Callback to add `rel=”noopener”` string to HTML A element.

Will not duplicate an existing ‘noopener’ value to avoid invalidating the HTML.

function wp_targeted_link_rel_callback( $matches ) {
	$link_html          = $matches[1];
	$original_link_html = $link_html;

	// Consider the HTML escaped if there are no unescaped quotes.
	$is_escaped = ! preg_match( '/(^|[^\])['"]/', $link_html );
	if ( $is_escaped ) {
		// Replace only the quotes so that they are parsable by wp_kses_hair(), leave the rest as is.
		$link_html = preg_replace( '/\(['"])/', '$1', $link_html );
	}

	$atts = wp_kses_hair( $link_html, wp_allowed_protocols() );

	/**
	 * Filters the rel values that are added to links with `target` attribute.
	 *
	 * @since 5.1.0
	 *
	 * @param string $rel       The rel values.
	 * @param string $link_html The matched content of the link tag including all HTML attributes.
	 */
	$rel = apply_filters( 'wp_targeted_link_rel', 'noopener', $link_html );

	// Return early if no rel values to be added or if no actual target attribute.
	if ( ! $rel || ! isset( $atts['target'] ) ) {
		return "";
	}

	if ( isset( $atts['rel'] ) ) {
		$all_parts = preg_split( '/s/', "{$atts['rel']['value']} $rel", -1, PREG_SPLIT_NO_EMPTY );
		$rel       = implode( ' ', array_unique( $all_parts ) );
	}

	$atts['rel']['whole'] = 'rel="' . esc_attr( $rel ) . '"';
	$link_html            = implode( ' ', array_column( $atts, 'whole' ) );

	if ( $is_escaped ) {
		$link_html = preg_replace( '/['"]/', '\$0', $link_html );
	}

	return "";
}