How to validate Textbox to allow only website url

Question

Hi,
I need to validate textbox so that it can only take website url up-to domain code only.
For e.g. User must insert website url as "http://www.google.com".

If user insert "http://www.codeproject.com/Articles/334310/Understanding-ASP-NET-Validation-Techniques" then it should not allow.

Allowed:-"http://www.google.com"

Not Allowed:-"http://www.codeproject.com/Articles/334310/Understanding-ASP-NET-Validation-Techniques"

Please help.