mod_rewrite: replace all occurance of a character

Hi,

I would like to replace all occurance of a character by another
character in mod_rewrite of .htaccess. However, I cannot find any
function to do this.

E.g. replace all "-" in "http://www.a-f.com/de-fg/d-f" by "_" and the
result should become "http://www.a_f.com/de_fg/d_f"

Thanks,
Jason