Interpolating environment variables into a string in Ruby using String#scan

Question

I'm trying to interpolate environment variables into a string in Ruby and not having much luck. One of my requirements is to do something (log an error, prompt for input, whatever) if a placeholder is found in the initial string that has no matching environment variable. It looks like the block form of String#scan is what I need. Below is an irb session of my failed attempt.

irb(main):014:0> raw_string = "need to replace %%FOO%% and %%BAR%% in here"
=> "need to replace %%FOO%% and %%BAR%% in here"
irb(main):015:0> cooked_string << raw_string
=> "need to replace %%FOO%% and %%BAR%% in here"
irb(main):016:0> raw_string.scan(/%%(.*?)%%/) do |var|
irb(main):017:1* cooked_string.sub!("%%#{var}%%", ENV[var])
irb(main):018:1> done
irb(main):019:1> end
TypeError: cannot convert Array into String
    from (irb):17:in `[]'
    from (irb):17
    from (irb):16:in `scan'
    from (irb):16
    from :0

If I use ENV["FOO"] to manually interpolate one of those, it works fine. I'm banging my head against the desk. What am I doing wrong?

Ruby is 1.8.1 on RHEL or 1.8.7 on Cygwin.

Answer 1

irb(main):018:0> "how %%EDITOR%% now %%HOME%% brown cow".gsub(/%%.*?%%/) do |e|
irb(main):019:1*   ENV[e.gsub '%', '']
irb(main):020:1> end
=> "how vi now /Users/ross brown cow"

Slightly different, and incorporating your default action...

irb(main):045:0> "how %%EDITOR%% now %%HOME%% brown %%X%% cow".gsub(/%%(.*?)%%/) do |e|
irb(main):046:1*   t = ENV[$1]
irb(main):047:1>   t.nil? ? "my action" : t
irb(main):048:1> end
=> "how vi now /Users/ross brown my action cow"
irb(main):049:0>