How would you modify the following validation expression in ASP.NET to allow for thousands separators and value must be > 0.00?

^[0-9]{1,15}(\.[0-9]{1,2})?$

That is different than your original statement....

99,999,999,999,999.99

and you have 6.845.88

But you deleted what you posted LOL.  Did the expression work for you?

I deleted what I posted because I noticed that I had entered the value with 2 decimal points  6.845.88   as you had stated:

I re-input the value as  6,845.88 and I get the error message that I had set up:  
ErrorMessage="Enter dollars, decimal, cents and commas. (ex: 0.36)."

I used the following code:

<td align="left" width="50%" style="font-family: Arial, Helvetica, sans-serif;
                           font-style: normal; font-weight: bold;color: #00356A; " class="style18"> Dollar Amount:
                           <asp:Label ID="Label4" Text = " " runat ="server" ForeColor="#00356A" Width="2%" />
                           <asp:TextBox ID="txtAmountS" Maxlength="21" runat="server"
                               style="margin-right: 0px" Width="25%" ></asp:TextBox>                  
                           <asp:RegularExpressionValidator ID="RegularExpressionValidator3" runat="server"
                           ErrorMessage="Enter dollars, decimal, cents and commas. (ex: 0.36)."
                           ControlToValidate="txtAmountS" ForeColor="#FF0066"
                           SetFocusOnError="true"  
                           ValidationExpression="^[0-9]{1,15}(\.[0-9]{1,2})?$">                  
                           </asp:RegularExpressionValidator>

                       &nbsp;<asp:Label ID="lblHowMany" runat="server" Text="How Many" Visible="False"
                               Width="10%"></asp:Label>
&nbsp;
                           <asp:TextBox ID="txtHowManyS" runat="server" Height="25px" Visible="False"
                               Width="12%"></asp:TextBox>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;                  
                           
                       </td>

^\d{0,15}(?:\.\d{0,2})?$

^\d{0,15}(?:\.\d{0,2})?$

Would this accommodate the thousands separator (commas)?

It appears I get the same error when entering a value with commas.

Here is another approach:
/^(?:0*[1-9]\d*\s*(?:,|$)\s*)+$/gm