Nested blocks:

<div>
        <div>
        tags for inner block must be indented.
        </div>
</div>

%r{
      ^                                             # Start of line
      <(#{StrictTagPattern})        # Start tag: \2
      \b                                            # word break
      (.*\n)*?                              # Any number of lines, minimal match
      </\1>                                 # Matching end tag
      [ ]*                                  # trailing spaces
      $                                             # End of line or document
}ix