Question

I want to prevent users from entering url’s (like a href=””) in a TextBox.

I want to use a regular expression validator but no idea what to write?

How can I do it?

Answer 1

Do you mean you literally want to prevent them from entering the text href=" in the TextBox, or you want to prevent URLs? Either way, a RegexValidator is one solution:

Actually, as far as I know there is not a very easy way to use an OOTB-regex validator to do a negative contains (i.e. “fail if any match”). Someone smarter may be able to correct me on that. But you can definitely use a custom validator:

<asp:TextBox runat="server" id="myTextBox" />
<asp:CustomValidator runat="server" OnServerValidate="ValidateNoUrls" ControlToValidate="myTextBox" ErrorMessage="URLs not allowed" />

Codebehind:

protected void ValidateNoUrls(object sender, ServerValidateEventArgs e)
{
    e.IsValid = !Regex.IsMatch(e.Value, @"(ht|f)tp(s?)\:\/\/[0-9a-zA-Z]([-.\w]*[0-9a-zA-Z])*(:(0-9)*)*(\/?)([a-zA-Z0-9\-\.\?\,\'\/\\\+&%\$#_]*)?");
}