assertEquals("", fessXpathTransformer.removeCommentTag(""));
        assertEquals(" ", fessXpathTransformer.removeCommentTag("<!-- - -->"));
        assertEquals("abc", fessXpathTransformer.removeCommentTag("abc"));
        assertEquals("abc ", fessXpathTransformer.removeCommentTag("abc<!-- foo -->"));
        assertEquals("abc 123", fessXpathTransformer.removeCommentTag("abc<!-- fo\no -->123"));

        } catch (final MalformedURLException e) {
            logger.warn("Invalid canonical url: {} : {}", baseUrl, canonicalUrl, e);
        }
        return null;
    }

    protected String removeCommentTag(final String content) {
        if (content == null) {
            return StringUtil.EMPTY;
        }
        String value = content;
        int pos = value.indexOf("<!--");
        while (pos >= 0) {