private static String getPlaceholder(final String rule) {
        int start = rule.indexOf("${");
        if (start < 0) {
            return null;
        }
        int counter = 1;
        for (int i = start + 2; i < rule.length(); i++) {
            char c = rule.charAt(i);
            if (c == '{') {
                counter++;
            } else if (c == '}') {
                counter--;
                if (counter == 0) {
                    return "${" + rule.substring(start + 2, i) + "}";
                }
            }
        }
        return null;
    }
replace placeholder in content.

@param content response content
@param mockRequest request
@return replace the content after the placeholder.
shenyu-plugin/shenyu-plugin-mock/src/main/java/org/apache/shenyu/plugin/mock/generator/GeneratorFactory.java