WordPress at a glance
function is not described

wp_rel_nofollow_callback() WP 2.3.0

Callback to add rel=nofollow string to HTML A element.

Will remove already existing rel="nofollow" and rel='nofollow' from the string to prevent from invalidating (X)HTML.

No Hooks.

Return

String. HTML A Element with rel nofollow.

Usage

wp_rel_nofollow_callback( $matches );
$matches(array) (required)
Single Match

Code of wp rel nofollow callback: wp-includes/formatting.php VER 5.0.3

<?php
function wp_rel_nofollow_callback( $matches ) {
	$text = $matches[1];
	$atts = shortcode_parse_atts( $matches[1] );
	$rel  = 'nofollow';

	if ( preg_match( '%href=["\'](' . preg_quote( set_url_scheme( home_url(), 'http' ) ) . ')%i', $text ) ||
	     preg_match( '%href=["\'](' . preg_quote( set_url_scheme( home_url(), 'https' ) ) . ')%i', $text )
	) {
		return "<a $text>";
	}

	if ( ! empty( $atts['rel'] ) ) {
		$parts = array_map( 'trim', explode( ' ', $atts['rel'] ) );
		if ( false === array_search( 'nofollow', $parts ) ) {
			$parts[] = 'nofollow';
		}
		$rel = implode( ' ', $parts );
		unset( $atts['rel'] );

		$html = '';
		foreach ( $atts as $name => $value ) {
			$html .= "{$name}=\"$value\" ";
		}
		$text = trim( $html );
	}
	return "<a $text rel=\"$rel\">";
}