title: String#count options
body: <p>From the <a href="http://www.ruby-doc.org/core-1.9.3/String.html#method-i-count">documentation for <code>String#count</code></a> I understand the first example, but I do not understand the rest of the examples:</p>
  
  <pre><code>a = "hello world"
  a.count "lo"            #=&gt; 5
  a.count "lo", "o"       #=&gt; 2
  a.count "hello", "^l"   #=&gt; 4
  a.count "ej-m"          #=&gt; 4
  </code></pre>
  
  <p>Any explanation will be helpful.</p>