#! /usr/bin/perl -w
use strict;
use HTML::Entities qw(encode_entities);

my @stack;
while (<>) {
  while (/\G([^{}]+)|\G([{}])/g) {
    if (length($1)) {
      print encode_entities($1);
    }
    elsif ("{" eq $2) {
      my $pos = pos($_);
      if (/\G(\w+)/g) {
        print "<$1>";
        push @stack, $1;
      }
      else {
        die "Unnamed opening brace at character $pos, line $.";
      }
    }
    elsif (@stack) {
      my $tag = pop(@stack);
      print "</$tag>";
    }
    else {
      my $pos = pos($_);
      die "Unmatched closing brace at character $pos, line $.";
    }
  }
}
if (@stack) {
  die "Unclosed tags at end of file: '@stack'";
}