You need to find the specific file and line of code within the plugin persian-woocommerce where the preg_match() function is being used. Since the error message states the offset as 51, you will want to look at the 51st character of the regex inside the preg_match function. The most common cause of this error is incorrect escaping of backslashes. In regex, a backslash \ is used as an escape character. If you want to match a literal backslash, you need to escape it with another backslash (\\).