Actually, I oversimplified my example.  I want to escape single quotes 
and question marks.

irb(main):001:0> a = "Can't we?"
=> "Can't we?"
irb(main):002:0> a.gsub(/([\'\?])/, '\\1')
=> "Can't we?"
irb(main):003:0> a.gsub(/([\'\?])/, '\\\1')
=> "Can\\1t we\\1"
irb(main):004:0> a.gsub(/([\'\?])/, '\\\\1')
=> "Can\\1t we\\1"
irb(main):005:0> a.gsub(/([\'\?])/, '\\\\\1')
=> "Can\\'t we\\?"
irb(main):006:0>