preg_replace_callback('{(?>
            (?>
                (?<=\n)            # Starting on its own line
                |                # or
                \A\n?            # the at beginning of the doc
            )
            (                        # save in $1

              # Match from `\n<tag>` to `</tag>\n`, handling nested tags
              # in between.

                        [ ]{0,' . $less_than_tab . '}
                        <(' . $block_tags_b_re . ')# start tag = $2
                        ' . $attr . '>            # attributes followed by > and \n
                        ' . $content . '        # content, support nesting
                        </\2>                # the matching end tag
                        [ ]*                # trailing spaces/tabs
                        (?=\n+|\Z)    # followed by a newline or end of document

            | # Special version for tags of group a.

                        [ ]{0,' . $less_than_tab . '}
                        <(' . $block_tags_a_re . ')# start tag = $3
                        ' . $attr . '>[ ]*\n    # attributes followed by >
                        ' . $content2 . '        # content, support nesting
                        </\3>                # the matching end tag
                        [ ]*                # trailing spaces/tabs
                        (?=\n+|\Z)    # followed by a newline or end of document

            | # Special case just for <hr />. It was easier to make a special
              # case than to make the other regex more complicated.

                        [ ]{0,' . $less_than_tab . '}
                        <(hr)                # start tag = $2
                        ' . $attr . '            # attributes
                        /?>                    # the matching end tag
                        [ ]*
                        (?=\n{2,}|\Z)        # followed by a blank line or end of document

            | # Special case for standalone HTML comments:

                    [ ]{0,' . $less_than_tab . '}
                    (?s:
                        
                    )
                    [ ]*
                    (?=\n{2,}|\Z)        # followed by a blank line or end of document

            | # PHP and ASP-style processor instructions (<? and <%)

                    [ ]{0,' . $less_than_tab . '}
                    (?s:
                        <([?%])            # $2
                        .*?
                        \2>
                    )
                    [ ]*
                    (?=\n{2,}|\Z)        # followed by a blank line or end of document

            )
            )}Sxmi', array($this, '_hashHTMLBlocks_callback'), $text)

Issues