Question

How would you calculate the intersection of characters between two strings?

For example (assuming we would have a method called String.intersection):

"abc".intersection("ab") = 2
"hello".intersection("hallo") = 4

Ok, boys and girls, thanks for your massive feedback. Some more examples:

"aaa".intersection("a") = 1
"foo".intersection("bar") = 0
"abc".intersection("bc") = 2
"abc".intersection("ac") = 2
"abba".intersection("aa") = 2

Some more notes:
Wikipedia defines intersection as follows:

Intersection of the sets A and B,
denoted A ∩ B, is the set of all
objects that are members of both A and
B. The intersection of {1, 2, 3} and
{2, 3, 4} is the set {2, 3}

Answer 1

This passes all your described test cases:

class String
  def intersection(other)
    str = self.dup
    other.split(//).inject(0) do |sum, char|
      sum += 1 if str.sub!(char,'')
      sum
    end
  end
end