private static function prepareRules(array $rules)
    {
        $mergedRules = [];

        foreach ($rules as $ruleFields => $ruleConditions) {

            //if set of fields like 'firstname, lastname...'
            if (strpos($ruleFields, ',') !== false) {
                foreach (explode(',', $ruleFields) as $fieldName) {
                    $fieldName = trim($fieldName);

                    if (!isset($mergedRules[$fieldName])) {
                        $mergedRules[$fieldName] = $ruleConditions;
                    } else {
                        $mergedRules[$fieldName] .= '|'.$ruleConditions;
                    }
                }
            } else {
                if (!isset($mergedRules[$ruleFields])) {
                    $mergedRules[$ruleFields] = $ruleConditions;
                } else {
                    $mergedRules[$ruleFields] .= '|'.$ruleConditions;
                }
            }
        }

        $finalRules = [];

        //remove duplicated rules, like 'required|alpha|required'
        foreach ($mergedRules as $newRule => $rule) {
            $finalRules[$newRule] = implode('|', array_unique(explode('|', $rule)));
        }

        return $finalRules;
    }