#!/usr/bin/perl

use strict;

$/=undef;
$_=<>; 

my @stack = ();
my $offset = 0;

while (( my $i = index( $_, "<" )) >= 0 ) {
    $offset += $i;
    $_ = substr( $_, $i );
    if ( s{^(<(\w+).*?>)}{} ) {
        $offset += length( $1 );
        push @stack, $2;
    }
    elsif( s{^(</(\w+)>)}{} ) {
        my $et = $2;
        if ( $stack[$#stack] eq $et ) {
            pop @stack;
        }
        elsif ( grep( /$et/, @stack )) {
            print "missing end-tags for:";
            while ( @stack and $stack[$#stack] ne $et ) {
                print " ".pop @stack;
            }
            print " at </$et> (offset: $offset)\n";
        }
        else {
            print "missing open-tag for $et (offset: $offset)\n";
        }
        $offset += length( $1 );
    }
}

### updated: added condition on inner while loop to check for empty stack