Your regular expression doesn't allow new-line characters. If you enter more than one line of text in the textbox, the validation will fail.
If you want the field to be required, you'll also need a required field validator.
<asp:requiredFieldValidator runat="server" controlToValidate="txtdetail" errorMessage="You must enter the detail." />
<asp:regularExpressionValidator runat="server" controlToValidate="txtdetail" validationExpression="^[a-zA-Z0-9 ,\n]+$" errorMessage="Alphabet, Numbers and Space are allowed." />
You also need to check the
property in your code-behind event handler.
protected void BtnGoToOtherDetail_Click(object sender, EventArgs e)
MultiView1.ActiveViewIndex = 2;