title: Kotlin: count occurrences of `charArray` in `String`
body: <p>I have two strings</p>
  
  <pre><code>val string1 = "Hello"
  val string2 = "Hello world"
  </code></pre>
  
  <p>I have to count existence of each letter from <code>string1</code> in <code>string2</code> in <code>Kotlin</code></p>
  
  <p>So far, I have written this much code and stuck with regex</p>
  
  <pre><code>val string1_array = string1.toCharArray()
  val pattern = Regex("") // Regex pattern here
  val matcher = string2
  
  val count = pattern.findAll(matcher).count()
  </code></pre>
  
  <p>What should be the appropriate Regex pattern to search for charArray? Is there some better way to do in Kotlin</p>