Let's explain this on a well known service - Twitter.
After you sign up for Twitter you are redirected to a "Welcome Page" which has an unique URL address. The only way you can end up on this URL is after you've signed up for Twitter.
And that's it.
If you still can't figure this out OR you need to use regular expressions, get in touch with us at firstname.lastname@example.org and we will see if we can help.