try this
<asp:TextBox ID="TextBox1" MaxLength="5" runat="server"></asp:TextBox><asp:RegularExpressionValidator
ID="RegularExpressionValidator1" ControlToValidate="TextBox1" ValidationExpression="^[A-Za-z]\w*" runat="server" ErrorMessage="RegularExpressionValidator"></asp:RegularExpressionValidator>
<asp:Button ID="btn" Text="Check" runat="server" />