Source

File: wp-includes/formatting.php

function wp_html_excerpt( $str, $count, $more = null ) {
	if ( null === $more ) {
		$more = '';
	}

	$str     = wp_strip_all_tags( $str, true );
	$excerpt = mb_substr( $str, 0, $count );

	// Remove part of an entity at the end.
	$excerpt = preg_replace( '/&[^;\s]{0,6}$/', '', $excerpt );
	if ( $str != $excerpt ) {
		$excerpt = trim( $excerpt ) . $more;
	}

	return $excerpt;
}