This feature is not available in classic Google Search and it's not in Google's roadmap. You can learn more about this topic watching the Google video Will Google implement the ability to search with regular expressions?
However, there's one exception. Google Code Search supports regular expressions. Of course, the search target for this topic search engine is reduced to source code only.
It is worth to mention that some Google search keywords can partially replace regular expressions. For example, if you want to search any two-word variation of "search TERM", you can use the wildcard operator.
will find results for search and any other (one) word. I often use it to check basic English grammar rules or synonyms (e.g., "as easy as *").