The Ruby Way: Solutions and Techniques in Ruby Programming, Second Edition

3.7. Accessing Backreferences

Each parenthesized piece of a regular expression will be a submatch of its own. These are numbered and can be referenced by these numbers in more than one way. Let’s examine the more traditional “ugly” ways first.

The special global variables $1, $2, and so on, can be used to reference matches:

str = "a123b45c678"
if /(a\d+)(b\d+)(c\d+)/ =~ str
  puts "Matches are: '#$1', '#$2', '#$3'"
  # Prints: Matches are: 'a123', 'b45', 'c768'
end

Within a substitution such as sub or gsub, these variables cannot be used:

str = "a123b45c678"
str.sub(/(a\d+)(b\d+)(c\d+)/, "1st=#$1, 2nd=#$2, 3rd=#$3")
# "1st=, 2nd=, 3rd="

Why didn’t this work? Because the arguments to sub are evaluated before sub is called. This code is equivalent: ...