You're not gonna like this, but what I got to work was:
$processed = ereg_replace( '\\' , '\\' , $raw );
In other words, the FROM argument is undergoing one more level
of backslash-interpretation that the TO argument is. The manual
is (of course) silent on this subject. I even added a paren-match
to the pattern and a \1 to the replacement, to see if that affected
things, but it didn't seem to.