REGEX in Page behind?

I am writing an app in ASP.NET 2.0. I want to have the code behind use a regex to validate a field. Any ideas on how to do this? The code is VB.NET

I need to do it in code behind because if I do it in the validation control it checks the field when the page loads.

thanks in advance

Needs to be posted in the VB.NET section.....

Kai

VB Code:

Dim regex As New RegularExpression("expressionhere")
If regex.IsMatch(TextBox1.Text) THen
'...
End If

I haven't checked it, but I think you can also make the regular expression validator work on postback too, instead of the page itself.