It's possible to ask auto-completion modes like helm to not suggest files with particular file extensions, by:

(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?