regular expression for only alpha characters in C#

<asp:RegularExpressionValidator ID="regName" runat="server" ControlToValidate="txtYourControlId"
                      ValidationGroup="ValidGroup" Display="Dynamic" ValidationExpression="^[a-zA-Z'.\s]"
                      Text="Please enter alphabets only" />

//use System.Text.RegularExpressions

if (Regex.IsMatch("^[a-zA-Z]", textBox1.Text))
   {
//assign error to label
          }