{
        ......

        Regex strPattern = new Regex("[0-9]*[A-Z]*[a-z]*[_]*");

        if (!strPattern.IsMatch(val))
        {
            return false;
        }

        return true
  }

Regex strPattern = new Regex("^[0-9A-Za-z_]*$");

new Regex("[0-9A-Za-z_]*");