public function countSubstring($substring, $caseSensitive = true) {
		$this->verifyNotEmpty($substring, '$substring');
		if ($caseSensitive) {
			return mb_substr_count($this->string, $substring, $this->encoding);
		}
		$str = mb_strtoupper($this->string, $this->encoding);
		$substring = mb_strtoupper($substring, $this->encoding);
		return mb_substr_count($str, $substring, $this->encoding);
	}