string.
	 * @param array        $m      Regular expression match array.
	 */
	return apply_filters( 'do_shortcode_tag', $output, $tag, $attr, $m );
}

/**
 * Search only inside HTML elements for shortcodes and process them.
 *
 * Any [ or ] characters remaining inside elements will be HTML encoded
 * to prevent interference with shortcodes that are outside the elements.
 * Assumes $content processed by KSES already.  Users with unfiltered_html
 * capability may get unexpected output if angle braces are nested in tags.
 *
 * @since 4.2.3
 *
 * @param string $content     Content to search for shortcodes.
 * @param bool   $ignore_html When true, all square braces inside elements will be encoded.
 * @param array  $tagnames    List of shortcodes to find.
 * @return string Content with shortcodes filtered out.
 */
function do_shortcodes_in_html_tags( $content, $ignore_html, $tagnames ) {
	// Normalize entities in unfiltered HTML before adding placeholders.
	$trans   = array(
		'[' => '[',
		']' => ']',
	);
	$content = strtr( $content, $trans );
	$trans   = array(
		'[' => '[',
		']' => ']',
	);

	$pattern = get_shortcode_regex( $tagnames );
	$textarr = wp_html_split( $content );

	foreach ( $textarr as &$element ) {
		if ( '' === $element || '<' !== $element[0] ) {
			continue;
		}

		$noopen  = false === strpos( $element, '[' );
		$noclose = false === strpos( $element, ']' );
		if ( $noopen || $noclose ) {
			// This element does not contain shortcodes.
			if ( $noopen xor $noclose ) {
				// Need to encode stray '[' or ']' chars.
				$element = strtr( $element, $trans );
			}
			continue;
		}

		if ( $ignore_html || '<!--' === substr( $element, 0, 4 ) || '<![CDATA[' === substr( $element, 0, 9 ) ) {
			// Encode all '[' and ']' chars.
			$element = strtr( $element, $trans );
			continue;
		}

		$attributes = wp_kses_attr_parse( $element );
		if ( false === $attributes ) {
			// Some plugins are doing things like [name] <[email]>.
			if ( 1 === preg_match( '%^<\s*\[\[?[^\[\]]+\]%', $element ) ) {
				$element = preg_replace_callback( "/$pattern/", 'do_shortcode_tag', $element );
			}

			// Looks like we found some crazy unfiltered HTML. Skipping it for sanity.
			$element = strtr( $element, $trans );
			continue;
		}

		// Get element name.
		$front   = array_shift( $attributes );
		$back    = array_pop( $attributes );
		$matches = array();
		preg_match( '%[a-zA-Z0-9]+%', $front, $matches );
		$elname = $matches[0];

		// Look for shortcodes in each attribute separately.
		foreach ( $attributes as &$attr ) {
			$open  = strpos( $attr, '[' );
			$close = strpos( $attr, ']' );
			if ( false === $open || false === $close ) {
				continue; // Go to next attribute. Square braces will be escaped at end of loop.
			}
			$double = strpos( $attr, '"' );
			$single = strpos( $attr, "'" );
			if ( ( false === $single || $open < $single ) && ( false === $double || $open < $double ) ) {
				/*
				 * $attr like '[shortcode]' or 'name = [shortcode]' implies unfiltered_html.
				 * In this specific situation we assume KSES did not run because the input
				 * was written by an administrator, so we should avoid changing the output
				 * and we do not need to run KSES here.
				 */
				$attr = preg_replace_callback( "/$pattern/", 'do_shortcode_tag', $attr );
			} else {
				// $attr like 'name = "[shortcode]"' or "name = '[shortcode]'".
				// We do not know if $content was unfiltered. Assume KSES ran before shortcodes.
				$count    = 0;
				$new_attr = preg_replace_callback( "/$pattern/", 'do_shortcode_tag', $attr, -1, $count );
				if ( $count > 0 ) {
					// Sanitize the shortcode output using KSES.
					$new_attr = wp_kses_one_attr( $new_attr, $elname );
					if ( '' !== trim( $new_attr ) ) {
						// The shortcode is safe to use now.
						$attr = $new_attr;
					}
				}
			}
		}
		$element = $front . implode( '', $attributes ) . $back;

		// Now encode any remaining '[' or ']' chars.
		$element = strtr( $element, $trans );
	}

	$content = implode( '', $textarr );

	return $content;
}

/**
 * Remove placeholders added by do_shortcodes_in_html_tags().
 *
 * @since 4.2.3
 *
 * @param string $content Content to search for placeholders.
 * @return string Content with placeholders removed.
 */
function unescape_invalid_shortcodes( $content ) {
	// Clean up entire string, avoids re-parsing HTML.
	$trans = array(
		'&#91;' => '[',
		'&#93;' => ']',
	);

	$content = strtr( $content, $trans );

	return $content;
}

/**
 * Retrieve the shortcode attributes regex.
 *
 * @since 4.4.0
 *
 * @return string The shortcode attribute regular expression
 */
function get_shortcode_atts_regex() {
	return '/([\w-]+)\s*=\s*"([^"]*)"(?:\s|$)|([\w-]+)\s*=\s*\'([^\']*)\'(?:\s|$)|([\w-]+)\s*=\s*([^\s\'"]+)(?:\s|$)|"([^"]*)"(?:\s|$)|\'([^\']*)\'(?:\s|$)|(\S+)(?:\s|$)/';
}

/**
 * Retrieve all attributes from the shortcodes tag.
 *
 * The attributes list has the attribute name as the key and the value of the
 * attribute as the value in the key/value pair. This allows for easier
 * retrieval of the attributes, since all attributes have to be known.
 *
 * @since 2.5.0
 *
 * @param string $text
 * @return array|string List of attribute values.
 *                      Returns empty array if '""' === trim( $text ).
 *                      Returns empty string if '' === trim( $text ).
 *                      All other matches are checked for not empty().
 */
function shortcode_parse_atts( $text ) {
	$atts    = array();
	$pattern = get_shortcode_atts_regex();
	$text    = preg_replace( "/[\x{00a0}\x{200b}]+/u", ' ', $text );
	if ( preg_match_all( $pattern, $text, $match, PREG_SET_ORDER ) ) {
		foreach ( $match as $m ) {
			if ( ! empty( $m[1] ) ) {
				$atts[ strtolower( $m[1] ) ] = stripcslashes( $m[2] );
			} elseif ( ! empty( $m[3] ) ) {
				$atts[ strtolower( $m[3] ) ] = stripcslashes( $m[4] );
			} elseif ( ! empty( $m[5] ) ) {
				$atts[ strtolower( $m[5] ) ] = stripcslashes( $m[6] );
			} elseif ( isset( $m[7] ) && strlen( $m[7] ) ) {
				$atts[] = stripcslashes( $m[7] );
			} elseif ( isset( $m[8] ) && strlen( $m[8] ) ) {
				$atts[] = stripcslashes( $m[8] );
			} elseif ( isset( $m[9] ) ) {
				$atts[] = stripcslashes( $m[9] );
			}
		}

		// Reject any unclosed HTML elements.
		foreach ( $atts as &$value ) {
			if ( false !== strpos( $value, '<' ) ) {
				if ( 1 !== preg_match( '/^[^<]*+(?:<[^>]*+>[^<]*+)*+$/', $value ) ) {
					$value = '';
				}
			}
		}
	} else {
		$atts = ltrim( $text );
	}

	return $atts;
}

/**
 * Combine user attributes with known attributes and fill in defaults when needed.
 *
 * The pairs should be considered to be all of the attributes which are
 * supported by the caller and given as a list. The returned attributes will
 * only contain the attributes in the $pairs list.
 *
 * If the $atts list has unsupported attributes, then they will be ignored and
 * removed from the final returned list.
 *
 * @since 2.5.0
 *
 * @param array  $pairs     Entire list of supported attributes and their defaults.
 * @param array  $atts      User defined attributes in shortcode tag.
 * @param string $shortcode Optional. The name of the shortcode, provided for context to enable filtering
 * @return array Combined and filtered attribute list.
 */
function shortcode_atts( $pairs, $atts, $shortcode = '' ) {
	$atts = (array) $atts;
	$out  = array();
	foreach ( $pairs as $name => $default ) {
		if ( array_key_exists( $name, $atts ) ) {
			$out[ $name ] = $atts[ $name ];
		} else {
			$out[ $name ] = $default;
		}
	}

	if ( $shortcode ) {
		/**
		 * Filters shortcode attributes.
		 *
		 * If the third parameter of the shortcode_atts() function is present then this filter is available.
		 * The third parameter, $shortcode, is the name of the shortcode.
		 *
		 * @since 3.6.0
		 * @since 4.4.0 Added the `$shortcode` parameter.
		 *
		 * @param array  $out       The output array of shortcode attributes.
		 * @param array  $pairs     The supported attributes and their defaults.
		 * @param array  $atts      The user defined shortcode attributes.
		 * @param string $shortcode The shortcode name.
		 */
		$out = apply_filters( "shortcode_atts_{$shortcode}", $out, $pairs, $atts, $shortcode );
	}

	return $out;
}

/**
 * Remove all shortcode tags from the given content.
 *
 * @since 2.5.0
 *
 * @global array $shortcode_tags
 *
 * @param string $content Content to remove shortcode tags.
 * @return string Content without shortcode tags.
 */
function strip_shortcodes( $content ) {
	global $shortcode_tags;

	if ( false === strpos( $content, '[' ) ) {
		return $content;
	}

	if ( empty( $shortcode_tags ) || ! is_array( $shortcode_tags ) ) {
		return $content;
	}

	// Find all registered tag names in $content.
	preg_match_all( '@\[([^<>&/\[\]\x00-\x20=]++)@', $content, $matches );

	$tags_to_remove = array_keys( $shortcode_tags );

	/**
	 * Filters the list of shortcode tags to remove from the content.
	 *
	 * @since 4.7.0
	 *
	 * @param array  $tags_to_remove Array of shortcode tags to remove.
	 * @param string $content        Content shortcodes are being removed from.
	 */
	$tags_to_remove = apply_filters( 'strip_shortcodes_tagnames', $tags_to_remove, $content );

	$tagnames = array_intersect( $tags_to_remove, $matches[1] );

	if ( empty( $tagnames ) ) {
		return $content;
	}

	$content = do_shortcodes_in_html_tags( $content, true, $tagnames );

	$pattern = get_shortcode_regex( $tagnames );
	$content = preg_replace_callback( "/$pattern/", 'strip_shortcode_tag', $content );

	// Always restore square braces so we don't break things like <!--[if IE ]>.
	$content = unescape_invalid_shortcodes( $content );

	return $content;
}

/**
 * Strips a shortcode tag based on RegEx matches against post content.
 *
 * @since 3.3.0
 *
 * @param array $m RegEx matches against post content.
 * @return string|false The content stripped of the tag, otherwise false.
 */
function strip_shortcode_tag( $m ) {
	// Allow [[foo]] syntax for escaping a tag.
	if ( '[' === $m[1] && ']' === $m[6] ) {
		return substr( $m[0], 1, -1 );
	}

	return $m[1] . $m[6];
}