This code checks for missing opening or closing HTML tags. At the minute, it only works with simple tags like B and U etc. It could be easily extended to work with tags with more attributes etc with some regexps.
# Enter the tags to check for. Can only handle tags with
# no attributes at the moment
@tags = qw(b i u); 

# Put it in some HTML 
$html = "<b>Hello</b><u>Hello</i>";  # i.e. no </u> and <i> tag

# Iterate through each tag 
foreach $tag (@tags) { 

    # If $html contains that tag 
    while (($html =~ /<$tag>/i) or ($html =~ /<\/$tag>/i)) { 

        # If it contains the opening tag, keep count
        # of many opening tags there are. 
        if ($html =~ /<$tag>/i) { 
            $opening++; 
        }

              # If it contains the closing tag, keep count
        # of many closing tags there are.
        if ($html =~ /<\/$tag>/i) {
            $closing++;
        }

                # Remove the tags for the next iteration
        $html =~ s/<$tag>//si;
        $html =~ s/<\/$tag>//si; 

    }

        # If the number of opening tags and closing tags 
    # don't match, then something is wrong 
    if ($opening != $closing) { 
        if ($opening > $closing) { 
                 # More opening than closing, therefore
            # missing closing
            print "Warning, missing </$tag> tag\n"; 
        } else {
            print "Warning, missing <$tag> tag\n"; 
        }
    }

     # Prepare for next iteration
    $opening = $closing = undef; 
}

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RE: Missing HTML tags
by turnstep (Parson) on Apr 27, 2000 at 02:21 UTC
    Here's something to try out: 
    $file = shift or die "Need a file name!\n";

@closedtags = qw(hmtl head body b i u);
@opentags = qw(a img);
open(HTMLFILE, "$file") || die "Could not open $file: $!\n";
undef $/;
$html=<HTMLFILE>;
close(HTMLFILE);

## The magic part:
while($html =~ m#<(/?)([^ >]*)[^>]*>#gi) {
  if ($1) { $tag{lc $2}--; }
  else    { $tag{lc $2}++; }
}

## Now we have lots of options to play with:


## Show ALL tags, matched then unmatched:
print "Matched tags:\n";
for $x (sort keys %tag) { print "$x\n" unless $tag{$x}; }
print "Unmatched tags:\n";
for $x (sort keys %tag) { print "$x\n" if $tag{$x}; }

## Go through our list of 'closed' and check each:
for $x (@closedtags) {
  printf "Results for html tag %5s: ", $x;
  if (defined $tag{$x}) {
    print $tag{$x} ? "NOT balanced\n" : "balanced\n";
  }
  else { print "None found.\n"; }
}

# Etc. The regexp does a fairly good job, but 
# misses weird cases like > embedded in quotes, etc.

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