You could just write a CustomValidator that checks for an (Integer?) value
greater than zero.

http://msdn2.microsoft.com/en-us/library/f5db6z8k.aspx
-- Peter
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What are you going to do with value from the textbox? If you want to
convert it to a numeric value then a RangeValidator will be good
enough, just set a maximum value to Int32.MaxValue or Int64.MaxValue
according to the required result. In that way, you also check whether
an entered value does not exceed its data type range.

But if you want a string that contains numbers only, starting from 1,
use a RegularExpressionValidator with ValidationExpression =
"[1-9]\d*"

Regards,
Mykola
http://marss.co.ua

You neither need a RegExValidator nor a custom validator for that. The
compareValidator can validate against a constant value. Your tag should look
like this:

<asp:CompareValidator ID="CV1" runat="server"
    Type="Double"
    Operator="GreaterThanEqual"
    ValueToCompare="1"
    ControlToValidate="myTextBox"
    ErrorMessage="Value must be greater than 0!" />

Hope that works for you. Eid Mubarak ;)