Exchancing characters in .htacess

Hi,
just another problem - I wanna exchange 1 oder more single characters using
htaccess

example:
site_1.htm =>seite-1.htm
the same thing I do in PHP using str_replace ("_", "-", "site_1.htm")

But I have to do it in .htaccess - is there any chance?

I thought about:
redirect seeother (.+)_(.+)_(.+).htm $1-$2-$3.htm

- but I don't know how often the "-"-type appears in every URL.

Any ideas, suggestions, solutions...?

Thanks!

Frank