RegularExpressionValidator is very useful and powerful validation control in asp.net We can check the value of an associated input control that matches the pattern specified by a regular expression or not. The Property which is specific to RegularExpressionValidator is Property="ValidationExpression".

This example, checks if the input matches the pattern of an email address

<asp:TextBox ID="txtEmail" runat="server" Width="150px">
</asp:TextBox>

<asp:RegularExpressionValidator ID="RegularExpressionValidatorEmail" runat="server" 
ControlToValidate="txtEmail" ValidationExpression="\w+([-+.']\w+)*@\w+([-.]\w+)*\.\w+([-.]\w+)*"
ErrorMessage="Invalid Email" ForeColor="Red"></asp:RegularExpressionValidator> 

By default client side validation is turned on. To disable client side validation set EnableClientScript o false. This property is supported by all validation controls.

To disable validation control set Enabled propety to false.