The RegularExpressionValidator control performs its validation according to the regular expression. You can check the predefined pattern, such as a phone number, postal code, e-mail address, dates, and so on. The control uses the ValidationExpression property to set a valid regular expression that is applied to the input data.
Example
<asp:TextBox ID="txtEmailID"
runat="server"
Width="200px">
</asp:TextBox>
<asp:RegularExpressionValidator ID="RegularExpressionValidator1"
runat="server"
ForeColor="Red"
ControlToValidate="txtEmailID"
ErrorMessage="Enter correct EmailID"
ValidationExpression="\w+([-+.']\w+)*@\w+([-.]\w+)*\.\w+([-.]\w+)*">
</asp:RegularExpressionValidator>
