In Ruby, why does "omega-3 (dHA)".gsub(/\b([a-z])/, '\0'.upcase) not work?

I want to capitalize the first character of each word and leave the rest intact (so it is different from Rails's titleize, which downcase the rest). The first line is a test, and third line works, but I wonder why the second line doesn't work?

ruby-1.9.2-p180 :026 > "omega-3 (dHA)".gsub(/\b([a-z])/, '#\0#')
 => "#o#mega-3 (#d#HA)" 

ruby-1.9.2-p180 :027 > "omega-3 (dHA)".gsub(/\b([a-z])/, '\0'.upcase)
 => "omega-3 (dHA)" 

ruby-1.9.2-p180 :02开发者_JS百科8 > "omega-3 (dHA)".gsub(/\b([a-z])/) {|s| s.upcase}
 => "Omega-3 (DHA)" 

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'\0'.upcase calls the upcase method of the string '\0' - parens for invokation are optional in Ruby. Which of course does nothing, so the second line is just .gsub(/.../, '\0')