RegularExpressionValidator in Asp.net - asp.net

Question

I have search textbox,which has default value Enter Month to View Profit. When I click search button without entering any data, the default value of textbox is posted to server for search. I want that RegularExpressionValidator do not validate default value of textbox.
<asp:TextBox ID="Tboxsearch" Text="Enter Month to View Profit" OnClick="this.value=''" CssClass="textboxinput" runat="server"></asp:TextBox>
<asp:Button ID="ButtonSearch" CssClass="btnLog" runat="server" Text="Search" onclick="ButtonSearch_Click" />
<asp:RequiredFieldValidator
ID="RequiredFieldValidatorname"
runat="server"
ControlToValidate="Tboxsearch"
ForeColor="Red"
Text="*"
>
</asp:RequiredFieldValidator>
<asp:RegularExpressionValidator
ID="RegularExpressionValidatorname"
runat="server"
ValidationExpression="[a-zA-Z0-9]+"
ForeColor="Red"
ControlToValidate="Tboxsearch"
ErrorMessage="Enter Valid Name!"
>
</asp:RegularExpressionValidator>

Answer 1

The default for all validators except the RequiredFieldValidator control if you post with empty field the validator will not trigger
you must use required field validator with other validators to prevent the postback to happen
from MSDN
Special-Case Validation Results for ASP.NET Server Controls
EDIT
Also if you add your controls as in your question it should work but if there is other controls like for example other button you should set the validationGroup Property to the group that you want to work together
ValidationGroup="vGrp"
and your code will be like this
<asp:TextBox ID="Tboxsearch" Text="Enter Month to View Profit" OnClick="this.value=''" CssClass="textboxinput" runat="server" ValidationGroup="vGrp"></asp:TextBox>
<asp:Button ID="ButtonSearch" CssClass="btnLog" runat="server" Text="Search" onclick="ButtonSearch_Click" ValidationGroup="vGrp" />
<asp:RequiredFieldValidator
ID="RequiredFieldValidatorname"
runat="server"
ControlToValidate="Tboxsearch"
ForeColor="Red"
Text="*"
ValidationGroup="vGrp">
</asp:RequiredFieldValidator>
<asp:RegularExpressionValidator
ID="RegularExpressionValidatorname"
runat="server"
ValidationExpression="[a-zA-Z0-9]+"
ForeColor="Red"
ControlToValidate="Tboxsearch"
ErrorMessage="Enter Valid Name!" ValidationGroup="vGrp"></asp:RegularExpressionValidator>
from MSDN about validationGroup