Javascript match, need help

[griffith]

I'm making a small function that should replace all characters that is not [a-f]|[A-F]. How can I make a function that replaces all charaters that isn't between the valid ones with ex. 0.?

ex:

string1 = "cfghhh";

want to replace it to be:
stiring1 = "cf0000";

[andrin]

Maybe:

var string1 = "cfghhh";
string1.replace(/[a-fA-F]/g ,'0');