How to split a CamelCase string in its substrings in Ruby?

I have a nice CamelCase string such as ImageWideNice or ImageNarrowUgly. Now I want to break that string in its substrings, such as Image, Wide or Narrow, and Nice or Ugly.

I thought this could be solved simply by

camelCaseString =~ /(Image)((Wide)|(Narrow))((Nice)|(Ugly))/

But strangely, this will only fill $1 and $2, but not $3.

Do you have a better idea for splitting that string?