Variable substitution in Ruby sub/gsub replacement strings

With the sub and gsub methods in Ruby you can extract components of the matching string and use them in the replacement string.

For example, with input string 'Hello World' this replacement:

mystring.sub(/(Wo)rld/, '\1bble') -> Hello Wobble

Note the single quotes - if you use double quotes this happens:

mystring.sub(/(Wo)rld/, "\1bble") -> Hello \u0001bble

You can use double quotes if you double escape the 'sequence' \1

mystring.sub(/(Wo)rld/, "\\1bble") -> Hello Wobble

This is very important if you want to include a ruby variable in the replacement string

s = "Mr."
mystring.sub(/(Wo)rld/, "#{s} \\1bble") -> Hello Mr. Wobble

Just remember 'double quotes' means 'double escapes'....