How can I use Regular Expressions in Spotlight on Mountain Lion?
Apple's tools for accessing the Spotlight index (Spotlight itself and the mdfind command) don't offer any regex options, nor do I know of any 3rd party tools that offer that (I'm not actually sure Apple has APIs that would make that possible).

However, the mdfind command does offer a much more powerful interface than the GUI Spotlight search. It offers proper binary operators and finer grained control over what metadata you search. Check out The Power of mdfind (via archive.org) for some good information, as well as the mdfind man page.