I don't know what users are going to enter as a URL in my circumventor. Therefore, I CANNOT use a simple str_replace() in my PHP script! I need to use preg_replace to search for a pattern for a partial URL, and replace that with the fulll URL of the domain.
A "partial" URL is not a pattern, so you simply do not need regular expressions for it. I would check if the URL provided is an absolute URL. If it is not, I would modify it to become an absolute URL. This certainly does not need regular expressions, unless you want to use preg_match() to check if it is an absolute URL (but even that may be overkill).
I don't think you understand what I'm trying to get at here.
Ah, the reason is that you did not provide a context for your problem. Earlier, you ask "how to replace X with Y?", and then you asked the same question, phrased differently, so without reading NogDog's reply, I gave pretty much the same reply. Nowhere was it mentioned anything about a "circumventor", whatever that is.
Instead of asking "How can I get it so that it replaces partial URLs with the full version, using preg_replace()?", for which I would just give you an answer that would be better implemented with str_replace(), present what you are really trying to do, and ask for suggestions on how to solve it appropriately.