Well, that’s odd… (ruby 1.8.7)

irb(main):021:0> '\\\\' + 'x'
=> "\\\\x"
irb(main):022:0> "x".gsub("x",'\\\\' + 'x')
=> "\\\\x"
irb(main):023:0> '\\\\' + '&'
=> "\\\\&"
irb(main):024:0> "&".gsub("&",'\\\\' + '&')
=> "&"