Source

File: wp-includes/formatting.php

function wp_rel_callback( $matches, $rel ) {
	$text = $matches[1];
	$atts = wp_kses_hair( $matches[1], wp_allowed_protocols() );

	if ( ! empty( $atts['href'] ) ) {
		if ( in_array( strtolower( wp_parse_url( $atts['href']['value'], PHP_URL_SCHEME ) ), array( 'http', 'https' ), true ) ) {
			if ( strtolower( wp_parse_url( $atts['href']['value'], PHP_URL_HOST ) ) === strtolower( wp_parse_url( home_url(), PHP_URL_HOST ) ) ) {
				return "<a $text>";
			}
		}
	}

	if ( ! empty( $atts['rel'] ) ) {
		$parts     = array_map( 'trim', explode( ' ', $atts['rel']['value'] ) );
		$rel_array = array_map( 'trim', explode( ' ', $rel ) );
		$parts     = array_unique( array_merge( $parts, $rel_array ) );
		$rel       = implode( ' ', $parts );
		unset( $atts['rel'] );

		$html = '';
		foreach ( $atts as $name => $value ) {
			if ( isset( $value['vless'] ) && 'y' === $value['vless'] ) {
				$html .= $name . ' ';
			} else {
				$html .= "{$name}=\"" . esc_attr( $value['value'] ) . '" ';
			}
		}
		$text = trim( $html );
	}
	return "<a $text rel=\"" . esc_attr( $rel ) . '">';
}