Ruby Regex Gsub when does not match

Question

I want to gsub all characters in a string that are not letters and replace with '#'. I think I need a regular expression that goes something like, "gsub() when this regex does not match."

Any ideas?

Answer 1

Look ma, no regex...

str.tr( '^A-Za-z', '#' )

Answer 2

str.gsub(/[^a-zA-Z]/, '#')

The ^ means doesn't match