Checking if String.scan does not match

Hi everyone,

I'm extracting 3 words from a string delimited with colons using the following 
approach:

  str = "test:string:here"
  str.scan(/(.+):(.+):(.+)/) do |a, b, c|
    puts a, b, c
  end

This works fine if all 3 words are present but if you pass a string with only 2 
words, e.g. "test:string", the block is not executed.

I need to report an error if the match fails.  This could be done like this:

  str = "test:string"
  matched = false
  str.scan(/(.+):(.+):(.+)/) do |a, b, c|
    puts a, b, c
    matched = true
  end
  if not matched
    puts "invalid format"
  end

but this is messy.  Is there a neater, more ruby-like, approach to this?

Any help is greatly appreciated,
Alex