse_markup(<<'TEXT');
Anyone who watches the Syfy channel knows that on
Monday nights they aired three television series
I<A<EurSUP<e>ka|href="Movies_by_series.pl?series=EWA#EUReKA">|class="title">,
I<A<Warehouse & 13|href="Movies_by_series.pl?series=EWA#Warehouse_13">>,
and I<A<Alphas|href="Movies_by_series.pl?series=EWA#Alphas">>.
Some might not be aware that these three series have formed a crossover
cosmology which I call A<EWA|href="Movies_by_series.pl?series=EWA">
<!-- This is a long string. -->
TEXT