How to enforce Minimum character length Validation or how to allow Minimum of 4 letters?

<asp:TextBox Id = "txt_employee_code" runat = "server"></asp:TextBox>



<asp:RegularExpressionValidator Display = "Dynamic" ControlToValidate = "txt_employee_code" Id = "reg_ex_employee_code" ValidationExpression = "^[\s\S]{4,}$" runat = "server" ErrorMessage = "Minimum of 4 letters allowed only"></asp:RegularExpressionValidator>