Based on search shown in https://gist.github.com/4698031
Generate new empty RubyString's with US-ASCII encoding.
I just reviewed the regexp newstring changes and I think you might need to audit these individually. One of the two changes is for 1.8 mode which I think newString() is correct for that. The second was for union19 which arguably is correct except that the method will unconditionally set the encoding on the string. So in that case one could argue the change is not needed.
After talking to Hiro we decided to close this and fix per reported case if there are any.