Replacing all occurrences is the default behaviour in
$rreplace. Just use a group like this:
It will replace all occurrences of
c with a
-. Of course use the characters you actually want to replace.
One important note: Your example of replacing
_ makes me wonder, if you actually saw Picard generating underscores in filenames and you want to get rid of that. If so, you have to replace the actual characters getting replaced with
_ by Picard in your script (if Windows compatible filenames is activated, Picard will replace
["*:<>?|] with an underscore).
Also note that Picard will always replace
\ with an underscore before applying the script, as those characters are used as directory separators (and you don't want to have AC/DC end up in two folders).