There's a method that lets you substitute a bunch of characters for a corresponding set of characters
(it's not gsub). Find that method and use it in a code sample.

# Element assignment ([]=) is a method:
irb> str = "rlabuonora@yahoo.com"
=> "rlabuonora@yahoo.com"
irb> str.methods
=> [:<=>, :==, :===, :eql?, :hash, :casecmp, :+, :*, :%, :[], :[]=,
   ...


# str[] is syntactic sugar for str.[]=
irb> str.[]=(0, "s")
=> "s"
irb> str
=> "slabuonora@yahoo.com"

# you can user integers in []
irb> str[0] = "r"
=> "r"
irb> str
=> "rlabuonora@yahoo.com"
irb>

# but also ranges
irb> str[(0..10)] = "*" * 10
=> "**********"
irb> str
=> "**********yahoo.com"

# and even regular expressions
irb> str = "rlabuonora@yahoo.com"
=> "rlabuonora@yahoo.com"
irb> str[/@(.*).com/] = "@gmail.com"
=> "@gmail.com"
irb> str
=> "rlabuonora@gmail.com"