Convert camelCase to dash-case (hypens) in pure ruby

There are plenty of posts about the opposite way.

But how to I convert camelCase to camel-case in ruby? My regex-game is pretty low ... here is it the other way around:

def underscore(string)  string.gsub(/::/, '/').  gsub(/([A-Z]+)([A-Z][a-z])/,'\1_\2').  gsub(/([a-z\d])([A-Z])/,'\1_\2').  tr("-", "_").  downcaseend