Source

File: wp-includes/class-wp-site-query.php

protected function get_search_sql( $string, $columns ) {
		global $wpdb;

		if ( false !== strpos( $string, '*' ) ) {
			$like = '%' . implode( '%', array_map( array( $wpdb, 'esc_like' ), explode( '*', $string ) ) ) . '%';
		} else {
			$like = '%' . $wpdb->esc_like( $string ) . '%';
		}

		$searches = array();
		foreach ( $columns as $column ) {
			$searches[] = $wpdb->prepare( "$column LIKE %s", $like );
		}

		return '(' . implode( ' OR ', $searches ) . ')';
	}