wp_rel_nofollow( string $text )

Source

File: wp-includes/formatting.php

function wp_rel_nofollow( $text ) {
	// This is a pre-save filter, so text is already escaped.
	$text = stripslashes( $text );
	$text = preg_replace_callback(
		'|<a (.+?)>|i',
		function( $matches ) {
			return wp_rel_callback( $matches, 'nofollow' );
		},
		$text
	);
	return wp_slash( $text );
}