Dan

# Construct a regex fragment for handling normal tags. These can
# be either a start or an end tag.
my $standard_tags_fragment = join '|', qw{
    h1  h2  h3  ul  ol  li
    b   i   strong  em
    s   sub super   strike
    p   pre code    blockquote
};;

# Define a regex to match standard, open or close tags
my $standard_tags_re = qr{
    <                               # Open tag
        /?                          # Optional leading slash
        (?:$standard_tags_fragment) # Regex fragment to match tagnames
    >                               # Close tag
}xms;

# Construct a regex fragment for handling single, self-closing tags
my $single_tags_fragment = join '|', qw{ br hr };

my $single_tags_re = qr{
    <                               # Open tag
        (?:$single_tags_fragment)   # Regex fragment to match single tags
        \s*/?                       # Optional self-closing slash, with optional space
    >                               # Close tag
};

# Construct the final whitespace regexp
my $whitelist_re = qr{
    $single_tags_re             # Match an open or close tag
    |   $standard_tags_re       # Match a self-closing single tag

    # Special cases
    |   <a[^>]+>                # Match 'a' tags regardless of attributes
    |   <img[^>]+>              # Match 'img' regardless of attributes
    |   </a>                    # Special case to catch close 'a' tags
}xms;