Counting sub-string occurrence in a string

Once upon a time.. I need a way to count number of occurrence of certain sub-string (not exclusively words) inside a string.

For example, string "Aya Hirano is the best seiyuu in the world! She was mostly known as her role as Haruhi." I wanted to know how many times string "he" occurred in there.
The result should be 4, 2 in the, 1 in She and 1 in her.

By using scan method we can do that easily:

class String
  def substr_count(pattern)
    self.scan(pattern).length
  end
end

"Aya Hirano is the best seiyuu in the world! She was mostly known as her role as Haruhi.".substr_count("he") # -> 4
"Aya Hirano is the best seiyuu in the world! She was mostly known as her role as Haruhi.".substr_count(/(!|\.|\?)/) # -> 2 -> counting sentences? :D

Too simple to become share-worthy article I guess.. :P