Please refer this article
Regular Expression (Regex) to accept only Alphabets and Space in TextBox in ASP.Net
Change the ValidationExpression to
ValidationExpression="[a-zA-Z0-9,_]*$"
<asp:RegularExpressionValidator ID="RegularExpressionValidator1" runat="server" ControlToValidate="TextBox1"
ValidationExpression="[a-zA-Z0-9,_]*$" ErrorMessage="*Valid characters: Alphabets and Special Characters (,_)." />