Allow Alphabets and Numbers (AlphaNumeric) with Comma and Underscore Special characters using RegularExpression in ASP.Net

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 (,_)." />