how to write filter regexp?

[Tom Roche]

How does one negate an entire regexp (not merely a character range)?
Why I ask:

I'd like to use ediff-directories to compare two directories that
contain (mostly) java file, and I'd like to compare _only_ the java
files.

C-h f ediff-directories
> (ediff-directories DIR1 DIR2 REGEXP)

> Run Ediff on a pair of directories, DIR1 and DIR2, comparing files
> that have the same name in both. The third argument, REGEXP, is a
> regular expression that can be used to filter out certain file
> names.

What would be the appropriate filter regexp, given that I want the
_complement_ of *.java?