[a-zA-Z.]+

var r = @"[a-zA-Z.]+";

<asp:TextBox ID="qualificationBox" runat="server" Width="250px"></asp:TextBox>
<asp:RegularExpressionValidator ID="NameRequiredValidator" runat="server"
 ErrorMessage="Please Enter Name" ControlToValidate="qualificationBox"
 ValidationExpression="[a-zA-Z.]+"></asp:RegularExpressionValidator>

[a-zA-Z\.]+