return Pattern.quote(s.substring(0, s.length() - 1)) + "$";
        }
        return Pattern.quote(s);
    }

    /**
     * Unescapes double backslashes in regex patterns.
     * Converts escaped backslashes (\\) back to single backslashes (\).
     *
     * @param s the string to unescape
     * @return the unescaped string
     */
    protected String unescape(final String s) {

        assertFalse(CharUtil.isUrlChar('<')); // less than
        assertFalse(CharUtil.isUrlChar('>')); // greater than
        assertFalse(CharUtil.isUrlChar('"')); // double quote
        assertFalse(CharUtil.isUrlChar('\\')); // backslash
        assertFalse(CharUtil.isUrlChar('`')); // backtick
    }

    public void test_isUrlChar_controlCharacters() {
        // Test control characters
        assertFalse(CharUtil.isUrlChar('\u0000')); // null

    /**
     * Escapes special characters in an LDAP search filter to prevent LDAP injection attacks.
     *
     * <p>This method escapes the following characters as per RFC 4515:
     * <ul>
     * <li>\ (backslash) → \5c</li>
     * <li>* (asterisk) → \2a</li>
     * <li>( (left parenthesis) → \28</li>
     * <li>) (right parenthesis) → \29</li>
     * <li>\0 (null character) → \00</li>
     * </ul>
     *