It's possible to ask completion to not suggest files with particular file extensions, by:

<!-- language: lang-lisp -->

    (setq completion-ignored-extensions
          (append completion-ignored-extensions
                  (quote
                   (".o" ".aux"))))

However, I'm not able to determine how to pattern-match on regular expressions *before* the "." and not just after it. For example, I don't want files that end with "_flymake.hs" to be suggested by completion modes. How could I accomplish this?