Balances tags of string using a modified stack.

function force_balance_tags( $text ) {
	$tagstack  = array();
	$stacksize = 0;
	$tagqueue  = '';
	$newtext   = '';
	// Known single-entity/self-closing tags.
	$single_tags = array( 'area', 'base', 'basefont', 'br', 'col', 'command', 'embed', 'frame', 'hr', 'img', 'input', 'isindex', 'link', 'meta', 'param', 'source', 'track', 'wbr' );
	// Tags that can be immediately nested within themselves.
	$nestable_tags = array( 'article', 'aside', 'blockquote', 'details', 'div', 'figure', 'object', 'q', 'section', 'span' );

	// WP bug fix for comments - in case you REALLY meant to type ' $s = [paste copied contents of expression below including parentheses];
	 * php > echo $s;
	 */
	$tag_pattern = (
		'#' and have nothing here.
			's*' .
			'(/?)' . // Group 3 - "attributes" for empty tag.
				'|' .
			// Or we must start with space characters to separate the tag name from the attributes (or whitespace).
			'(s+)' . // Group 4 - Pre-attribute whitespace.
			'([^>]*)' . // Group 5 - Attributes.
		')' .
		'>#' // End with a closing bracket.
	);

	while ( preg_match( $tag_pattern, $text, $regex ) ) {
		$full_match        = $regex[0];
		$has_leading_slash = ! empty( $regex[1] );
		$tag_name          = $regex[2];
		$tag               = strtolower( $tag_name );
		$is_single_tag     = in_array( $tag, $single_tags, true );
		$pre_attribute_ws  = isset( $regex[4] ) ? $regex[4] : '';
		$attributes        = trim( isset( $regex[5] ) ? $regex[5] : $regex[3] );
		$has_self_closer   = '/' === substr( $attributes, -1 );

		$newtext .= $tagqueue;

		$i = strpos( $text, $full_match );
		$l = strlen( $full_match );

		// Clear the shifter.
		$tagqueue = '';
		if ( $has_leading_slash ) { // End tag.
			// If too many closing tags.
			if ( $stacksize '; // Close tag.
				array_pop( $tagstack );
				$stacksize--;
			} else { // Closing tag not at top, search for it.
				for ( $j = $stacksize - 1; $j >= 0; $j-- ) {
					if ( $tagstack[ $j ] === $tag ) {
						// Add tag to tagqueue.
						for ( $k = $stacksize - 1; $k >= $j; $k-- ) {
							$tagqueue .= '' . array_pop( $tagstack ) . '>';
							$stacksize--;
						}
						break;
					}
				}
				$tag = '';
			}
		} else { // Begin tag.
			if ( $has_self_closer ) { // If it presents itself as a self-closing tag...
				// ...but it isn't a known single-entity self-closing tag, then don't let it be treated as such
				// and immediately close it with a closing tag (the tag will encapsulate no text as a result).
				if ( ! $is_single_tag ) {
					$attributes = trim( substr( $attributes, 0, -1 ) ) . ">$tag";
				}
			} elseif ( $is_single_tag ) { // Else if it's a known single-entity tag but it doesn't close itself, do so.
				$pre_attribute_ws = ' ';
				$attributes      .= '/';
			} else { // It's not a single-entity tag.
				// If the top of the stack is the same as the tag we want to push, close previous tag.
				if ( $stacksize > 0 && ! in_array( $tag, $nestable_tags, true ) && $tagstack[ $stacksize - 1 ] === $tag ) {
					$tagqueue = '' . array_pop( $tagstack ) . '>';
					$stacksize--;
				}
				$stacksize = array_push( $tagstack, $tag );
			}

			// Attributes.
			if ( $has_self_closer && $is_single_tag ) {
				// We need some space - avoid 
 and prefer 
.
				$pre_attribute_ws = ' ';
			}

			$tag = '';
			// If already queuing a close tag, then put this tag on too.
			if ( ! empty( $tagqueue ) ) {
				$tagqueue .= $tag;
				$tag       = '';
			}
		}
		$newtext .= substr( $text, 0, $i ) . $tag;
		$text     = substr( $text, $i + $l );
	}

	// Clear tag queue.
	$newtext .= $tagqueue;

	// Add remaining text.
	$newtext .= $text;

	while ( $x = array_pop( $tagstack ) ) {
		$newtext .= '' . $x . '>'; // Add remaining tags to close.
	}

	// WP fix for the bug with HTML comments.
	$newtext = str_replace( '