[Rails] What is the question mark inside this regex doing?

['John Merlino' via Ruby on Rails: Talk]

I thought the ? matches zero or one occurrence of a pattern. However in 
this example:

def show_regexp(string, pattern)
  match = pattern.match(string)
  if match
    "#{match.pre_match}->#{match[0]}<-#{match.post_match}"
  else
    "no match"
  end
end

a = "The moon is made of cheese"
show_regexp(a, /\s.*?\s/) #=> The-> moon <-is made of cheese

What exactly is the ? doing?

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